30 seconds of code

Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less.

Use Ctrl + F or command + F to search for a snippet.
Contributions welcome, please read the contribution guide.
Snippets are written in ES6, use the Babel transpiler to ensure backwards-compatibility.
You can import these snippets into your text editor of choice (VSCode, Atom, Sublime) using the files found in this repo.
You can import these snippets into Alfred 3, using this file.
If you want to follow 30-seconds-of-code on social media, you can find us on Facebook, Instagram and Twitter.

Related projects

Package

⚠️ NOTICE: A few of our snippets are not yet optimized for production (see disclaimers for individual snippet issues).

You can find a package with all the snippets on npm.

# With npm
npm install 30-seconds-of-code

# With yarn
yarn add 30-seconds-of-code

CDN links

Details

Browser

IMPORTANT: replace the src with the full version link and desired target spec (such as ES5 minified)):

<script src="https://unpkg.com/30-seconds-of-code"></script>
<script>
  _30s.average(1, 2, 3);
</script>

Node

// CommonJS
const _30s = require('30-seconds-of-code');
_30s.average(1, 2, 3);

// ES Modules
import _30s from '30-seconds-of-code';
_30s.average(1, 2, 3);

To import snippets directly:

// CommonJS
const { average } = require('30-seconds-of-code');
average(1, 2, 3);

// ES Modules
import { average } from '30-seconds-of-code';
average(1, 2, 3);

Table of Contents

🔌 Adapter

View contents

ary
call
collectInto
flip
over
overArgs
pipeAsyncFunctions
pipeFunctions
promisify
rearg
spreadOver
unary

📚 Array

View contents

all
allEqual
any
arrayToCSV
bifurcate
bifurcateBy
chunk
compact
countBy
countOccurrences
deepFlatten
difference
differenceBy
differenceWith
drop
dropRight
dropRightWhile
dropWhile
everyNth
filterNonUnique
filterNonUniqueBy
findLast
findLastIndex
flatten
forEachRight
groupBy
head
indexOfAll
initial
initialize2DArray
initializeArrayWithRange
initializeArrayWithRangeRight
initializeArrayWithValues
initializeNDArray
intersection
intersectionBy
intersectionWith
isSorted
join
JSONtoCSV
last
longestItem
mapObject
maxN
minN
none
nthElement
offset
partition
permutations
pull
pullAtIndex
pullAtValue
pullBy
reducedFilter
reduceSuccessive
reduceWhich
reject
remove
sample
sampleSize
shank
shuffle
similarity
sortedIndex
sortedIndexBy
sortedLastIndex
sortedLastIndexBy
stableSort
symmetricDifference
symmetricDifferenceBy
symmetricDifferenceWith
tail
take
takeRight
takeRightWhile
takeWhile
toHash
union
unionBy
unionWith
uniqueElements
uniqueElementsBy
uniqueElementsByRight
uniqueSymmetricDifference
unzip
unzipWith
without
xProd
zip
zipObject
zipWith

🌐 Browser

View contents

arrayToHtmlList
bottomVisible
copyToClipboard
counter
createElement
createEventHub
currentURL
detectDeviceType
elementContains
elementIsVisibleInViewport
getScrollPosition
getStyle
hasClass
hashBrowser
hide
httpsRedirect
insertAfter
insertBefore
isBrowserTabFocused
nodeListToArray
observeMutations
off
on
onUserInputChange
prefix
recordAnimationFrames
redirect
runAsync
scrollToTop
setStyle
show
smoothScroll
toggleClass
triggerEvent
UUIDGeneratorBrowser

⏱️ Date

View contents

dayOfYear
formatDuration
getColonTimeFromDate
getDaysDiffBetweenDates
getMeridiemSuffixOfInteger
isAfterDate
isBeforeDate
isSameDate
maxDate
minDate
tomorrow

🎛️ Function

View contents

attempt
bind
bindKey
chainAsync
compose
composeRight
converge
curry
debounce
defer
delay
functionName
hz
memoize
negate
once
partial
partialRight
runPromisesInSeries
sleep
throttle
times
uncurry
unfold
when

➗ Math

View contents

approximatelyEqual
average
averageBy
binomialCoefficient
clampNumber
degreesToRads
digitize
distance
elo
factorial
fibonacci
gcd
geometricProgression
hammingDistance
inRange
isDivisible
isEven
isPrime
lcm
luhnCheck
maxBy
median
minBy
percentile
powerset
primes
radsToDegrees
randomIntArrayInRange
randomIntegerInRange
randomNumberInRange
round
sdbm
standardDeviation
sum
sumBy
sumPower
toSafeInteger

📦 Node

View contents

atob
btoa
colorize
hasFlags
hashNode
isTravisCI
JSONToFile
readFileLines
untildify
UUIDGeneratorNode

🗃️ Object

View contents

bindAll
deepClone
deepFreeze
defaults
dig
equals
findKey
findLastKey
flattenObject
forOwn
forOwnRight
functions
get
invertKeyValues
lowercaseKeys
mapKeys
mapValues
matches
matchesWith
merge
nest
objectFromPairs
objectToPairs
omit
omitBy
orderBy
pick
pickBy
renameKeys
shallowClone
size
transform
truthCheckCollection
unflattenObject

📜 String

View contents

byteSize
capitalize
capitalizeEveryWord
CSVToArray
CSVToJSON
decapitalize
escapeHTML
escapeRegExp
fromCamelCase
indentString
isAbsoluteURL
isAnagram
isLowerCase
isUpperCase
mapString
mask
pad
palindrome
pluralize
removeNonASCII
reverseString
sortCharactersInString
splitLines
stringPermutations
stripHTMLTags
toCamelCase
toKebabCase
toSnakeCase
truncateString
unescapeHTML
URLJoin
words

📃 Type

View contents

getType
is
isArrayLike
isBoolean
isEmpty
isFunction
isNil
isNull
isNumber
isObject
isObjectLike
isPlainObject
isPrimitive
isPromiseLike
isString
isSymbol
isUndefined
isValidJSON

🔧 Utility

View contents

castArray
cloneRegExp
coalesce
coalesceFactory
extendHex
getURLParameters
hexToRGB
httpGet
httpPost
isBrowser
mostPerformant
nthArg
parseCookie
prettyBytes
randomHexColorCode
RGBToHex
serializeCookie
timeTaken
toCurrency
toDecimalMark
toOrdinalSuffix
validateNumber
yesNo

🔌 Adapter

ary

Creates a function that accepts up to n arguments, ignoring any additional arguments.

Call the provided function, fn, with up to n arguments, using Array.prototype.slice(0,n) and the spread operator (...).

const ary = (fn, n) => (...args) => fn(...args.slice(0, n));

Examples

const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [8, 4, 6], [10]].map(x => firstTwoMax(...x)); // [6, 8, 10]

⬆ Back to top

call

Given a key and a set of arguments, call them when given a context. Primarily useful in composition.

Use a closure to call a stored key with stored arguments.

const call = (key, ...args) => context => context[key](...args);

Examples

Promise.resolve([1, 2, 3])
  .then(call('map', x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
  .then(map(x => 2 * x))
  .then(console.log); //[ 2, 4, 6 ]

⬆ Back to top

collectInto

Changes a function that accepts an array into a variadic function.

Given a function, return a closure that collects all inputs into an array-accepting function.

const collectInto = fn => (...args) => fn(args);

Examples

const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)

⬆ Back to top

flip

Flip takes a function as an argument, then makes the first argument the last.

Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest.

const flip = fn => (first, ...rest) => fn(...rest, first);

Examples

let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b

⬆ Back to top

over

Creates a function that invokes each provided function with the arguments it receives and returns the results.

Use Array.prototype.map() and Function.prototype.apply() to apply each function to the given arguments.

const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));

Examples

const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1,5]

⬆ Back to top

overArgs

Creates a function that invokes the provided function with its arguments transformed.

Use Array.prototype.map() to apply transforms to args in combination with the spread operator (...) to pass the transformed arguments to fn.

const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val)));

Examples

const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]

⬆ Back to top

pipeAsyncFunctions

Performs left-to-right function composition for asynchronous functions.

Use Array.prototype.reduce() with the spread operator (...) to perform left-to-right function composition using Promise.then(). The functions can return a combination of: simple values, Promise's, or they can be defined as async ones returning through await. All functions must be unary.

const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));

Examples

const sum = pipeAsyncFunctions(
  x => x + 1,
  x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
  x => x + 3,
  async x => (await x) + 4
);
(async () => {
  console.log(await sum(5)); // 15 (after one second)
})();

⬆ Back to top

pipeFunctions

Performs left-to-right function composition.

Use Array.prototype.reduce() with the spread operator (...) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));

Examples

const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15

⬆ Back to top

promisify

Converts an asynchronous function to return a promise.

Use currying to return a function returning a Promise that calls the original function. Use the ...rest operator to pass in all the parameters.

In Node 8+, you can use util.promisify

const promisify = func => (...args) =>
  new Promise((resolve, reject) =>
    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
  );

Examples

const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s

⬆ Back to top

rearg

Creates a function that invokes the provided function with its arguments arranged according to the specified indexes.

Use Array.prototype.map() to reorder arguments based on indexes in combination with the spread operator (...) to pass the transformed arguments to fn.

const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));

Examples

var rearged = rearg(
  function(a, b, c) {
    return [a, b, c];
  },
  [2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']

⬆ Back to top

spreadOver

Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function.

Use closures and the spread operator (...) to map the array of arguments to the inputs of the function.

const spreadOver = fn => argsArr => fn(...argsArr);

Examples

const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3

⬆ Back to top

unary

Creates a function that accepts up to one argument, ignoring any additional arguments.

Call the provided function, fn, with just the first argument given.

const unary = fn => val => fn(val);

Examples

['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]

⬆ Back to top

📚 Array

all

Returns true if the provided predicate function returns true for all elements in a collection, false otherwise.

Use Array.prototype.every() to test if all elements in the collection return true based on fn. Omit the second argument, fn, to use Boolean as a default.

const all = (arr, fn = Boolean) => arr.every(fn);

Examples

all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true

⬆ Back to top

allEqual

Check if all elements in an array are equal.

Use Array.prototype.every() to check if all the elements of the array are the same as the first one.

const allEqual = arr => arr.every(val => val === arr[0]);

Examples

allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true

⬆ Back to top

any

Returns true if the provided predicate function returns true for at least one element in a collection, false otherwise.

Use Array.prototype.some() to test if any elements in the collection return true based on fn. Omit the second argument, fn, to use Boolean as a default.

const any = (arr, fn = Boolean) => arr.some(fn);

Examples

any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true

⬆ Back to top

arrayToCSV

Converts a 2D array to a comma-separated values (CSV) string.

Use Array.prototype.map() and Array.prototype.join(delimiter) to combine individual 1D arrays (rows) into strings. Use Array.prototype.join('\n') to combine all rows into a CSV string, separating each row with a newline. Omit the second argument, delimiter, to use a default delimiter of ,.

const arrayToCSV = (arr, delimiter = ',') =>
  arr.map(v => v.map(x => `"${x}"`).join(delimiter)).join('\n');

Examples

arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'

⬆ Back to top

bifurcate

Splits values into two groups. If an element in filter is truthy, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group.

Use Array.prototype.reduce() and Array.prototype.push() to add elements to groups, based on filter.

const bifurcate = (arr, filter) =>
  arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]);

Examples

bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ]

⬆ Back to top

bifurcateBy

Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.

Use Array.prototype.reduce() and Array.prototype.push() to add elements to groups, based on the value returned by fn for each element.

const bifurcateBy = (arr, fn) =>
  arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);

Examples

bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]

⬆ Back to top

chunk

Chunks an array into smaller arrays of a specified size.

Use Array.from() to create a new array, that fits the number of chunks that will be produced. Use Array.prototype.slice() to map each element of the new array to a chunk the length of size. If the original array can't be split evenly, the final chunk will contain the remaining elements.

const chunk = (arr, size) =>
  Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
    arr.slice(i * size, i * size + size)
  );

Examples

chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]]

⬆ Back to top

compact

Removes falsey values from an array.

Use Array.prototype.filter() to filter out falsey values (false, null, 0, "", undefined, and NaN).

const compact = arr => arr.filter(Boolean);

Examples

compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ]

⬆ Back to top

countBy

Groups the elements of an array based on the given function and returns the count of elements in each group.

Use Array.prototype.map() to map the values of an array to a function or property name. Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.

const countBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || 0) + 1;
    return acc;
  }, {});

Examples

countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}

⬆ Back to top

countOccurrences

Counts the occurrences of a value in an array.

Use Array.prototype.reduce() to increment a counter each time you encounter the specific value inside the array.

const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);

Examples

countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3

⬆ Back to top

deepFlatten

Deep flattens an array.

Use recursion. Use Array.prototype.concat() with an empty array ([]) and the spread operator (...) to flatten an array. Recursively flatten each element that is an array.

const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));

Examples

deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]

⬆ Back to top

difference

Returns the difference between two arrays.

Create a Set from b, then use Array.prototype.filter() on a to only keep values not contained in b.

const difference = (a, b) => {
  const s = new Set(b);
  return a.filter(x => !s.has(x));
};

Examples

difference([1, 2, 3], [1, 2, 4]); // [3]

⬆ Back to top

differenceBy

Returns the difference between two arrays, after applying the provided function to each array element of both.

Create a Set by applying fn to each element in b, then use Array.prototype.filter() in combination with fn on a to only keep values not contained in the previously created set.

const differenceBy = (a, b, fn) => {
  const s = new Set(b.map(v => fn(v)));
  return a.filter(x => !s.has(fn(x)));
};

Examples

differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]

⬆ Back to top

differenceWith

Filters out all values from an array for which the comparator function does not return true.

Use Array.prototype.filter() and Array.prototype.findIndex() to find the appropriate values.

const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);

Examples

differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]

⬆ Back to top

drop

Returns a new array with n elements removed from the left.

Use Array.prototype.slice() to slice the remove the specified number of elements from the left.

const drop = (arr, n = 1) => arr.slice(n);

Examples

drop([1, 2, 3]); // [2,3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []

⬆ Back to top

dropRight

Returns a new array with n elements removed from the right.

Use Array.prototype.slice() to slice the remove the specified number of elements from the right.

const dropRight = (arr, n = 1) => arr.slice(0, -n);

Examples

dropRight([1, 2, 3]); // [1,2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []

⬆ Back to top

dropRightWhile

Removes elements from the end of an array until the passed function returns true. Returns the remaining elements in the array.

Loop through the array, using Array.prototype.slice() to drop the last element of the array until the returned value from the function is true. Returns the remaining elements.

const dropRightWhile = (arr, func) => {
  while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1);
  return arr;
};

Examples

dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]

⬆ Back to top

dropWhile

Removes elements in an array until the passed function returns true. Returns the remaining elements in the array.

Loop through the array, using Array.prototype.slice() to drop the first element of the array until the returned value from the function is true. Returns the remaining elements.

const dropWhile = (arr, func) => {
  while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
  return arr;
};

Examples

dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4]

⬆ Back to top

everyNth

Returns every nth element in an array.

Use Array.prototype.filter() to create a new array that contains every nth element of a given array.

const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);

Examples

everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]

⬆ Back to top

filterNonUnique

Filters out the non-unique values in an array.

Use Array.prototype.filter() for an array containing only the unique values.

const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));

Examples

filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]

⬆ Back to top

filterNonUniqueBy

Filters out the non-unique values in an array, based on a provided comparator function.

Use Array.prototype.filter() and Array.prototype.every() for an array containing only the unique values, based on the comparator function, fn. The comparator function takes four arguments: the values of the two elements being compared and their indexes.

const filterNonUniqueBy = (arr, fn) =>
  arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));

Examples

filterNonUniqueBy(
  [
    { id: 0, value: 'a' },
    { id: 1, value: 'b' },
    { id: 2, value: 'c' },
    { id: 1, value: 'd' },
    { id: 0, value: 'e' }
  ],
  (a, b) => a.id == b.id
); // [ { id: 2, value: 'c' } ]

⬆ Back to top

findLast

Returns the last element for which the provided function returns a truthy value.

Use Array.prototype.filter() to remove elements for which fn returns falsey values, Array.prototype.pop() to get the last one.

const findLast = (arr, fn) => arr.filter(fn).pop();

Examples

findLast([1, 2, 3, 4], n => n % 2 === 1); // 3

⬆ Back to top

findLastIndex

Returns the index of the last element for which the provided function returns a truthy value.

Use Array.prototype.map() to map each element to an array with its index and value. Use Array.prototype.filter() to remove elements for which fn returns falsey values, Array.prototype.pop() to get the last one.

const findLastIndex = (arr, fn) =>
  arr
    .map((val, i) => [i, val])
    .filter(([i, val]) => fn(val, i, arr))
    .pop()[0];

Examples

findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)

⬆ Back to top

flatten

Flattens an array up to the specified depth.

Use recursion, decrementing depth by 1 for each level of depth. Use Array.prototype.reduce() and Array.prototype.concat() to merge elements or arrays. Base case, for depth equal to 1 stops recursion. Omit the second argument, depth to flatten only to a depth of 1 (single flatten).

const flatten = (arr, depth = 1) =>
  arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);

Examples

flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]

⬆ Back to top

forEachRight

Executes a provided function once for each array element, starting from the array's last element.

Use Array.prototype.slice(0) to clone the given array, Array.prototype.reverse() to reverse it and Array.prototype.forEach() to iterate over the reversed array.

const forEachRight = (arr, callback) =>
  arr
    .slice(0)
    .reverse()
    .forEach(callback);

Examples

forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'

⬆ Back to top

groupBy

Groups the elements of an array based on the given function.

Use Array.prototype.map() to map the values of an array to a function or property name. Use Array.prototype.reduce() to create an object, where the keys are produced from the mapped results.

const groupBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
    acc[val] = (acc[val] || []).concat(arr[i]);
    return acc;
  }, {});

Examples

groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}

⬆ Back to top

head

Returns the head of a list.

Use arr[0] to return the first element of the passed array.

const head = arr => arr[0];

Examples

head([1, 2, 3]); // 1

⬆ Back to top

indexOfAll

Returns all indices of val in an array. If val never occurs, returns [].

Use Array.prototype.reduce() to loop over elements and store indices for matching elements. Return the array of indices.

const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);

Examples

indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
indexOfAll([1, 2, 3], 4); // []

⬆ Back to top

initial

Returns all the elements of an array except the last one.

Use arr.slice(0,-1) to return all but the last element of the array.

const initial = arr => arr.slice(0, -1);

Examples

initial([1, 2, 3]); // [1,2]

⬆ Back to top

initialize2DArray

Initializes a 2D array of given width and height and value.

Use Array.prototype.map() to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to null.

const initialize2DArray = (w, h, val = null) =>
  Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));

Examples

initialize2DArray(2, 2, 0); // [[0,0], [0,0]]

⬆ Back to top

initializeArrayWithRange

Initializes an array containing the numbers in the specified range where start and end are inclusive with their common difference step.

Use Array.from() to create an array of the desired length, (end - start + 1)/step, and a map function to fill it with the desired values in the given range. You can omit start to use a default value of 0. You can omit step to use a default value of 1.

const initializeArrayWithRange = (end, start = 0, step = 1) =>
  Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start);

Examples

initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]
initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]

⬆ Back to top

initializeArrayWithRangeRight

Initializes an array containing the numbers in the specified range (in reverse) where start and end are inclusive with their common difference step.

Use Array.from(Math.ceil((end+1-start)/step)) to create an array of the desired length(the amounts of elements is equal to (end-start)/step or (end+1-start)/step for inclusive end), Array.prototype.map() to fill with the desired values in a range. You can omit start to use a default value of 0. You can omit step to use a default value of 1.

const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
  Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
    (v, i, arr) => (arr.length - i - 1) * step + start
  );

Examples

initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]

⬆ Back to top

initializeArrayWithValues

Initializes and fills an array with the specified values.

Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values. You can omit val to use a default value of 0.

const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);

Examples

initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]

⬆ Back to top

initializeNDArray

Create a n-dimensional array with given value.

Use recursion. Use Array.prototype.map() to generate rows where each is a new array initialized using initializeNDArray.

const initializeNDArray = (val, ...args) =>
  args.length === 0
    ? val
    : Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1)));

Examples

initializeNDArray(1, 3); // [1,1,1]
initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]]

⬆ Back to top

intersection

Returns a list of elements that exist in both arrays.

Create a Set from b, then use Array.prototype.filter() on a to only keep values contained in b.

const intersection = (a, b) => {
  const s = new Set(b);
  return a.filter(x => s.has(x));
};

Examples

intersection([1, 2, 3], [4, 3, 2]); // [2, 3]

⬆ Back to top

intersectionBy

Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both.

Create a Set by applying fn to all elements in b, then use Array.prototype.filter() on a to only keep elements, which produce values contained in b when fn is applied to them.

const intersectionBy = (a, b, fn) => {
  const s = new Set(b.map(fn));
  return a.filter(x => s.has(fn(x)));
};

Examples

intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]

⬆ Back to top

intersectionWith

Returns a list of elements that exist in both arrays, using a provided comparator function.

Use Array.prototype.filter() and Array.prototype.findIndex() in combination with the provided comparator to determine intersecting values.

const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1);

Examples

intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0]

⬆ Back to top

isSorted

Returns 1 if the array is sorted in ascending order, -1 if it is sorted in descending order or 0 if it is not sorted.

Calculate the ordering direction for the first two elements. Use Object.entries() to loop over array objects and compare them in pairs. Return 0 if the direction changes or the direction if the last element is reached.

const isSorted = arr => {
  let direction = -(arr[0] - arr[1]);
  for (let [i, val] of arr.entries()) {
    direction = !direction ? -(arr[i - 1] - arr[i]) : direction;
    if (i === arr.length - 1) return !direction ? 0 : direction;
    else if ((val - arr[i + 1]) * direction > 0) return 0;
  }
};

Examples

isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0

⬆ Back to top

join

Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.

Use Array.prototype.reduce() to combine elements into a string. Omit the second argument, separator, to use a default separator of ','. Omit the third argument, end, to use the same value as separator by default.

const join = (arr, separator = ',', end = separator) =>
  arr.reduce(
    (acc, val, i) =>
      i === arr.length - 2
        ? acc + val + end
        : i === arr.length - 1
          ? acc + val
          : acc + val + separator,
    ''
  );

Examples

join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"

⬆ Back to top

JSONtoCSV

Converts an array of objects to a comma-separated values (CSV) string that contains only the columns specified.

Use Array.prototype.join(demiliter) to combine all the names in columns to create the first row. Use Array.prototype.map() and Array.prototype.reduce() to create a row for each object, substituting non-existent values with empty strings and only mapping values in columns. Use Array.prototype.join('\n') to combine all rows into a string. Omit the third argument, delimiter, to use a default delimiter of ,.

const JSONtoCSV = (arr, columns, delimiter = ',') =>
  [
    columns.join(delimiter),
    ...arr.map(obj =>
      columns.reduce(
        (acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
        ''
      )
    )
  ].join('\n');

Examples

JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'

⬆ Back to top

last

Returns the last element in an array.

Use arr.length - 1 to compute the index of the last element of the given array and returning it.

const last = arr => arr[arr.length - 1];

Examples

last([1, 2, 3]); // 3

⬆ Back to top

longestItem

Takes any number of iterable objects or objects with a length property and returns the longest one. If multiple objects have the same length, the first one will be returned. Returns undefined if no arguments are provided.

Use Array.prototype.reduce(), comparing the length of objects to find the longest one.

const longestItem = (val, ...vals) =>
  [val, ...vals].reduce((a, x) => (x.length > a.length ? x : a));

Examples

longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'

⬆ Back to top

mapObject

Maps the values of an array to an object using a function, where the key-value pairs consist of the original value as the key and the mapped value.

Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new Array to store the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations).

const mapObject = (arr, fn) =>
  (a => (
    (a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {})
  ))();

Examples

const squareIt = arr => mapObject(arr, a => a * a);
squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 }

⬆ Back to top

maxN

Returns the n maximum elements from the provided array. If n is greater than or equal to the provided array's length, then return the original array (sorted in descending order).

Use Array.prototype.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in descending order. Use Array.prototype.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);

Examples

maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3,2]

⬆ Back to top

minN

Returns the n minimum elements from the provided array. If n is greater than or equal to the provided array's length, then return the original array (sorted in ascending order).

Use Array.prototype.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in ascending order. Use Array.prototype.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);

Examples

minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1,2]

⬆ Back to top

none

Returns true if the provided predicate function returns false for all elements in a collection, false otherwise.

Use Array.prototype.some() to test if any elements in the collection return true based on fn. Omit the second argument, fn, to use Boolean as a default.

const none = (arr, fn = Boolean) => !arr.some(fn);

Examples

none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // true

⬆ Back to top

nthElement

Returns the nth element of an array.

Use Array.prototype.slice() to get an array containing the nth element at the first place. If the index is out of bounds, return undefined. Omit the second argument, n, to get the first element of the array.

const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];

Examples

nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'

⬆ Back to top

offset

Moves the specified amount of elements to the end of the array.

Use Array.prototype.slice() twice to get the elements after the specified index and the elements before that. Use the spread operator(...) to combine the two into one array. If offset is negative, the elements will be moved from end to start.

const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];

Examples

offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]

⬆ Back to top

partition

Groups the elements into two arrays, depending on the provided function's truthiness for each element.

Use Array.prototype.reduce() to create an array of two arrays. Use Array.prototype.push() to add elements for which fn returns true to the first array and elements for which fn returns false to the second one.

const partition = (arr, fn) =>
  arr.reduce(
    (acc, val, i, arr) => {
      acc[fn(val, i, arr) ? 0 : 1].push(val);
      return acc;
    },
    [[], []]
  );

Examples

const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred',    'age': 40, 'active': true }],[{ 'user': 'barney',  'age': 36, 'active': false }]]

⬆ Back to top

permutations

⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations.

Generates all permutations of an array's elements (contains duplicates).

Use recursion. For each element in the given array, create all the partial permutations for the rest of its elements. Use Array.prototype.map() to combine the element with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array. Base cases are for array length equal to 2 or 1.

const permutations = arr => {
  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
  return arr.reduce(
    (acc, item, i) =>
      acc.concat(
        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
      ),
    []
  );
};

Examples

permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]

⬆ Back to top

pull

Mutates the original array to filter out the values specified.

Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed. Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.

(For a snippet that does not mutate the original array see without)

const pull = (arr, ...args) => {
  let argState = Array.isArray(args[0]) ? args[0] : args;
  let pulled = arr.filter((v, i) => !argState.includes(v));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
};

Examples

let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]

⬆ Back to top

pullAtIndex

Mutates the original array to filter out the values at the specified indexes.

Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed. Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values. Use Array.prototype.push() to keep track of pulled values

const pullAtIndex = (arr, pullArr) => {
  let removed = [];
  let pulled = arr
    .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
    .filter((v, i) => !pullArr.includes(i));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
  return removed;
};

Examples

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

⬆ Back to top

pullAtValue

Mutates the original array to filter out the values specified. Returns the removed elements.

Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed. Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values. Use Array.prototype.push() to keep track of pulled values

const pullAtValue = (arr, pullArr) => {
  let removed = [],
    pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
    mutateTo = arr.filter((v, i) => !pullArr.includes(v));
  arr.length = 0;
  mutateTo.forEach(v => arr.push(v));
  return removed;
};

Examples

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]

⬆ Back to top

pullBy

Mutates the original array to filter out the values specified, based on a given iterator function.

Check if the last argument provided in a function. Use Array.prototype.map() to apply the iterator function fn to all array elements. Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed. Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.

const pullBy = (arr, ...args) => {
  const length = args.length;
  let fn = length > 1 ? args[length - 1] : undefined;
  fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
  let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
  let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
  arr.length = 0;
  pulled.forEach(v => arr.push(v));
};

Examples

var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]

⬆ Back to top

reducedFilter

Filter an array of objects based on a condition while also filtering out unspecified keys.

Use Array.prototype.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value. On the filtered array, use Array.prototype.map() to return the new object using Array.prototype.reduce() to filter out the keys which were not supplied as the keys argument.

const reducedFilter = (data, keys, fn) =>
  data.filter(fn).map(el =>
    keys.reduce((acc, key) => {
      acc[key] = el[key];
      return acc;
    }, {})
  );

Examples

const data = [
  {
    id: 1,
    name: 'john',
    age: 24
  },
  {
    id: 2,
    name: 'mike',
    age: 50
  }
];

reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]

⬆ Back to top

reduceSuccessive

Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.

Use Array.prototype.reduce() to apply the given function to the given array, storing each new result.

const reduceSuccessive = (arr, fn, acc) =>
  arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);

Examples

reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21]

⬆ Back to top

reduceWhich

Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule.

Use Array.prototype.reduce() in combination with the comparator function to get the appropriate element in the array. You can omit the second parameter, comparator, to use the default one that returns the minimum element in the array.

const reduceWhich = (arr, comparator = (a, b) => a - b) =>
  arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));

Examples

reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
  [{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
  (a, b) => a.age - b.age
); // {name: "Lucy", age: 9}

⬆ Back to top

reject

Takes a predicate and array, like Array.prototype.filter(), but only keeps x if pred(x) === false.

const reject = (pred, array) => array.filter((...args) => !pred(...args));

Examples

reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi']

⬆ Back to top

remove

Removes elements from an array for which the given function returns false.

Use Array.prototype.filter() to find array elements that return truthy values and Array.prototype.reduce() to remove elements using Array.prototype.splice(). The func is invoked with three arguments (value, index, array).

const remove = (arr, func) =>
  Array.isArray(arr)
    ? arr.filter(func).reduce((acc, val) => {
        arr.splice(arr.indexOf(val), 1);
        return acc.concat(val);
      }, [])
    : [];

Examples

remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]

⬆ Back to top

sample

Returns a random element from an array.

Use Math.random() to generate a random number, multiply it by length and round it of to the nearest whole number using Math.floor(). This method also works with strings.

const sample = arr => arr[Math.floor(Math.random() * arr.length)];

Examples

sample([3, 7, 9, 11]); // 9

⬆ Back to top

sampleSize

Gets n random elements at unique keys from array up to the size of array.

Shuffle the array using the Fisher-Yates algorithm. Use Array.prototype.slice() to get the first n elements. Omit the second argument, n to get only one element at random from the array.

const sampleSize = ([...arr], n = 1) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr.slice(0, n);
};

Examples

sampleSize([1, 2, 3], 2); // [3,1]
sampleSize([1, 2, 3], 4); // [2,3,1]

⬆ Back to top

shank

Has the same functionality as Array.prototype.prototype.splice(), but returning a new array instead of mutating the original array.

Use Array.prototype.slice() and Array.prototype.concat() to get a new array with the new contents after removing existing elements and/or adding new elements. Omit the second argument, index, to start at 0. Omit the third argument, delCount, to remove 0 elements. Omit the fourth argument, elements, in order to not add any new elements.

const shank = (arr, index = 0, delCount = 0, ...elements) =>
  arr
    .slice(0, index)
    .concat(elements)
    .concat(arr.slice(index + delCount));

Examples

const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']

⬆ Back to top

shuffle

Randomizes the order of the values of an array, returning a new array.

Uses the Fisher-Yates algorithm to reorder the elements of the array.

const shuffle = ([...arr]) => {
  let m = arr.length;
  while (m) {
    const i = Math.floor(Math.random() * m--);
    [arr[m], arr[i]] = [arr[i], arr[m]];
  }
  return arr;
};

Examples

const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]

⬆ Back to top

similarity

Returns an array of elements that appear in both arrays.

Use Array.prototype.filter() to remove values that are not part of values, determined using Array.prototype.includes().

const similarity = (arr, values) => arr.filter(v => values.includes(v));

Examples

similarity([1, 2, 3], [1, 2, 4]); // [1, 2]

⬆ Back to top

sortedIndex

Returns the lowest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.

const sortedIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};

Examples

sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1

⬆ Back to top

sortedIndexBy

Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.

Check if the array is sorted in descending order (loosely). Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted, based on the iterator function fn.

const sortedIndexBy = (arr, n, fn) => {
  const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
  const val = fn(n);
  const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
  return index === -1 ? arr.length : index;
};

Examples

sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0

⬆ Back to top

sortedLastIndex

Returns the highest index at which value should be inserted into array in order to maintain its sort order.

Check if the array is sorted in descending order (loosely). Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted.

const sortedLastIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el));
  return index === -1 ? 0 : arr.length - index;
};

Examples

sortedLastIndex([10, 20, 30, 30, 40], 30); // 4

⬆ Back to top

sortedLastIndexBy

Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.

Check if the array is sorted in descending order (loosely). Use Array.prototype.map() to apply the iterator function to all elements of the array. Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted, based on the provided iterator function.

const sortedLastIndexBy = (arr, n, fn) => {
  const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
  const val = fn(n);
  const index = arr
    .map(fn)
    .reverse()
    .findIndex(el => (isDescending ? val <= el : val >= el));
  return index === -1 ? 0 : arr.length - index;
};

Examples

sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1

⬆ Back to top

stableSort

Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead.

Use Array.prototype.map() to pair each element of the input array with its corresponding index. Use Array.prototype.sort() and a compare function to sort the list, preserving their initial order if the items compared are equal. Use Array.prototype.map() to convert back to the initial array items.

const stableSort = (arr, compare) =>
  arr
    .map((item, index) => ({ item, index }))
    .sort((a, b) => compare(a.item, b.item) || a.index - b.index)
    .map(({ item }) => item);

Examples

const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

⬆ Back to top

symmetricDifference

Returns the symmetric difference between two arrays, without filtering out duplicate values.

Create a Set from each array, then use Array.prototype.filter() on each of them to only keep values not contained in the other.

const symmetricDifference = (a, b) => {
  const sA = new Set(a),
    sB = new Set(b);
  return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};

Examples

symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]

⬆ Back to top

symmetricDifferenceBy

Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.

Create a Set by applying fn to each array's elements, then use Array.prototype.filter() on each of them to only keep values not contained in the other.

const symmetricDifferenceBy = (a, b, fn) => {
  const sA = new Set(a.map(v => fn(v))),
    sB = new Set(b.map(v => fn(v)));
  return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};

Examples

symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]

⬆ Back to top

symmetricDifferenceWith

Returns the symmetric difference between two arrays, using a provided function as a comparator.

Use Array.prototype.filter() and Array.prototype.findIndex() to find the appropriate values.

const symmetricDifferenceWith = (arr, val, comp) => [
  ...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
  ...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];

Examples

symmetricDifferenceWith(
  [1, 1.2, 1.5, 3, 0],
  [1.9, 3, 0, 3.9],
  (a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]

⬆ Back to top

tail

Returns all elements in an array except for the first one.

Return Array.prototype.slice(1) if the array's length is more than 1, otherwise, return the whole array.

const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);

Examples

tail([1, 2, 3]); // [2,3]
tail([1]); // [1]

⬆ Back to top

take

Returns an array with n elements removed from the beginning.

Use Array.prototype.slice() to create a slice of the array with n elements taken from the beginning.

const take = (arr, n = 1) => arr.slice(0, n);

Examples

take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []

⬆ Back to top

takeRight

Returns an array with n elements removed from the end.

Use Array.prototype.slice() to create a slice of the array with n elements taken from the end.

const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);

Examples

takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]

⬆ Back to top

takeRightWhile

Removes elements from the end of an array until the passed function returns true. Returns the removed elements.

Loop through the array, using a Array.prototype.reduceRight() and accumulating elements while the function returns falsy value.

const takeRightWhile = (arr, func) =>
  arr.reduceRight((acc, el) => (func(el) ? acc : [el, ...acc]), []);

Examples

takeRightWhile([1, 2, 3, 4], n => n < 3); // [3, 4]

⬆ Back to top

takeWhile

Removes elements in an array until the passed function returns true. Returns the removed elements.

Loop through the array, using a for...of loop over Array.prototype.entries() until the returned value from the function is true. Return the removed elements, using Array.prototype.slice().

const takeWhile = (arr, func) => {
  for (const [i, val] of arr.entries()) if (func(val)) return arr.slice(0, i);
  return arr;
};

Examples

takeWhile([1, 2, 3, 4], n => n >= 3); // [1, 2]

⬆ Back to top

toHash

Reduces a given Array-like into a value hash (keyed data store).

Given an Iterable or Array-like structure, call Array.prototype.prototype.reduce.call() on the provided object to step over it and return an Object, keyed by the reference value.

const toHash = (object, key) =>
  Array.prototype.reduce.call(
    object,
    (acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
    {}
  );

Examples

toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 1: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [{ id: 1, first: 'Jon' }, { id: 2, first: 'Joe' }, { id: 3, first: 'Moe' }];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference, but a closure referencing the hash would work, too.
managers.forEach(
  manager =>
    (manager.employees = manager.employees.map(function(id) {
      return this[id];
    }, toHash(users, 'id')))
);
managers; // [ { manager:1, employees: [ { id: 2, first: "Joe" }, { id: 3, first: "Moe" } ] } ]

⬆ Back to top

union

Returns every element that exists in any of the two arrays once.

Create a Set with all values of a and b and convert to an array.

const union = (a, b) => Array.from(new Set([...a, ...b]));

Examples

union([1, 2, 3], [4, 3, 2]); // [1,2,3,4]

⬆ Back to top

unionBy

Returns every element that exists in any of the two arrays once, after applying the provided function to each array element of both.

Create a Set by applying all fn to all values of a. Create a Set from a and all elements in b whose value, after applying fn does not match a value in the previously created set. Return the last set converted to an array.

const unionBy = (a, b, fn) => {
  const s = new Set(a.map(v => fn(v)));
  return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};

Examples

unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]

⬆ Back to top

unionWith

Returns every element that exists in any of the two arrays once, using a provided comparator function.

Create a Set with all values of a and values in b for which the comparator finds no matches in a, using Array.prototype.findIndex().

const unionWith = (a, b, comp) =>
  Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)]));

Examples

unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9]

⬆ Back to top

uniqueElements

Returns all unique values of an array.

Use ES6 Set and the ...rest operator to discard all duplicated values.

const uniqueElements = arr => [...new Set(arr)];

Examples

uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]

⬆ Back to top

uniqueElementsBy

Returns all unique values of an array, based on a provided comparator function.

Use Array.prototype.reduce() and Array.prototype.some() for an array containing only the first unique occurence of each value, based on the comparator function, fn. The comparator function takes two arguments: the values of the two elements being compared.

const uniqueElementsBy = (arr, fn) =>
  arr.reduce((acc, v) => {
    if (!acc.some(x => fn(v, x))) acc.push(v);
    return acc;
  }, []);

Examples

uniqueElementsBy(
  [
    { id: 0, value: 'a' },
    { id: 1, value: 'b' },
    { id: 2, value: 'c' },
    { id: 1, value: 'd' },
    { id: 0, value: 'e' }
  ],
  (a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]

⬆ Back to top

uniqueElementsByRight

Returns all unique values of an array, based on a provided comparator function.

Use Array.prototype.reduce() and Array.prototype.some() for an array containing only the last unique occurence of each value, based on the comparator function, fn. The comparator function takes two arguments: the values of the two elements being compared.

const uniqueElementsByRight = (arr, fn) =>
  arr.reduceRight((acc, v) => {
    if (!acc.some(x => fn(v, x))) acc.push(v);
    return acc;
  }, []);

Examples

uniqueElementsByRight(
  [
    { id: 0, value: 'a' },
    { id: 1, value: 'b' },
    { id: 2, value: 'c' },
    { id: 1, value: 'd' },
    { id: 0, value: 'e' }
  ],
  (a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]

⬆ Back to top

uniqueSymmetricDifference

Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.

Use Array.prototype.filter() and Array.prototype.includes() on each array to remove values contained in the other, then create a Set from the results, removing duplicate values.

const uniqueSymmetricDifference = (a, b) => [
  ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
];

Examples

uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]

⬆ Back to top

unzip

Creates an array of arrays, ungrouping the elements in an array produced by zip.

Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array. Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays.

const unzip = arr =>
  arr.reduce(
    (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
    Array.from({
      length: Math.max(...arr.map(x => x.length))
    }).map(x => [])
  );

Examples

unzip([['a', 1, true], ['b', 2, false]]); //[['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); //[['a', 'b'], [1, 2], [true]]

⬆ Back to top

unzipWith

Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.

Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array. Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays. Use Array.prototype.map() and the spread operator (...) to apply fn to each individual group of elements.

const unzipWith = (arr, fn) =>
  arr
    .reduce(
      (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
      Array.from({
        length: Math.max(...arr.map(x => x.length))
      }).map(x => [])
    )
    .map(val => fn(...val));

Examples

unzipWith([[1, 10, 100], [2, 20, 200]], (...args) => args.reduce((acc, v) => acc + v, 0)); // [3, 30, 300]

⬆ Back to top

without

Filters out the elements of an array, that have one of the specified values.

Use Array.prototype.filter() to create an array excluding(using !Array.includes()) all given values.

(For a snippet that mutates the original array see pull)

const without = (arr, ...args) => arr.filter(v => !args.includes(v));

Examples

without([2, 1, 2, 3], 1, 2); // [3]

⬆ Back to top

xProd

Creates a new array out of the two supplied by creating each possible pair from the arrays.

Use Array.prototype.reduce(), Array.prototype.map() and Array.prototype.concat() to produce every possible pair from the elements of the two arrays and save them in an array.

const xProd = (a, b) => a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);

Examples

xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]

⬆ Back to top

zip

Creates an array of elements, grouped based on the position in the original arrays.

Use Math.max.apply() to get the longest array in the arguments. Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, undefined is used where no value could be found.

const zip = (...arrays) => {
  const maxLength = Math.max(...arrays.map(x => x.length));
  return Array.from({ length: maxLength }).map((_, i) => {
    return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
  });
};

Examples

zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]

⬆ Back to top

zipObject

Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.

Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using Array.prototype.reduce().

const zipObject = (props, values) =>
  props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});

Examples

zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}

⬆ Back to top

zipWith

Creates an array of elements, grouped based on the position in the original arrays and using function as the last value to specify how grouped values should be combined.

Check if the last argument provided is a function. Use Math.max() to get the longest array in the arguments. Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, undefined is used where no value could be found. The function is invoked with the elements of each group (...group).

const zipWith = (...array) => {
  const fn = typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
  return Array.from(
    { length: Math.max(...array.map(a => a.length)) },
    (_, i) => (fn ? fn(...array.map(a => a[i])) : array.map(a => a[i]))
  );
};

Examples

zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111,222]
zipWith(
  [1, 2, 3],
  [10, 20],
  [100, 200],
  (a, b, c) => (a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']

⬆ Back to top

🌐 Browser

arrayToHtmlList

Converts the given array elements into <li> tags and appends them to the list of the given id.

Use Array.prototype.map(), document.querySelector(), and an anonymous inner closure to create a list of html tags.

const arrayToHtmlList = (arr, listID) =>
  (el => (
    (el = document.querySelector('#' + listID)),
    (el.innerHTML += arr.map(item => `<li>${item}</li>`).join(''))
  ))();

Examples

arrayToHtmlList(['item 1', 'item 2'], 'myListID');

⬆ Back to top

bottomVisible

Returns true if the bottom of the page is visible, false otherwise.

Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >=
  (document.documentElement.scrollHeight || document.documentElement.clientHeight);

Examples

bottomVisible(); // true

⬆ Back to top

copyToClipboard

⚠️ NOTICE: The same functionality can be easily implemented by using the new asynchronous Clipboard API, which is still experimental but should be used in the future instead of this snippet. Find out more about it here.

Copy a string to the clipboard. Only works as a result of user action (i.e. inside a click event listener).

Create a new <textarea> element, fill it with the supplied data and add it to the HTML document. Use Selection.getRangeAt()to store the selected range (if any). Use document.execCommand('copy') to copy to the clipboard. Remove the <textarea> element from the HTML document. Finally, use Selection().addRange() to recover the original selected range (if any).

const copyToClipboard = str => {
  const el = document.createElement('textarea');
  el.value = str;
  el.setAttribute('readonly', '');
  el.style.position = 'absolute';
  el.style.left = '-9999px';
  document.body.appendChild(el);
  const selected =
    document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
  el.select();
  document.execCommand('copy');
  document.body.removeChild(el);
  if (selected) {
    document.getSelection().removeAllRanges();
    document.getSelection().addRange(selected);
  }
};

Examples

copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.

⬆ Back to top

counter

Creates a counter with the specified range, step and duration for the specified selector.

Check if step has the proper sign and change it accordingly. Use setInterval() in combination with Math.abs() and Math.floor() to calculate the time between each new text draw. Use document.querySelector().innerHTML to update the value of the selected element. Omit the fourth parameter, step, to use a default step of 1. Omit the fifth parameter, duration, to use a default duration of 2000ms.

const counter = (selector, start, end, step = 1, duration = 2000) => {
  let current = start,
    _step = (end - start) * step < 0 ? -step : step,
    timer = setInterval(() => {
      current += _step;
      document.querySelector(selector).innerHTML = current;
      if (current >= end) document.querySelector(selector).innerHTML = end;
      if (current >= end) clearInterval(timer);
    }, Math.abs(Math.floor(duration / (end - start))));
  return timer;
};

Examples

counter('#my-id', 1, 1000, 5, 2000); // Creates a 2-second timer for the element with id="my-id"

⬆ Back to top

createElement

Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.

Use document.createElement() to create a new element. Set its innerHTML to the string supplied as the argument. Use ParentNode.firstElementChild to return the element version of the string.

const createElement = str => {
  const el = document.createElement('div');
  el.innerHTML = str;
  return el.firstElementChild;
};

Examples

const el = createElement(
  `<div class="container">
    <p>Hello!</p>
  </div>`
);
console.log(el.className); // 'container'

⬆ Back to top

createEventHub

Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.

Use Object.create(null) to create an empty hub object that does not inherit properties from Object.prototype. For emit, resolve the array of handlers based on the event argument and then run each one with Array.prototype.forEach() by passing in the data as an argument. For on, create an array for the event if it does not yet exist, then use Array.prototype.push() to add the handler to the array. For off, use Array.prototype.findIndex() to find the index of the handler in the event array and remove it using Array.prototype.splice().

const createEventHub = () => ({
  hub: Object.create(null),
  emit(event, data) {
    (this.hub[event] || []).forEach(handler => handler(data));
  },
  on(event, handler) {
    if (!this.hub[event]) this.hub[event] = [];
    this.hub[event].push(handler);
  },
  off(event, handler) {
    const i = (this.hub[event] || []).findIndex(h => h === handler);
    if (i > -1) this.hub[event].splice(i, 1);
  }
});

Examples

const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;

// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);

// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1

// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);

⬆ Back to top

currentURL

Returns the current URL.

Use window.location.href to get current URL.

const currentURL = () => window.location.href;

Examples

currentURL(); // 'https://google.com'

⬆ Back to top

detectDeviceType

Detects wether the website is being opened in a mobile device or a desktop/laptop.

Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.

const detectDeviceType = () =>
  /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
    ? 'Mobile'
    : 'Desktop';

Examples

detectDeviceType(); // "Mobile" or "Desktop"

⬆ Back to top

elementContains

Returns true if the parent element contains the child element, false otherwise.

Check that parent is not the same element as child, use parent.contains(child) to check if the parent element contains the child element.

const elementContains = (parent, child) => parent !== child && parent.contains(child);

Examples

elementContains(document.querySelector('head'), document.querySelector('title')); // true
elementContains(document.querySelector('body'), document.querySelector('body')); // false

⬆ Back to top

elementIsVisibleInViewport

Returns true if the element specified is visible in the viewport, false otherwise.

Use Element.getBoundingClientRect() and the window.inner(Width|Height) values to determine if a given element is visible in the viewport. Omit the second argument to determine if the element is entirely visible, or specify true to determine if it is partially visible.

const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
  const { top, left, bottom, right } = el.getBoundingClientRect();
  const { innerHeight, innerWidth } = window;
  return partiallyVisible
    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
        ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};

Examples

// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)

⬆ Back to top

getScrollPosition

Returns the scroll position of the current page.

Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop. You can omit el to use a default value of window.

const getScrollPosition = (el = window) => ({
  x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
  y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});

Examples

getScrollPosition(); // {x: 0, y: 200}

⬆ Back to top

getStyle

Returns the value of a CSS rule for the specified element.

Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.

const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];

Examples

getStyle(document.querySelector('p'), 'font-size'); // '16px'

⬆ Back to top

hasClass

Returns true if the element has the specified class, false otherwise.

Use element.classList.contains() to check if the element has the specified class.

const hasClass = (el, className) => el.classList.contains(className);

Examples

hasClass(document.querySelector('p.special'), 'special'); // true

⬆ Back to top

hashBrowser

Creates a hash for a value using the SHA-256 algorithm. Returns a promise.

Use the SubtleCrypto API to create a hash for the given value.

const hashBrowser = val =>
  crypto.subtle.digest('SHA-256', new TextEncoder('utf-8').encode(val)).then(h => {
    let hexes = [],
      view = new DataView(h);
    for (let i = 0; i < view.byteLength; i += 4)
      hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
    return hexes.join('');
  });

Examples

hashBrowser(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'

⬆ Back to top

hide

Hides all the elements specified.

Use NodeList.prototype.forEach() to apply display: none to each element specified.

const hide = els => els.forEach(e => (e.style.display = 'none'));

Examples

hide(document.querySelectorAll('img')); // Hides all <img> elements on the page

⬆ Back to top

httpsRedirect

Redirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.

Use location.protocol to get the protocol currently being used. If it's not HTTPS, use location.replace() to replace the existing page with the HTTPS version of the page. Use location.href to get the full address, split it with String.prototype.split() and remove the protocol part of the URL.

const httpsRedirect = () => {
  if (location.protocol !== 'https:') location.replace('https://' + location.href.split('//')[1]);
};

Examples

httpsRedirect(); // If you are on http://mydomain.com, you are redirected to https://mydomain.com

⬆ Back to top

insertAfter

Inserts an HTML string after the end of the specified element.

Use el.insertAdjacentHTML() with a position of 'afterend' to parse htmlString and insert it after the end of el.

const insertAfter = (el, htmlString) => el.insertAdjacentHTML('afterend', htmlString);

Examples

insertAfter(document.getElementById('myId'), '<p>after</p>'); // <div id="myId">...</div> <p>after</p>

⬆ Back to top

insertBefore

Inserts an HTML string before the start of the specified element.

Use el.insertAdjacentHTML() with a position of 'beforebegin' to parse htmlString and insert it before the start of el.

const insertBefore = (el, htmlString) => el.insertAdjacentHTML('beforebegin', htmlString);

Examples

insertBefore(document.getElementById('myId'), '<p>before</p>'); // <p>before</p> <div id="myId">...</div>

⬆ Back to top

isBrowserTabFocused

Returns true if the browser tab of the page is focused, false otherwise.

Use the Document.hidden property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.

const isBrowserTabFocused = () => !document.hidden;

Examples

isBrowserTabFocused(); // true

⬆ Back to top

nodeListToArray

Converts a NodeList to an array.

Use spread operator inside new array to convert a NodeList to an array.

const nodeListToArray = nodeList => [...nodeList];

Examples

nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]

⬆ Back to top

observeMutations

Returns a new MutationObserver and runs the provided callback for each mutation on the specified element.

Use a MutationObserver to observe mutations on the given element. Use Array.prototype.forEach() to run the callback for each mutation that is observed. Omit the third argument, options, to use the default options (all true).

const observeMutations = (element, callback, options) => {
  const observer = new MutationObserver(mutations => mutations.forEach(m => callback(m)));
  observer.observe(
    element,
    Object.assign(
      {
        childList: true,
        attributes: true,
        attributeOldValue: true,
        characterData: true,
        characterDataOldValue: true,
        subtree: true
      },
      options
    )
  );
  return observer;
};

Examples

const obs = observeMutations(document, console.log); // Logs all mutations that happen on the page
obs.disconnect(); // Disconnects the observer and stops logging mutations on the page

⬆ Back to top

off

Removes an event listener from an element.

Use EventTarget.removeEventListener() to remove an event listener from an element. Omit the fourth argument opts to use false or specify it based on the options used when the event listener was added.

const off = (el, evt, fn, opts = false) => el.removeEventListener(evt, fn, opts);

Examples

const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the page

⬆ Back to top

on

Adds an event listener to an element with the ability to use event delegation.

Use EventTarget.addEventListener() to add an event listener to an element. If there is a target property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correct this context. Returns a reference to the custom delegator function, in order to be possible to use with off. Omit opts to default to non-delegation behavior and event bubbling.

const on = (el, evt, fn, opts = {}) => {
  const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
  el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
  if (opts.target) return delegatorFn;
};

Examples

const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' }); // logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true }); // use capturing instead of bubbling

⬆ Back to top

onUserInputChange

Run the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).

Use two event listeners. Assume mouse input initially and bind a touchstart event listener to the document. On touchstart, add a mousemove event listener to listen for two consecutive mousemove events firing within 20ms, using performance.now(). Run the callback with the input type as an argument in either of these situations.

const onUserInputChange = callback => {
  let type = 'mouse',
    lastTime = 0;
  const mousemoveHandler = () => {
    const now = performance.now();
    if (now - lastTime < 20)
      (type = 'mouse'), callback(type), document.removeEventListener('mousemove', mousemoveHandler);
    lastTime = now;
  };
  document.addEventListener('touchstart', () => {
    if (type === 'touch') return;
    (type = 'touch'), callback(type), document.addEventListener('mousemove', mousemoveHandler);
  });
};

Examples

onUserInputChange(type => {
  console.log('The user is now using', type, 'as an input method.');
});

⬆ Back to top

prefix

Returns the prefixed version (if necessary) of a CSS property that the browser supports.

Use Array.prototype.findIndex() on an array of vendor prefix strings to test if document.body has one of them defined in its CSSStyleDeclaration object, otherwise return null. Use String.prototype.charAt() and String.prototype.toUpperCase() to capitalize the property, which will be appended to the vendor prefix string.

const prefix = prop => {
  const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
  const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
  const i = prefixes.findIndex(
    prefix => typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !== 'undefined'
  );
  return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};

Examples

prefix('appearance'); // 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'

⬆ Back to top

recordAnimationFrames

Invokes the provided callback on each animation frame.

Use recursion. Provided that running is true, continue invoking window.requestAnimationFrame() which invokes the provided callback. Return an object with two methods start and stop to allow manual control of the recording. Omit the second argument, autoStart, to implicitly call start when the function is invoked.

const recordAnimationFrames = (callback, autoStart = true) => {
  let running = true,
    raf;
  const stop = () => {
    running = false;
    cancelAnimationFrame(raf);
  };
  const start = () => {
    running = true;
    run();
  };
  const run = () => {
    raf = requestAnimationFrame(() => {
      callback();
      if (running) run();
    });
  };
  if (autoStart) start();
  return { start, stop };
};

Examples

const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb); // logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false); // `start` needs to be explicitly called to begin recording frames

⬆ Back to top

redirect

Redirects to a specified URL.

Use window.location.href or window.location.replace() to redirect to url. Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).

const redirect = (url, asLink = true) =>
  asLink ? (window.location.href = url) : window.location.replace(url);

Examples

redirect('https://google.com');

⬆ Back to top

runAsync

Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.

Create a new Worker using a Blob object URL, the contents of which should be the stringified version of the supplied function. Immediately post the return value of calling the function back. Return a promise, listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.

const runAsync = fn => {
  const worker = new Worker(
    URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
      type: 'application/javascript; charset=utf-8'
    })
  );
  return new Promise((res, rej) => {
    worker.onmessage = ({ data }) => {
      res(data), worker.terminate();
    };
    worker.onerror = err => {
      rej(err), worker.terminate();
    };
  });
};

Examples

const longRunningFunction = () => {
  let result = 0;
  for (let i = 0; i < 1000; i++) {
    for (let j = 0; j < 700; j++) {
      for (let k = 0; k < 300; k++) {
        result = result + i + j + k;
      }
    }
  }
  return result;
};
/*
  NOTE: Since the function is running in a different context, closures are not supported.
  The function supplied to `runAsync` gets stringified, so everything becomes literal.
  All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'

⬆ Back to top

scrollToTop

Smooth-scrolls to the top of the page.

Get distance from top using document.documentElement.scrollTop or document.body.scrollTop. Scroll by a fraction of the distance from the top. Use window.requestAnimationFrame() to animate the scrolling.

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c / 8);
  }
};

Examples

scrollToTop();

⬆ Back to top

setStyle

Sets the value of a CSS rule for the specified element.

Use element.style to set the value of the CSS rule for the specified element to val.

const setStyle = (el, ruleName, val) => (el.style[ruleName] = val);

Examples

setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20px

⬆ Back to top

show

Shows all the elements specified.

Use the spread operator (...) and Array.prototype.forEach() to clear the display property for each element specified.

const show = (...el) => [...el].forEach(e => (e.style.display = ''));

Examples

show(...document.querySelectorAll('img')); // Shows all <img> elements on the page

⬆ Back to top

smoothScroll

Smoothly scrolls the element on which it's called into the visible area of the browser window.

Use .scrollIntoView method to scroll the element. Pass { behavior: 'smooth' } to .scrollIntoView so it scrolls smoothly.

const smoothScroll = element =>
  document.querySelector(element).scrollIntoView({
    behavior: 'smooth'
  });

Examples

smoothScroll('#fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar'); // scrolls smoothly to the first element with a class of fooBar

⬆ Back to top

toggleClass

Toggle a class for an element.

Use element.classList.toggle() to toggle the specified class for the element.

const toggleClass = (el, className) => el.classList.toggle(className);

Examples

toggleClass(document.querySelector('p.special'), 'special'); // The paragraph will not have the 'special' class anymore

⬆ Back to top

triggerEvent

Triggers a specific event on a given element, optionally passing custom data.

Use new CustomEvent() to create an event from the specified eventType and details. Use el.dispatchEvent() to trigger the newly created event on the given element. Omit the third argument, detail, if you do not want to pass custom data to the triggered event.

const triggerEvent = (el, eventType, detail = undefined) =>
  el.dispatchEvent(new CustomEvent(eventType, { detail: detail }));

Examples

triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });

⬆ Back to top

UUIDGeneratorBrowser

Generates a UUID in a browser.

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

const UUIDGeneratorBrowser = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ (crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))).toString(16)
  );

Examples

UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'

⬆ Back to top

⏱️ Date

dayOfYear

Gets the day of the year from a Date object.

Use new Date() and Date.prototype.getFullYear() to get the first day of the year as a Date object, subtract it from the provided date and divide with the milliseconds in each day to get the result. Use Math.floor() to appropriately round the resulting day count to an integer.

const dayOfYear = date =>
  Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);

Examples

dayOfYear(new Date()); // 272

⬆ Back to top

formatDuration

Returns the human readable format of the given number of milliseconds.

Divide ms with the appropriate values to obtain the appropriate values for day, hour, minute, second and millisecond. Use Object.entries() with Array.prototype.filter() to keep only non-zero values. Use Array.prototype.map() to create the string for each value, pluralizing appropriately. Use String.prototype.join(', ') to combine the values into a string.

const formatDuration = ms => {
  if (ms < 0) ms = -ms;
  const time = {
    day: Math.floor(ms / 86400000),
    hour: Math.floor(ms / 3600000) % 24,
    minute: Math.floor(ms / 60000) % 60,
    second: Math.floor(ms / 1000) % 60,
    millisecond: Math.floor(ms) % 1000
  };
  return Object.entries(time)
    .filter(val => val[1] !== 0)
    .map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
    .join(', ');
};

Examples

formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574); // '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'

⬆ Back to top

getColonTimeFromDate

Returns a string of the form HH:MM:SS from a Date object.

Use Date.prototype.toString() and String.prototype.slice() to get the HH:MM:SS part of a given Date object.

const getColonTimeFromDate = date => date.toTimeString().slice(0, 8);

Examples

getColonTimeFromDate(new Date()); // "08:38:00"

⬆ Back to top

getDaysDiffBetweenDates

Returns the difference (in days) between two dates.

Calculate the difference (in days) between two Date objects.

const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / (1000 * 3600 * 24);

Examples

getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9

⬆ Back to top

getMeridiemSuffixOfInteger

Converts an integer to a suffixed string, adding am or pm based on its value.

Use the modulo operator (%) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.

const getMeridiemSuffixOfInteger = num =>
  num === 0 || num === 24
    ? 12 + 'am'
    : num === 12
      ? 12 + 'pm'
      : num < 12
        ? (num % 12) + 'am'
        : (num % 12) + 'pm';

Examples

getMeridiemSuffixOfInteger(0); // "12am"
getMeridiemSuffixOfInteger(11); // "11am"
getMeridiemSuffixOfInteger(13); // "1pm"
getMeridiemSuffixOfInteger(25); // "1pm"

⬆ Back to top

isAfterDate

Check if a date is after another date.

Use the greater than operator (>) to check if the first date comes after the second one.

const isAfterDate = (dateA, dateB) => dateA > dateB;

Examples

isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // true

⬆ Back to top

isBeforeDate

Check if a date is before another date.

Use the less than operator (<) to check if the first date comes before the second one.

const isBeforeDate = (dateA, dateB) => dateA < dateB;

Examples

isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // true

⬆ Back to top

isSameDate

Check if a date is the same as another date.

Use Date.prototype.toISOString() and strict equality checking (===) to check if the first date is the same as the second one.

const isSameDate = (dateA, dateB) => dateA.toISOString() === dateB.toISOString();

Examples

isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // true

⬆ Back to top

maxDate

Returns the maximum of the given dates.

Use Math.max.apply() to find the maximum date value, new Date() to convert it to a Date object.

const maxDate = (...dates) => new Date(Math.max.apply(null, ...dates));

Examples

const array = [
  new Date(2017, 4, 13),
  new Date(2018, 2, 12),
  new Date(2016, 0, 10),
  new Date(2016, 0, 9)
];
maxDate(array); // 2018-03-11T22:00:00.000Z

⬆ Back to top

minDate

Returns the minimum of the given dates.

Use Math.min.apply() to find the minimum date value, new Date() to convert it to a Date object.

const minDate = (...dates) => new Date(Math.min.apply(null, ...dates));

Examples

const array = [
  new Date(2017, 4, 13),
  new Date(2018, 2, 12),
  new Date(2016, 0, 10),
  new Date(2016, 0, 9)
];
minDate(array); // 2016-01-08T22:00:00.000Z

⬆ Back to top

tomorrow

Results in a string representation of tomorrow's date.

Use new Date() to get today's date, adding one day using Date.getDate() and Date.setDate(), and converting the Date object to a string.

const tomorrow = (long = false) => {
  let t = new Date();
  t.setDate(t.getDate() + 1);
  const ret = `${t.getFullYear()}-${String(t.getMonth() + 1).padStart(2, '0')}-${String(
    t.getDate()
  ).padStart(2, '0')}`;
  return !long ? ret : `${ret}T00:00:00`;
};

Examples

tomorrow(); // 2017-12-27 (if current date is 2017-12-26)
tomorrow(true); // 2017-12-27T00:00:00 (if current date is 2017-12-26)

⬆ Back to top

🎛️ Function

attempt

Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.

Use a try... catch block to return either the result of the function or an appropriate error.

const attempt = (fn, ...args) => {
  try {
    return fn(...args);
  } catch (e) {
    return e instanceof Error ? e : new Error(e);
  }
};

Examples

var elements = attempt(function(selector) {
  return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []

⬆ Back to top

bind

Creates a function that invokes fn with a given context, optionally adding any additional supplied parameters to the beginning of the arguments.

Return a function that uses Function.prototype.apply() to apply the given context to fn. Use Array.prototype.concat() to prepend any additional supplied parameters to the arguments.

const bind = (fn, context, ...boundArgs) => (...args) => fn.apply(context, [...boundArgs, ...args]);

Examples

function greet(greeting, punctuation) {
  return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'

⬆ Back to top

bindKey

Creates a function that invokes the method at a given key of an object, optionally adding any additional supplied parameters to the beginning of the arguments.

Return a function that uses Function.prototype.apply() to bind context[fn] to context. Use the spread operator (...) to prepend any additional supplied parameters to the arguments.

const bindKey = (context, fn, ...boundArgs) => (...args) =>
  context[fn].apply(context, [...boundArgs, ...args]);

Examples

const freddy = {
  user: 'fred',
  greet: function(greeting, punctuation) {
    return greeting + ' ' + this.user + punctuation;
  }
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'

⬆ Back to top

chainAsync

Chains asynchronous functions.

Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.

const chainAsync = fns => {
  let curr = 0;
  const next = () => fns[curr++](next);
  next();
};

Examples

chainAsync([
  next => {
    console.log('0 seconds');
    setTimeout(next, 1000);
  },
  next => {
    console.log('1 second');
  }
]);

⬆ Back to top

compose

Performs right-to-left function composition.

Use Array.prototype.reduce() to perform right-to-left function composition. The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));

Examples

const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
  add5,
  multiply
);
multiplyAndAdd5(5, 2); // 15

⬆ Back to top

composeRight

Performs left-to-right function composition.

Use Array.prototype.reduce() to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.

const composeRight = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));

Examples

const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9

⬆ Back to top

converge

Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.

Use Array.prototype.map() and Function.prototype.apply() to apply each function to the given arguments. Use the spread operator (...) to call coverger with the results of all other functions.

const converge = (converger, fns) => (...args) => converger(...fns.map(fn => fn.apply(null, args)));

Examples

const average = converge((a, b) => a / b, [
  arr => arr.reduce((a, v) => a + v, 0),
  arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4

⬆ Back to top

curry

Curries a function.

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function fn. Otherwise, return a curried function fn that expects the rest of the arguments. If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity.

const curry = (fn, arity = fn.length, ...args) =>
  arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);

Examples

curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2

⬆ Back to top

debounce

Creates a debounced function that delays invoking the provided function until at least ms milliseconds have elapsed since the last time it was invoked.

Each time the debounced function is invoked, clear the current pending timeout with clearTimeout() and use setTimeout() to create a new timeout that delays invoking the function until at least ms milliseconds has elapsed. Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments. Omit the second argument, ms, to set the timeout at a default of 0 ms.

const debounce = (fn, ms = 0) => {
  let timeoutId;
  return function(...args) {
    clearTimeout(timeoutId);
    timeoutId = setTimeout(() => fn.apply(this, args), ms);
  };
};

Examples

window.addEventListener(
  'resize',
  debounce(() => {
    console.log(window.innerWidth);
    console.log(window.innerHeight);
  }, 250)
); // Will log the window dimensions at most every 250ms

⬆ Back to top

defer

Defers invoking a function until the current call stack has cleared.

Use setTimeout() with a timeout of 1ms to add a new event to the browser event queue and allow the rendering engine to complete its work. Use the spread (...) operator to supply the function with an arbitrary number of arguments.

const defer = (fn, ...args) => setTimeout(fn, 1, ...args);

Examples

// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'

// Example B:
document.querySelector('#someElement').innerHTML = 'Hello';
longRunningFunction(); //Browser will not update the HTML until this has finished
defer(longRunningFunction); // Browser will update the HTML then run the function

⬆ Back to top

delay

Invokes the provided function after wait milliseconds.

Use setTimeout() to delay execution of fn. Use the spread (...) operator to supply the function with an arbitrary number of arguments.

const delay = (fn, wait, ...args) => setTimeout(fn, wait, ...args);

Examples

delay(
  function(text) {
    console.log(text);
  },
  1000,
  'later'
); // Logs 'later' after one second.

⬆ Back to top

functionName

Logs the name of a function.

Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.

const functionName = fn => (console.debug(fn.name), fn);

Examples

functionName(Math.max); // max (logged in debug channel of console)

⬆ Back to top

hz

Returns the number of times a function executed per second. hz is the unit for hertz, the unit of frequency defined as one cycle per second.

Use performance.now() to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the function iterations times. Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed. Omit the second argument, iterations, to use the default of 100 iterations.

const hz = (fn, iterations = 100) => {
  const before = performance.now();
  for (let i = 0; i < iterations; i++) fn();
  return (1000 * iterations) / (performance.now() - before);
};

Examples

// 10,000 element array
const numbers = Array(10000)
  .fill()
  .map((_, i) => i);

// Test functions with the same goal: sum up the elements in the array
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
  let sum = 0;
  for (let i = 0; i < numbers.length; i++) sum += numbers[i];
  return sum;
};

// `sumForLoop` is nearly 10 times faster
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784

⬆ Back to top

memoize

Returns the memoized (cached) function.

Create an empty cache by instantiating a new Map object. Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not. The function keyword must be used in order to allow the memoized function to have its this context changed if necessary. Allow access to the cache by setting it as a property on the returned function.

const memoize = fn => {
  const cache = new Map();
  const cached = function(val) {
    return cache.has(val) ? cache.get(val) : cache.set(val, fn.call(this, val)) && cache.get(val);
  };
  cached.cache = cache;
  return cached;
};

Examples

// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's now cached
console.log(anagramsCached.cache); // The cached anagrams map

⬆ Back to top

negate

Negates a predicate function.

Take a predicate function and apply the not operator (!) to it with its arguments.

const negate = func => (...args) => !func(...args);

Examples

[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]

⬆ Back to top

once

Ensures a function is called only once.

Utilizing a closure, use a flag, called, and set it to true once the function is called for the first time, preventing it from being called again. In order to allow the function to have its this context changed (such as in an event listener), the function keyword must be used, and the supplied function must have the context applied. Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (...) operator.

const once = fn => {
  let called = false;
  return function(...args) {
    if (called) return;
    called = true;
    return fn.apply(this, args);
  };
};

Examples

const startApp = function(event) {
  console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp)); // only runs `startApp` once upon click

⬆ Back to top

partial

Creates a function that invokes fn with partials prepended to the arguments it receives.

Use the spread operator (...) to prepend partials to the list of arguments of fn.

const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);

Examples

const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'

⬆ Back to top

partialRight

Creates a function that invokes fn with partials appended to the arguments it receives.

Use the spread operator (...) to append partials to the list of arguments of fn.

const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);

Examples

const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'

⬆ Back to top

runPromisesInSeries

Runs an array of promises in series.

Use Array.prototype.reduce() to create a promise chain, where each promise returns the next promise when resolved.

const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());

Examples

const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]); // Executes each promise sequentially, taking a total of 3 seconds to complete

⬆ Back to top

sleep

Delays the execution of an asynchronous function.

Delay executing part of an async function, by putting it to sleep, returning a Promise.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

Examples

async function sleepyWork() {
  console.log("I'm going to sleep for 1 second.");
  await sleep(1000);
  console.log('I woke up after 1 second.');
}

⬆ Back to top

throttle

Creates a throttled function that only invokes the provided function at most once per every wait milliseconds

Use setTimeout() and clearTimeout() to throttle the given method, fn. Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments. Use Date.now() to keep track of the last time the throttled function was invoked. Omit the second argument, wait, to set the timeout at a default of 0 ms.

const throttle = (fn, wait) => {
  let inThrottle, lastFn, lastTime;
  return function() {
    const context = this,
      args = arguments;
    if (!inThrottle) {
      fn.apply(context, args);
      lastTime = Date.now();
      inThrottle = true;
    } else {
      clearTimeout(lastFn);
      lastFn = setTimeout(function() {
        if (Date.now() - lastTime >= wait) {
          fn.apply(context, args);
          lastTime = Date.now();
        }
      }, wait - (Date.now() - lastTime));
    }
  };
};

Examples

window.addEventListener(
  'resize',
  throttle(function(evt) {
    console.log(window.innerWidth);
    console.log(window.innerHeight);
  }, 250)
); // Will log the window dimensions at most every 250ms

⬆ Back to top

times

Iterates over a callback n times

Use Function.call() to call fn n times or until it returns false. Omit the last argument, context, to use an undefined object (or the global object in non-strict mode).

const times = (n, fn, context = undefined) => {
  let i = 0;
  while (fn.call(context, i) !== false && ++i < n) {}
};

Examples

var output = '';
times(5, i => (output += i));
console.log(output); // 01234

⬆ Back to top

uncurry

Uncurries a function up to depth n.

Return a variadic function. Use Array.prototype.reduce() on the provided arguments to call each subsequent curry level of the function. If the length of the provided arguments is less than n throw an error. Otherwise, call fn with the proper amount of arguments, using Array.prototype.slice(0, n). Omit the second argument, n, to uncurry up to depth 1.

const uncurry = (fn, n = 1) => (...args) => {
  const next = acc => args => args.reduce((x, y) => x(y), acc);
  if (n > args.length) throw new RangeError('Arguments too few!');
  return next(fn)(args.slice(0, n));
};

Examples

const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6

⬆ Back to top

unfold

Builds an array, using an iterator function and an initial seed value.

Use a while loop and Array.prototype.push() to call the function repeatedly until it returns false. The iterator function accepts one argument (seed) and must always return an array with two elements ([value, nextSeed]) or false to terminate.

const unfold = (fn, seed) => {
  let result = [],
    val = [null, seed];
  while ((val = fn(val[1]))) result.push(val[0]);
  return result;
};

Examples

var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]

⬆ Back to top

when

Tests a value, x, against a predicate function. If true, return fn(x). Else, return x.

Return a function expecting a single value, x, that returns the appropriate value based on pred.

const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);

Examples

const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1

⬆ Back to top

➗ Math

approximatelyEqual

Checks if two numbers are approximately equal to each other.

Use Math.abs() to compare the absolute difference of the two values to epsilon. Omit the third parameter, epsilon, to use a default value of 0.001.

const approximatelyEqual = (v1, v2, epsilon = 0.001) => Math.abs(v1 - v2) < epsilon;

Examples

approximatelyEqual(Math.PI / 2.0, 1.5708); // true

⬆ Back to top

average

Returns the average of two or more numbers.

Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

const average = (...nums) => nums.reduce((acc, val) => acc + val, 0) / nums.length;

Examples

average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2

⬆ Back to top

averageBy

Returns the average of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

const averageBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
  arr.length;

Examples

averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5

⬆ Back to top

binomialCoefficient

Evaluates the binomial coefficient of two integers n and k.

Use Number.isNaN() to check if any of the two values is NaN. Check if k is less than 0, greater than or equal to n, equal to 1 or n - 1 and return the appropriate result. Check if n - k is less than k and switch their values accordingly. Loop from 2 through k and calculate the binomial coefficient. Use Math.round() to account for rounding errors in the calculation.

const binomialCoefficient = (n, k) => {
  if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
  if (k < 0 || k > n) return 0;
  if (k === 0 || k === n) return 1;
  if (k === 1 || k === n - 1) return n;
  if (n - k < k) k = n - k;
  let res = n;
  for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
  return Math.round(res);
};

Examples

binomialCoefficient(8, 2); // 28

⬆ Back to top

clampNumber

Clamps num within the inclusive range specified by the boundary values a and b.

If num falls within the range, return num. Otherwise, return the nearest number in the range.

const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));

Examples

clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1

⬆ Back to top

degreesToRads

Converts an angle from degrees to radians.

Use Math.PI and the degree to radian formula to convert the angle from degrees to radians.

const degreesToRads = deg => (deg * Math.PI) / 180.0;

Examples

degreesToRads(90.0); // ~1.5708

⬆ Back to top

digitize

Converts a number to an array of digits.

Convert the number to a string, using the spread operator (...) to build an array. Use Array.prototype.map() and parseInt() to transform each value to an integer.

const digitize = n => [...`${n}`].map(i => parseInt(i));

Examples

digitize(123); // [1, 2, 3]

⬆ Back to top

distance

Returns the distance between two points.

Use Math.hypot() to calculate the Euclidean distance between two points.

const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);

Examples

distance(1, 1, 2, 3); // 2.23606797749979

⬆ Back to top

elo

Computes the new ratings between two or more opponents using the Elo rating system. It takes an array of pre-ratings and returns an array containing post-ratings. The array should be ordered from best performer to worst performer (winner -> loser).

Use the exponent ** operator and math operators to compute the expected score (chance of winning). of each opponent and compute the new rating for each. Loop through the ratings, using each permutation to compute the post-Elo rating for each player in a pairwise fashion. Omit the second argument to use the default kFactor of 32.

const elo = ([...ratings], kFactor = 32, selfRating) => {
  const [a, b] = ratings;
  const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
  const newRating = (rating, i) =>
    (selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
  if (ratings.length === 2) {
    return [newRating(a, 1), newRating(b, 0)];
  }
  for (let i = 0, len = ratings.length; i < len; i++) {
    let j = i;
    while (j < len - 1) {
      j++;
      [ratings[i], ratings[j]] = elo([ratings[i], ratings[j]], kFactor);
    }
  }
  return ratings;
};

Examples

// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/

⬆ Back to top

factorial

Calculates the factorial of a number.

Use recursion. If n is less than or equal to 1, return 1. Otherwise, return the product of n and the factorial of n - 1. Throws an exception if n is a negative number.

const factorial = n =>
  n < 0
    ? (() => {
        throw new TypeError('Negative numbers are not allowed!');
      })()
    : n <= 1
      ? 1
      : n * factorial(n - 1);

Examples

factorial(6); // 720

⬆ Back to top

fibonacci

Generates an array, containing the Fibonacci sequence, up until the nth term.

Create an empty array of the specific length, initializing the first two values (0 and 1). Use Array.prototype.reduce() to add values into the array, using the sum of the last two values, except for the first two.

const fibonacci = n =>
  Array.from({ length: n }).reduce(
    (acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
    []
  );

Examples

fibonacci(6); // [0, 1, 1, 2, 3, 5]

⬆ Back to top

gcd

Calculates the greatest common divisor between two or more numbers/arrays.

The inner _gcd function uses recursion. Base case is when y equals 0. In this case, return x. Otherwise, return the GCD of y and the remainder of the division x/y.

const gcd = (...arr) => {
  const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
  return [...arr].reduce((a, b) => _gcd(a, b));
};

Examples

gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4

⬆ Back to top

geometricProgression

Initializes an array containing the numbers in the specified range where start and end are inclusive and the ratio between two terms is step. Returns an error if step equals 1.

Use Array.from(), Math.log() and Math.floor() to create an array of the desired length, Array.prototype.map() to fill with the desired values in a range. Omit the second argument, start, to use a default value of 1. Omit the third argument, step, to use a default value of 2.

const geometricProgression = (end, start = 1, step = 2) =>
  Array.from({ length: Math.floor(Math.log(end / start) / Math.log(step)) + 1 }).map(
    (v, i) => start * step ** i
  );

Examples

geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]

⬆ Back to top

hammingDistance

Calculates the Hamming distance between two values.

Use XOR operator (^) to find the bit difference between the two numbers, convert to a binary string using toString(2). Count and return the number of 1s in the string, using match(/1/g).

const hammingDistance = (num1, num2) => ((num1 ^ num2).toString(2).match(/1/g) || '').length;

Examples

hammingDistance(2, 3); // 1

⬆ Back to top

inRange

Checks if the given number falls within the given range.

Use arithmetic comparison to check if the given number is in the specified range. If the second parameter, end, is not specified, the range is considered to be from 0 to start.

const inRange = (n, start, end = null) => {
  if (end && start > end) [end, start] = [start, end];
  return end == null ? n >= 0 && n < start : n >= start && n < end;
};

Examples

inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // false

⬆ Back to top

isDivisible

Checks if the first numeric argument is divisible by the second one.

Use the modulo operator (%) to check if the remainder is equal to 0.

const isDivisible = (dividend, divisor) => dividend % divisor === 0;

Examples

isDivisible(6, 3); // true

⬆ Back to top

isEven

Returns true if the given number is even, false otherwise.

Checks whether a number is odd or even using the modulo (%) operator. Returns true if the number is even, false if the number is odd.

const isEven = num => num % 2 === 0;

Examples

isEven(3); // false

⬆ Back to top

isPrime

Checks if the provided integer is a prime number.

Check numbers from 2 to the square root of the given number. Return false if any of them divides the given number, else return true, unless the number is less than 2.

const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (var i = 2; i <= boundary; i++) if (num % i === 0) return false;
  return num >= 2;
};

Examples

isPrime(11); // true

⬆ Back to top

lcm

Returns the least common multiple of two or more numbers.

Use the greatest common divisor (GCD) formula and the fact that lcm(x,y) = x * y / gcd(x,y) to determine the least common multiple. The GCD formula uses recursion.

const lcm = (...arr) => {
  const gcd = (x, y) => (!y ? x : gcd(y, x % y));
  const _lcm = (x, y) => (x * y) / gcd(x, y);
  return [...arr].reduce((a, b) => _lcm(a, b));
};

Examples

lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60

⬆ Back to top

luhnCheck

Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

Use String.prototype.split(''), Array.prototype.reverse() and Array.prototype.map() in combination with parseInt() to obtain an array of digits. Use Array.prototype.splice(0,1) to obtain the last digit. Use Array.prototype.reduce() to implement the Luhn Algorithm. Return true if sum is divisible by 10, false otherwise.

const luhnCheck = num => {
  let arr = (num + '')
    .split('')
    .reverse()
    .map(x => parseInt(x));
  let lastDigit = arr.splice(0, 1)[0];
  let sum = arr.reduce((acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9), 0);
  sum += lastDigit;
  return sum % 10 === 0;
};

Examples

luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); //  false
luhnCheck(123456789); // false

⬆ Back to top

maxBy

Returns the maximum value of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Math.max() to get the maximum value.

const maxBy = (arr, fn) => Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));

Examples

maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8

⬆ Back to top

median

Returns the median of an array of numbers.

Find the middle of the array, use Array.prototype.sort() to sort the values. Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.

const median = arr => {
  const mid = Math.floor(arr.length / 2),
    nums = [...arr].sort((a, b) => a - b);
  return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};

Examples

median([5, 6, 50, 1, -5]); // 5

⬆ Back to top

minBy

Returns the minimum value of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Math.min() to get the maximum value.

const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));

Examples

minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2

⬆ Back to top

percentile

Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.

Use Array.prototype.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) =>
  (100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0)) / arr.length;

Examples

percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55

⬆ Back to top

powerset

Returns the powerset of a given array of numbers.

Use Array.prototype.reduce() combined with Array.prototype.map() to iterate over elements and combine into an array containing all combinations.

const powerset = arr => arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);

Examples

powerset([1, 2]); // [[], [1], [2], [2, 1]]

⬆ Back to top

primes

Generates primes up to a given number, using the Sieve of Eratosthenes.

Generate an array from 2 to the given number. Use Array.prototype.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
  return arr;
};

Examples

primes(10); // [2,3,5,7]

⬆ Back to top

radsToDegrees

Converts an angle from radians to degrees.

Use Math.PI and the radian to degree formula to convert the angle from radians to degrees.

const radsToDegrees = rad => (rad * 180.0) / Math.PI;

Examples

radsToDegrees(Math.PI / 2); // 90

⬆ Back to top

randomIntArrayInRange

Returns an array of n random integers in the specified range.

Use Array.from() to create an empty array of the specific length, Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntArrayInRange = (min, max, n = 1) =>
  Array.from({ length: n }, () => Math.floor(Math.random() * (max - min + 1)) + min);

Examples

randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]

⬆ Back to top

randomIntegerInRange

Returns a random integer in the specified range.

Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;

Examples

randomIntegerInRange(0, 5); // 2

⬆ Back to top

randomNumberInRange

Returns a random number in the specified range.

Use Math.random() to generate a random value, map it to the desired range using multiplication.

const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;

Examples

randomNumberInRange(2, 10); // 6.0211363285087005

⬆ Back to top

round

Rounds a number to a specified amount of digits.

Use Math.round() and template literals to round the number to the specified number of digits. Omit the second argument, decimals to round to an integer.

const round = (n, decimals = 0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);

Examples

round(1.005, 2); // 1.01

⬆ Back to top

sdbm

Hashes the input string into a whole number.

Use String.prototype.split('') and Array.prototype.reduce() to create a hash of the input string, utilizing bit shifting.

const sdbm = str => {
  let arr = str.split('');
  return arr.reduce(
    (hashCode, currentVal) =>
      (hashCode = currentVal.charCodeAt(0) + (hashCode << 6) + (hashCode << 16) - hashCode),
    0
  );
};

Examples

sdbm('name'); // -3521204949

⬆ Back to top

standardDeviation

Returns the standard deviation of an array of numbers.

Use Array.prototype.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then determine the standard deviation. You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.

const standardDeviation = (arr, usePopulation = false) => {
  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
  return Math.sqrt(
    arr.reduce((acc, val) => acc.concat((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
      (arr.length - (usePopulation ? 0 : 1))
  );
};

Examples

standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true); // 12.29899614287479 (population)

⬆ Back to top

sum

Returns the sum of two or more numbers/arrays.

Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.

const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);

Examples

sum(...[1, 2, 3, 4]); // 10

⬆ Back to top

sumBy

Returns the sum of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.

const sumBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);

Examples

sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20

⬆ Back to top

sumPower

Returns the sum of the powers of all the numbers from start to end (both inclusive).

Use Array.prototype.fill() to create an array of all the numbers in the target range, Array.prototype.map() and the exponent operator (**) to raise them to power and Array.prototype.reduce() to add them together. Omit the second argument, power, to use a default power of 2. Omit the third argument, start, to use a default starting value of 1.

const sumPower = (end, power = 2, start = 1) =>
  Array(end + 1 - start)
    .fill(0)
    .map((x, i) => (i + start) ** power)
    .reduce((a, b) => a + b, 0);

Examples

sumPower(10); // 385
sumPower(10, 3); //3025
sumPower(10, 3, 5); //2925

⬆ Back to top

toSafeInteger

Converts a value to a safe integer.

Use Math.max() and Math.min() to find the closest safe value. Use Math.round() to convert to an integer.

const toSafeInteger = num =>
  Math.round(Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER));

Examples

toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991

⬆ Back to top

📦 Node

atob

Decodes a string of data which has been encoded using base-64 encoding.

Create a Buffer for the given string with base-64 encoding and use Buffer.toString('binary') to return the decoded string.

const atob = str => new Buffer(str, 'base64').toString('binary');

Examples

atob('Zm9vYmFy'); // 'foobar'

⬆ Back to top

btoa

Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.

Create a Buffer for the given string with binary encoding and use Buffer.toString('base64') to return the encoded string.

const btoa = str => new Buffer(str, 'binary').toString('base64');

Examples

btoa('foobar'); // 'Zm9vYmFy'

⬆ Back to top

colorize

Add special characters to text to print in color in the console (combined with console.log()).

Use template literals and special characters to add the appropriate color code to the string output. For background colors, add a special character that resets the background color at the end of the string.

const colorize = (...args) => ({
  black: `\x1b[30m${args.join(' ')}`,
  red: `\x1b[31m${args.join(' ')}`,
  green: `\x1b[32m${args.join(' ')}`,
  yellow: `\x1b[33m${args.join(' ')}`,
  blue: `\x1b[34m${args.join(' ')}`,
  magenta: `\x1b[35m${args.join(' ')}`,
  cyan: `\x1b[36m${args.join(' ')}`,
  white: `\x1b[37m${args.join(' ')}`,
  bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
  bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
  bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
  bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
  bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
  bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
  bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
  bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});

Examples

console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite); // 'foo bar' (first word in yellow letters, second word in green letters, white background for both)

⬆ Back to top

hasFlags

Check if the current process's arguments contain the specified flags.

Use Array.prototype.every() and Array.prototype.includes() to check if process.argv contains all the specified flags. Use a regular expression to test if the specified flags are prefixed with - or -- and prefix them accordingly.

const hasFlags = (...flags) =>
  flags.every(flag => process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag));

Examples

// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // false

⬆ Back to top

hashNode

Creates a hash for a value using the SHA-256 algorithm. Returns a promise.

Use crypto API to create a hash for the given value.

const crypto = require('crypto');
const hashNode = val =>
  new Promise(resolve =>
    setTimeout(
      () =>
        resolve(
          crypto
            .createHash('sha256')
            .update(val)
            .digest('hex')
        ),
      0
    )
  );

Examples

hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'

⬆ Back to top

isTravisCI

Checks if the current environment is Travis CI.

Checks if the current environment has the TRAVIS and CI environment variables (reference).

const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;

Examples

isTravisCI(); // true (if code is running on Travis CI)

⬆ Back to top

JSONToFile

Writes a JSON object to a file.

Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.

const fs = require('fs');
const JSONToFile = (obj, filename) =>
  fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2));

Examples

JSONToFile({ test: 'is passed' }, 'testJsonFile'); // writes the object to 'testJsonFile.json'

⬆ Back to top

readFileLines

Returns an array of lines from the specified file.

Use readFileSync function in fs node package to create a Buffer from a file. convert buffer to string using toString(encoding) function. creating an array from contents of file by spliting file content line by line (each \n).

const fs = require('fs');
const readFileLines = filename =>
  fs
    .readFileSync(filename)
    .toString('UTF8')
    .split('\n');

Examples

/*
contents of test.txt :
  line1
  line2
  line3
  ___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']

⬆ Back to top

untildify

Converts a tilde path to an absolute path.

Use String.prototype.replace() with a regular expression and OS.homedir() to replace the ~ in the start of the path with the home directory.

const untildify = str => str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);

Examples

untildify('~/node'); // '/Users/aUser/node'

⬆ Back to top

UUIDGeneratorNode

Generates a UUID in Node.JS.

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

const crypto = require('crypto');
const UUIDGeneratorNode = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
  );

Examples

UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'

⬆ Back to top

🗃️ Object

bindAll

Binds methods of an object to the object itself, overwriting the existing method.

Use Array.prototype.forEach() to return a function that uses Function.prototype.apply() to apply the given context (obj) to fn for each function specified.

const bindAll = (obj, ...fns) =>
  fns.forEach(
    fn => (
      (f = obj[fn]),
      (obj[fn] = function() {
        return f.apply(obj);
      })
    )
  );

Examples

var view = {
  label: 'docs',
  click: function() {
    console.log('clicked ' + this.label);
  }
};
bindAll(view, 'click');
jQuery(element).on('click', view.click); // Logs 'clicked docs' when clicked.

⬆ Back to top

deepClone

Creates a deep clone of an object.

Use recursion. Use Object.assign() and an empty object ({}) to create a shallow clone of the original. Use Object.keys() and Array.prototype.forEach() to determine which key-value pairs need to be deep cloned.

const deepClone = obj => {
  let clone = Object.assign({}, obj);
  Object.keys(clone).forEach(
    key => (clone[key] = typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
  );
  return Array.isArray(obj) ? (clone.length = obj.length) && Array.from(clone) : clone;
};

Examples

const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.obj

⬆ Back to top

deepFreeze

Deep freezes an object.

Calls Object.freeze(obj) recursively on all unfrozen properties of passed object that are instanceof object.

const deepFreeze = obj =>
  Object.keys(obj).forEach(
    prop =>
      !obj[prop] instanceof Object || Object.isFrozen(obj[prop]) ? null : deepFreeze(obj[prop])
  ) || Object.freeze(obj);

Examples

'use strict';

const o = deepFreeze([1, [2, 3]]);

o[0] = 3; // not allowed
o[1][0] = 4; // not allowed as well

⬆ Back to top

defaults

Assigns default values for all properties in an object that are undefined.

Use Object.assign() to create a new empty object and copy the original one to maintain key order, use Array.prototype.reverse() and the spread operator ... to combine the default values from left to right, finally use obj again to overwrite properties that originally had a value.

const defaults = (obj, ...defs) => Object.assign({}, obj, ...defs.reverse(), obj);

Examples

defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }

⬆ Back to top

dig

Returns the target value in a nested JSON object, based on the given key.

Use the in operator to check if target exists in obj. If found, return the value of obj[target], otherwise use Object.values(obj) and Array.prototype.reduce() to recursively call dig on each nested object until the first matching key/value pair is found.

const dig = (obj, target) =>
  target in obj
    ? obj[target]
    : Object.values(obj).reduce((acc, val) => {
        if (acc !== undefined) return acc;
        if (typeof val === 'object') return dig(val, target);
      }, undefined);

Examples

const data = {
  level1: {
    level2: {
      level3: 'some data'
    }
  }
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefined

⬆ Back to top

equals

Performs a deep comparison between two values to determine if they are equivalent.

Check if the two values are identical, if they are both Date objects with the same time, using Date.getTime() or if they are both non-object values with an equivalent value (strict comparison). Check if only one value is null or undefined or if their prototypes differ. If none of the above conditions are met, use Object.keys() to check if both values have the same number of keys, then use Array.prototype.every() to check if every key in the first value exists in the second one and if they are equivalent by calling this method recursively.

const equals = (a, b) => {
  if (a === b) return true;
  if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
  if (!a || !b || (typeof a !== 'object' && typeof b !== 'object')) return a === b;
  if (a === null || a === undefined || b === null || b === undefined) return false;
  if (a.prototype !== b.prototype) return false;
  let keys = Object.keys(a);
  if (keys.length !== Object.keys(b).length) return false;
  return keys.every(k => equals(a[k], b[k]));
};

Examples

equals({ a: [2, { e: 3 }], b: [4], c: 'foo' }, { a: [2, { e: 3 }], b: [4], c: 'foo' }); // true

⬆ Back to top

findKey

Returns the first key that satisfies the provided testing function. Otherwise undefined is returned.

Use Object.keys(obj) to get all the properties of the object, Array.prototype.find() to test the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const findKey = (obj, fn) => Object.keys(obj).find(key => fn(obj[key], key, obj));

Examples

findKey(
  {
    barney: { age: 36, active: true },
    fred: { age: 40, active: false },
    pebbles: { age: 1, active: true }
  },
  o => o['active']
); // 'barney'

⬆ Back to top

findLastKey

Returns the last key that satisfies the provided testing function. Otherwise undefined is returned.

Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.find() to test the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const findLastKey = (obj, fn) =>
  Object.keys(obj)
    .reverse()
    .find(key => fn(obj[key], key, obj));

Examples

findLastKey(
  {
    barney: { age: 36, active: true },
    fred: { age: 40, active: false },
    pebbles: { age: 1, active: true }
  },
  o => o['active']
); // 'pebbles'

⬆ Back to top

flattenObject

Flatten an object with the paths for keys.

Use recursion. Use Object.keys(obj) combined with Array.prototype.reduce() to convert every leaf node to a flattened path node. If the value of a key is an object, the function calls itself with the appropriate prefix to create the path using Object.assign(). Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object. You should always omit the second argument, prefix, unless you want every key to have a prefix.

const flattenObject = (obj, prefix = '') =>
  Object.keys(obj).reduce((acc, k) => {
    const pre = prefix.length ? prefix + '.' : '';
    if (typeof obj[k] === 'object') Object.assign(acc, flattenObject(obj[k], pre + k));
    else acc[pre + k] = obj[k];
    return acc;
  }, {});

Examples

flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }

⬆ Back to top

forOwn

Iterates over all own properties of an object, running a callback for each one.

Use Object.keys(obj) to get all the properties of the object, Array.prototype.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const forOwn = (obj, fn) => Object.keys(obj).forEach(key => fn(obj[key], key, obj));

Examples

forOwn({ foo: 'bar', a: 1 }, v => console.log(v)); // 'bar', 1

⬆ Back to top

forOwnRight

Iterates over all own properties of an object in reverse, running a callback for each one.

Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const forOwnRight = (obj, fn) =>
  Object.keys(obj)
    .reverse()
    .forEach(key => fn(obj[key], key, obj));

Examples

forOwnRight({ foo: 'bar', a: 1 }, v => console.log(v)); // 1, 'bar'

⬆ Back to top

functions

Returns an array of function property names from own (and optionally inherited) enumerable properties of an object.

Use Object.keys(obj) to iterate over the object's own properties. If inherited is true, use Object.get.PrototypeOf(obj) to also get the object's inherited properties. Use Array.prototype.filter() to keep only those properties that are functions. Omit the second argument, inherited, to not include inherited properties by default.

const functions = (obj, inherited = false) =>
  (inherited
    ? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
    : Object.keys(obj)
  ).filter(key => typeof obj[key] === 'function');

Examples

function Foo() {
  this.a = () => 1;
  this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']

⬆ Back to top

get

Retrieve a set of properties indicated by the given selectors from an object.

Use Array.prototype.map() for each selector, String.prototype.replace() to replace square brackets with dots, String.prototype.split('.') to split each selector, Array.prototype.filter() to remove empty values and Array.prototype.reduce() to get the value indicated by it.

const get = (from, ...selectors) =>
  [...selectors].map(s =>
    s
      .replace(/\[([^\[\]]*)\]/g, '.$1.')
      .split('.')
      .filter(t => t !== '')
      .reduce((prev, cur) => prev && prev[cur], from)
  );

Examples

const obj = { selector: { to: { val: 'val to select' } }, target: [1, 2, { a: 'test' }] };
get(obj, 'selector.to.val', 'target[0]', 'target[2].a'); // ['val to select', 1, 'test']

⬆ Back to top

invertKeyValues

Inverts the key-value pairs of an object, without mutating it. The corresponding inverted value of each inverted key is an array of keys responsible for generating the inverted value. If a function is supplied, it is applied to each inverted key.

Use Object.keys() and Array.prototype.reduce() to invert the key-value pairs of an object and apply the function provided (if any). Omit the second argument, fn, to get the inverted keys without applying a function to them.

const invertKeyValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, key) => {
    const val = fn ? fn(obj[key]) : obj[key];
    acc[val] = acc[val] || [];
    acc[val].push(key);
    return acc;
  }, {});

Examples

invertKeyValues({ a: 1, b: 2, c: 1 }); // { 1: [ 'a', 'c' ], 2: [ 'b' ] }
invertKeyValues({ a: 1, b: 2, c: 1 }, value => 'group' + value); // { group1: [ 'a', 'c' ], group2: [ 'b' ] }

⬆ Back to top

lowercaseKeys

Creates a new object from the specified object, where all the keys are in lowercase.

Use Object.keys() and Array.prototype.reduce() to create a new object from the specified object. Convert each key in the original object to lowercase, using String.toLowerCase().

const lowercaseKeys = obj =>
  Object.keys(obj).reduce((acc, key) => {
    acc[key.toLowerCase()] = obj[key];
    return acc;
  }, {});

Examples

const myObj = { Name: 'Adam', sUrnAME: 'Smith' };
const myObjLower = lowercaseKeys(myObj); // {name: 'Adam', surname: 'Smith'};

⬆ Back to top

mapKeys

Creates an object with keys generated by running the provided function for each key and the same values as the provided object.

Use Object.keys(obj) to iterate over the object's keys. Use Array.prototype.reduce() to create a new object with the same values and mapped keys using fn.

const mapKeys = (obj, fn) =>
  Object.keys(obj).reduce((acc, k) => {
    acc[fn(obj[k], k, obj)] = obj[k];
    return acc;
  }, {});

Examples

mapKeys({ a: 1, b: 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }

⬆ Back to top

mapValues

Creates an object with the same keys as the provided object and values generated by running the provided function for each value.

Use Object.keys(obj) to iterate over the object's keys. Use Array.prototype.reduce() to create a new object with the same keys and mapped values using fn.

const mapValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, k) => {
    acc[k] = fn(obj[k], k, obj);
    return acc;
  }, {});

Examples

const users = {
  fred: { user: 'fred', age: 40 },
  pebbles: { user: 'pebbles', age: 1 }
};
mapValues(users, u => u.age); // { fred: 40, pebbles: 1 }

⬆ Back to top

matches

Compares two objects to determine if the first one contains equivalent property values to the second one.

Use Object.keys(source) to get all the keys of the second object, then Array.prototype.every(), Object.hasOwnProperty() and strict comparison to determine if all keys exist in the first object and have the same values.

const matches = (obj, source) =>
  Object.keys(source).every(key => obj.hasOwnProperty(key) && obj[key] === source[key]);

Examples

matches({ age: 25, hair: 'long', beard: true }, { hair: 'long', beard: true }); // true
matches({ hair: 'long', beard: true }, { age: 25, hair: 'long', beard: true }); // false

⬆ Back to top

matchesWith

Compares two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.

Use Object.keys(source) to get all the keys of the second object, then Array.prototype.every(), Object.hasOwnProperty() and the provided function to determine if all keys exist in the first object and have equivalent values. If no function is provided, the values will be compared using the equality operator.

const matchesWith = (obj, source, fn) =>
  Object.keys(source).every(
    key =>
      obj.hasOwnProperty(key) && fn
        ? fn(obj[key], source[key], key, obj, source)
        : obj[key] == source[key]
  );

Examples

const isGreeting = val => /^h(?:i|ello)$/.test(val);
matchesWith(
  { greeting: 'hello' },
  { greeting: 'hi' },
  (oV, sV) => isGreeting(oV) && isGreeting(sV)
); // true

⬆ Back to top

merge

Creates a new object from the combination of two or more objects.

Use Array.prototype.reduce() combined with Object.keys(obj) to iterate over all objects and keys. Use hasOwnProperty() and Array.prototype.concat() to append values for keys existing in multiple objects.

const merge = (...objs) =>
  [...objs].reduce(
    (acc, obj) =>
      Object.keys(obj).reduce((a, k) => {
        acc[k] = acc.hasOwnProperty(k) ? [].concat(acc[k]).concat(obj[k]) : obj[k];
        return acc;
      }, {}),
    {}
  );

Examples

const object = {
  a: [{ x: 2 }, { y: 4 }],
  b: 1
};
const other = {
  a: { z: 3 },
  b: [2, 3],
  c: 'foo'
};
merge(object, other); // { a: [ { x: 2 }, { y: 4 }, { z: 3 } ], b: [ 1, 2, 3 ], c: 'foo' }

⬆ Back to top

nest

Given a flat array of objects linked to one another, it will nest them recursively. Useful for nesting comments, such as the ones on reddit.com.

Use recursion. Use Array.prototype.filter() to filter the items where the id matches the link, then Array.prototype.map() to map each one to a new object that has a children property which recursively nests the items based on which ones are children of the current item. Omit the second argument, id, to default to null which indicates the object is not linked to another one (i.e. it is a top level object). Omit the third argument, link, to use 'parent_id' as the default property which links the object to another one by its id.

const nest = (items, id = null, link = 'parent_id') =>
  items
    .filter(item => item[link] === id)
    .map(item => ({ ...item, children: nest(items, item.id) }));

Examples

// One top level comment
const comments = [
  { id: 1, parent_id: null },
  { id: 2, parent_id: 1 },
  { id: 3, parent_id: 1 },
  { id: 4, parent_id: 2 },
  { id: 5, parent_id: 4 }
];
const nestedComments = nest(comments); // [{ id: 1, parent_id: null, children: [...] }]

⬆ Back to top

objectFromPairs

Creates an object from the given key-value pairs.

Use Array.prototype.reduce() to create and combine key-value pairs.

const objectFromPairs = arr => arr.reduce((a, [key, val]) => ((a[key] = val), a), {});

Examples

objectFromPairs([['a', 1], ['b', 2]]); // {a: 1, b: 2}

⬆ Back to top

objectToPairs

Creates an array of key-value pair arrays from an object.

Use Object.keys() and Array.prototype.map() to iterate over the object's keys and produce an array with key-value pairs.

const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);

Examples

objectToPairs({ a: 1, b: 2 }); // [ ['a', 1], ['b', 2] ]

⬆ Back to top

omit

Omits the key-value pairs corresponding to the given keys from an object.

Use Object.keys(obj), Array.prototype.filter() and Array.prototype.includes() to remove the provided keys. Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const omit = (obj, arr) =>
  Object.keys(obj)
    .filter(k => !arr.includes(k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

Examples

omit({ a: 1, b: '2', c: 3 }, ['b']); // { 'a': 1, 'c': 3 }

⬆ Back to top

omitBy

Creates an object composed of the properties the given function returns falsey for. The function is invoked with two arguments: (value, key).

Use Object.keys(obj) and Array.prototype.filter()to remove the keys for which fn returns a truthy value. Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const omitBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => !fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

Examples

omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }

⬆ Back to top

orderBy

Returns a sorted array of objects ordered by properties and orders.

Uses Array.prototype.sort(), Array.prototype.reduce() on the props array with a default value of 0, use array destructuring to swap the properties position depending on the order passed. If no orders array is passed it sort by 'asc' by default.

const orderBy = (arr, props, orders) =>
  [...arr].sort((a, b) =>
    props.reduce((acc, prop, i) => {
      if (acc === 0) {
        const [p1, p2] = orders && orders[i] === 'desc' ? [b[prop], a[prop]] : [a[prop], b[prop]];
        acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;
      }
      return acc;
    }, 0)
  );

Examples

const users = [{ name: 'fred', age: 48 }, { name: 'barney', age: 36 }, { name: 'fred', age: 40 }];
orderBy(users, ['name', 'age'], ['asc', 'desc']); // [{name: 'barney', age: 36}, {name: 'fred', age: 48}, {name: 'fred', age: 40}]
orderBy(users, ['name', 'age']); // [{name: 'barney', age: 36}, {name: 'fred', age: 40}, {name: 'fred', age: 48}]

⬆ Back to top

pick

Picks the key-value pairs corresponding to the given keys from an object.

Use Array.prototype.reduce() to convert the filtered/picked keys back to an object with the corresponding key-value pairs if the key exists in the object.

const pick = (obj, arr) =>
  arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});

Examples

pick({ a: 1, b: '2', c: 3 }, ['a', 'c']); // { 'a': 1, 'c': 3 }

⬆ Back to top

pickBy

Creates an object composed of the properties the given function returns truthy for. The function is invoked with two arguments: (value, key).

Use Object.keys(obj) and Array.prototype.filter()to remove the keys for which fn returns a falsey value. Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.

const pickBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

Examples

pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { 'a': 1, 'c': 3 }

⬆ Back to top

renameKeys

Replaces the names of multiple object keys with the values provided.

Use Object.keys() in combination with Array.prototype.reduce() and the spread operator (...) to get the object's keys and rename them according to keysMap.

const renameKeys = (keysMap, obj) =>
  Object.keys(obj).reduce(
    (acc, key) => ({
      ...acc,
      ...{ [keysMap[key] || key]: obj[key] }
    }),
    {}
  );

Examples

const obj = { name: 'Bobo', job: 'Front-End Master', shoeSize: 100 };
renameKeys({ name: 'firstName', job: 'passion' }, obj); // { firstName: 'Bobo', passion: 'Front-End Master', shoeSize: 100 }

⬆ Back to top

shallowClone

Creates a shallow clone of an object.

Use Object.assign() and an empty object ({}) to create a shallow clone of the original.

const shallowClone = obj => Object.assign({}, obj);

Examples

const a = { x: true, y: 1 };
const b = shallowClone(a); // a !== b

⬆ Back to top

size

Get size of arrays, objects or strings.

Get type of val (array, object or string). Use length property for arrays. Use length or size value if available or number of keys for objects. Use size of a Blob object created from val for strings.

Split strings into array of characters with split('') and return its length.

const size = val =>
  Array.isArray(val)
    ? val.length
    : val && typeof val === 'object'
      ? val.size || val.length || Object.keys(val).length
      : typeof val === 'string'
        ? new Blob([val]).size
        : 0;

Examples

size([1, 2, 3, 4, 5]); // 5
size('size'); // 4
size({ one: 1, two: 2, three: 3 }); // 3

⬆ Back to top

transform

Applies a function against an accumulator and each key in the object (from left to right).

Use Object.keys(obj) to iterate over each key in the object, Array.prototype.reduce() to call the apply the specified function against the given accumulator.

const transform = (obj, fn, acc) => Object.keys(obj).reduce((a, k) => fn(a, obj[k], k, obj), acc);

Examples

transform(
  { a: 1, b: 2, c: 1 },
  (r, v, k) => {
    (r[v] || (r[v] = [])).push(k);
    return r;
  },
  {}
); // { '1': ['a', 'c'], '2': ['b'] }

⬆ Back to top

truthCheckCollection

Checks if the predicate (second argument) is truthy on all elements of a collection (first argument).

Use Array.prototype.every() to check if each passed object has the specified property and if it returns a truthy value.

const truthCheckCollection = (collection, pre) => collection.every(obj => obj[pre]);

Examples

truthCheckCollection([{ user: 'Tinky-Winky', sex: 'male' }, { user: 'Dipsy', sex: 'male' }], 'sex'); // true

⬆ Back to top

unflattenObject

Unflatten an object with the paths for keys.

Use Object.keys(obj) combined with Array.prototype.reduce() to convert flattened path node to a leaf node. If the value of a key contains a dot delimiter (.), use Array.prototype.split('.'), string transformations and JSON.parse() to create an object, then Object.assign() to create the leaf node. Otherwise, add the appropriate key-value pair to the accumulator object.

const unflattenObject = obj =>
  Object.keys(obj).reduce((acc, k) => {
    if (k.indexOf('.') !== -1) {
      const keys = k.split('.');
      Object.assign(
        acc,
        JSON.parse(
          '{' +
            keys.map((v, i) => (i !== keys.length - 1 ? `"${v}":{` : `"${v}":`)).join('') +
            obj[k] +
            '}'.repeat(keys.length)
        )
      );
    } else acc[k] = obj[k];
    return acc;
  }, {});

Examples

unflattenObject({ 'a.b.c': 1, d: 1 }); // { a: { b: { c: 1 } }, d: 1 }

⬆ Back to top

📜 String

byteSize

Returns the length of a string in bytes.

Convert a given string to a Blob Object and find its size.

const byteSize = str => new Blob([str]).size;

Examples

byteSize('😀'); // 4
byteSize('Hello World'); // 11

⬆ Back to top

capitalize

Capitalizes the first letter of a string.

Use array destructuring and String.prototype.toUpperCase() to capitalize first letter, ...rest to get array of characters after first letter and then Array.prototype.join('') to make it a string again. Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lowercase.

const capitalize = ([first, ...rest], lowerRest = false) =>
  first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));

Examples

capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'

⬆ Back to top

capitalizeEveryWord

Capitalizes the first letter of every word in a string.

Use String.prototype.replace() to match the first character of each word and String.prototype.toUpperCase() to capitalize it.

const capitalizeEveryWord = str => str.replace(/\b[a-z]/g, char => char.toUpperCase());

Examples

capitalizeEveryWord('hello world!'); // 'Hello World!'

⬆ Back to top

CSVToArray

Converts a comma-separated values (CSV) string to a 2D array.

Use Array.prototype.slice() and Array.prototype.indexOf('\n') to remove the first row (title row) if omitFirstRow is true. Use String.prototype.split('\n') to create a string for each row, then String.prototype.split(delimiter) to separate the values in each row. Omit the second argument, delimiter, to use a default delimiter of ,. Omit the third argument, omitFirstRow, to include the first row (title row) of the CSV string.

const CSVToArray = (data, delimiter = ',', omitFirstRow = false) =>
  data
    .slice(omitFirstRow ? data.indexOf('\n') + 1 : 0)
    .split('\n')
    .map(v => v.split(delimiter));

Examples

CSVToArray('a,b\nc,d'); // [['a','b'],['c','d']];
CSVToArray('a;b\nc;d', ';'); // [['a','b'],['c','d']];
CSVToArray('col1,col2\na,b\nc,d', ',', true); // [['a','b'],['c','d']];

⬆ Back to top

CSVToJSON

Converts a comma-separated values (CSV) string to a 2D array of objects. The first row of the string is used as the title row.

Use Array.prototype.slice() and Array.prototype.indexOf('\n') and String.prototype.split(delimiter) to separate the first row (title row) into values. Use String.prototype.split('\n') to create a string for each row, then Array.prototype.map() and String.prototype.split(delimiter) to separate the values in each row. Use Array.prototype.reduce() to create an object for each row's values, with the keys parsed from the title row. Omit the second argument, delimiter, to use a default delimiter of ,.

const CSVToJSON = (data, delimiter = ',') => {
  const titles = data.slice(0, data.indexOf('\n')).split(delimiter);
  return data
    .slice(data.indexOf('\n') + 1)
    .split('\n')
    .map(v => {
      const values = v.split(delimiter);
      return titles.reduce((obj, title, index) => ((obj[title] = values[index]), obj), {});
    });
};

Examples

CSVToJSON('col1,col2\na,b\nc,d'); // [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];
CSVToJSON('col1;col2\na;b\nc;d', ';'); // [{'col1': 'a', 'col2': 'b'}, {'col1': 'c', 'col2': 'd'}];

⬆ Back to top

decapitalize

Decapitalizes the first letter of a string.

Use array destructuring and String.toLowerCase() to decapitalize first letter, ...rest to get array of characters after first letter and then Array.prototype.join('') to make it a string again. Omit the upperRest parameter to keep the rest of the string intact, or set it to true to convert to uppercase.

const decapitalize = ([first, ...rest], upperRest = false) =>
  first.toLowerCase() + (upperRest ? rest.join('').toUpperCase() : rest.join(''));

Examples

decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'

⬆ Back to top

escapeHTML

Escapes a string for use in HTML.

Use String.prototype.replace() with a regexp that matches the characters that need to be escaped, using a callback function to replace each character instance with its associated escaped character using a dictionary (object).

const escapeHTML = str =>
  str.replace(
    /[&<>'"]/g,
    tag =>
      ({
        '&': '&amp;',
        '<': '&lt;',
        '>': '&gt;',
        "'": '&#39;',
        '"': '&quot;'
      }[tag] || tag)
  );

Examples

escapeHTML('<a href="#">Me & you</a>'); // '&lt;a href=&quot;#&quot;&gt;Me &amp; you&lt;/a&gt;'

⬆ Back to top

escapeRegExp

Escapes a string to use in a regular expression.

Use String.prototype.replace() to escape special characters.

const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');

Examples

escapeRegExp('(test)'); // \\(test\\)

⬆ Back to top

fromCamelCase

Converts a string from camelcase.

Use String.prototype.replace() to remove underscores, hyphens, and spaces and convert words to camelcase. Omit the second argument to use a default separator of _.

const fromCamelCase = (str, separator = '_') =>
  str
    .replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
    .replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2')
    .toLowerCase();

Examples

fromCamelCase('someDatabaseFieldName', ' '); // 'some database field name'
fromCamelCase('someLabelThatNeedsToBeCamelized', '-'); // 'some-label-that-needs-to-be-camelized'
fromCamelCase('someJavascriptProperty', '_'); // 'some_javascript_property'

⬆ Back to top

indentString

Indents each line in the provided string.

Use String.replace and a regular expression to add the character specified by indent count times at the start of each line. Omit the third parameter, indent, to use a default indentation character of ' '.

const indentString = (str, count, indent = ' ') => str.replace(/^/gm, indent.repeat(count));

Examples

indentString('Lorem\nIpsum', 2); // '  Lorem\n  Ipsum'
indentString('Lorem\nIpsum', 2, '_'); // '__Lorem\n__Ipsum'

⬆ Back to top

isAbsoluteURL

Returns true if the given string is an absolute URL, false otherwise.

Use a regular expression to test if the string is an absolute URL.

const isAbsoluteURL = str => /^[a-z][a-z0-9+.-]*:/.test(str);

Examples

isAbsoluteURL('https://google.com'); // true
isAbsoluteURL('ftp://www.myserver.net'); // true
isAbsoluteURL('/foo/bar'); // false

⬆ Back to top

isAnagram

Checks if a string is an anagram of another string (case-insensitive, ignores spaces, punctuation and special characters).

Use String.toLowerCase(), String.prototype.replace() with an appropriate regular expression to remove unnecessary characters, String.prototype.split(''), Array.prototype.sort() and Array.prototype.join('') on both strings to normalize them, then check if their normalized forms are equal.

const isAnagram = (str1, str2) => {
  const normalize = str =>
    str
      .toLowerCase()
      .replace(/[^a-z0-9]/gi, '')
      .split('')
      .sort()
      .join('');
  return normalize(str1) === normalize(str2);
};

Examples

isAnagram('iceman', 'cinema'); // true

⬆ Back to top

isLowerCase

Checks if a string is lower case.

Convert the given string to lower case, using String.toLowerCase() and compare it to the original.

const isLowerCase = str => str === str.toLowerCase();

Examples

isLowerCase('abc'); // true
isLowerCase('a3@$'); // true
isLowerCase('Ab4'); // false

⬆ Back to top

isUpperCase

Checks if a string is upper case.

Convert the given string to upper case, using String.prototype.toUpperCase() and compare it to the original.

const isUpperCase = str => str === str.toUpperCase();

Examples

isUpperCase('ABC'); // true
isLowerCase('A3@$'); // true
isLowerCase('aB4'); // false

⬆ Back to top

mapString

Creates a new string with the results of calling a provided function on every character in the calling string.

Use String.prototype.split('') and Array.prototype.map() to call the provided function, fn, for each character in str. Use Array.prototype.join('') to recombine the array of characters into a string. The callback function, fn, takes three arguments (the current character, the index of the current character and the string mapString was called upon).

const mapString = (str, fn) =>
  str
    .split('')
    .map((c, i) => fn(c, i, str))
    .join('');

Examples

mapString('lorem ipsum', c => c.toUpperCase()); // 'LOREM IPSUM'

⬆ Back to top

mask

Replaces all but the last num of characters with the specified mask character.

Use String.prototype.slice() to grab the portion of the characters that will remain unmasked and use String.padStart() to fill the beginning of the string with the mask character up to the original length. Omit the second argument, num, to keep a default of 4 characters unmasked. If num is negative, the unmasked characters will be at the start of the string. Omit the third argument, mask, to use a default character of '*' for the mask.

const mask = (cc, num = 4, mask = '*') => `${cc}`.slice(-num).padStart(`${cc}`.length, mask);

Examples

mask(1234567890); // '******7890'
mask(1234567890, 3); // '*******890'
mask(1234567890, -4, '$'); // '$$$$567890'

⬆ Back to top

pad

Pads a string on both sides with the specified character, if it's shorter than the specified length.

Use String.padStart() and String.padEnd() to pad both sides of the given string. Omit the third argument, char, to use the whitespace character as the default padding character.

const pad = (str, length, char = ' ') =>
  str.padStart((str.length + length) / 2, char).padEnd(length, char);

Examples

pad('cat', 8); // '  cat   '
pad(String(42), 6, '0'); // '004200'
pad('foobar', 3); // 'foobar'

⬆ Back to top

palindrome

Returns true if the given string is a palindrome, false otherwise.

Convert string String.toLowerCase() and use String.prototype.replace() to remove non-alphanumeric characters from it. Then, use the spread operator (...) to split string into individual characters, Array.prototype.reverse(), String.prototype.join('') and compare to the original, unreversed string, after converting it String.tolowerCase().

const palindrome = str => {
  const s = str.toLowerCase().replace(/[\W_]/g, '');
  return s === [...s].reverse().join('');
};

Examples

palindrome('taco cat'); // true

⬆ Back to top

pluralize

Returns the singular or plural form of the word based on the input number. If the first argument is an object, it will use a closure by returning a function that can auto-pluralize words that don't simply end in s if the supplied dictionary contains the word.

If num is either -1 or 1, return the singular form of the word. If num is any other number, return the plural form. Omit the third argument to use the default of the singular word + s, or supply a custom pluralized word when necessary. If the first argument is an object, utilize a closure by returning a function which can use the supplied dictionary to resolve the correct plural form of the word.

const pluralize = (val, word, plural = word + 's') => {
  const _pluralize = (num, word, plural = word + 's') =>
    [1, -1].includes(Number(num)) ? word : plural;
  if (typeof val === 'object') return (num, word) => _pluralize(num, word, val[word]);
  return _pluralize(val, word, plural);
};

Examples

pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'

const PLURALS = {
  person: 'people',
  radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'

⬆ Back to top

removeNonASCII

Removes non-printable ASCII characters.

Use a regular expression to remove non-printable ASCII characters.

const removeNonASCII = str => str.replace(/[^\x20-\x7E]/g, '');

Examples

removeNonASCII('äÄçÇéÉêlorem-ipsumöÖÐþúÚ'); // 'lorem-ipsum'

⬆ Back to top

reverseString

Reverses a string.

Use the spread operator (...) and Array.prototype.reverse() to reverse the order of the characters in the string. Combine characters to get a string using String.prototype.join('').

const reverseString = str => [...str].reverse().join('');

Examples

reverseString('foobar'); // 'raboof'

⬆ Back to top

sortCharactersInString

Alphabetically sorts the characters in a string.

Use the spread operator (...), Array.prototype.sort() and String.localeCompare() to sort the characters in str, recombine using String.prototype.join('').

const sortCharactersInString = str => [...str].sort((a, b) => a.localeCompare(b)).join('');

Examples

sortCharactersInString('cabbage'); // 'aabbceg'

⬆ Back to top

splitLines

Splits a multiline string into an array of lines.

Use String.prototype.split() and a regular expression to match line breaks and create an array.

const splitLines = str => str.split(/\r?\n/);

Examples

splitLines('This\nis a\nmultiline\nstring.\n'); // ['This', 'is a', 'multiline', 'string.' , '']

⬆ Back to top

stringPermutations

⚠️ WARNING: This function's execution time increases exponentially with each character. Anything more than 8 to 10 characters will cause your browser to hang as it tries to solve all the different combinations.

Generates all permutations of a string (contains duplicates).

Use recursion. For each letter in the given string, create all the partial permutations for the rest of its letters. Use Array.prototype.map() to combine the letter with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array. Base cases are for string length equal to 2 or 1.

const stringPermutations = str => {
  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
  return str
    .split('')
    .reduce(
      (acc, letter, i) =>
        acc.concat(stringPermutations(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)),
      []
    );
};

Examples

stringPermutations('abc'); // ['abc','acb','bac','bca','cab','cba']

⬆ Back to top

stripHTMLTags

Removes HTML/XML tags from string.

Use a regular expression to remove HTML/XML tags from a string.

const stripHTMLTags = str => str.replace(/<[^>]*>/g, '');

Examples

stripHTMLTags('<p><em>lorem</em> <strong>ipsum</strong></p>'); // 'lorem ipsum'

⬆ Back to top

toCamelCase

Converts a string to camelcase.

Break the string into words and combine them capitalizing the first letter of each word, using a regexp.

const toCamelCase = str => {
  let s =
    str &&
    str
      .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
      .map(x => x.slice(0, 1).toUpperCase() + x.slice(1).toLowerCase())
      .join('');
  return s.slice(0, 1).toLowerCase() + s.slice(1);
};

Examples

toCamelCase('some_database_field_name'); // 'someDatabaseFieldName'
toCamelCase('Some label that needs to be camelized'); // 'someLabelThatNeedsToBeCamelized'
toCamelCase('some-javascript-property'); // 'someJavascriptProperty'
toCamelCase('some-mixed_string with spaces_underscores-and-hyphens'); // 'someMixedStringWithSpacesUnderscoresAndHyphens'

⬆ Back to top

toKebabCase

Converts a string to kebab case.

Break the string into words and combine them adding - as a separator, using a regexp.

const toKebabCase = str =>
  str &&
  str
    .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
    .map(x => x.toLowerCase())
    .join('-');

Examples

toKebabCase('camelCase'); // 'camel-case'
toKebabCase('some text'); // 'some-text'
toKebabCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some-mixed-string-with-spaces-underscores-and-hyphens'
toKebabCase('AllThe-small Things'); // "all-the-small-things"
toKebabCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i-am-listening-to-fm-while-loading-different-url-on-my-browser-and-also-editing-xml-and-html"

⬆ Back to top

toSnakeCase

Converts a string to snake case.

Break the string into words and combine them adding _ as a separator, using a regexp.

const toSnakeCase = str =>
  str &&
  str
    .match(/[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+/g)
    .map(x => x.toLowerCase())
    .join('_');

Examples

toSnakeCase('camelCase'); // 'camel_case'
toSnakeCase('some text'); // 'some_text'
toSnakeCase('some-mixed_string With spaces_underscores-and-hyphens'); // 'some_mixed_string_with_spaces_underscores_and_hyphens'
toSnakeCase('AllThe-small Things'); // "all_the_smal_things"
toSnakeCase('IAmListeningToFMWhileLoadingDifferentURLOnMyBrowserAndAlsoEditingSomeXMLAndHTML'); // "i_am_listening_to_fm_while_loading_different_url_on_my_browser_and_also_editing_some_xml_and_html"

⬆ Back to top

truncateString

Truncates a string up to a specified length.

Determine if the string's length is greater than num. Return the string truncated to the desired length, with '...' appended to the end or the original string.

const truncateString = (str, num) =>
  str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;

Examples

truncateString('boomerang', 7); // 'boom...'

⬆ Back to top

unescapeHTML

Unescapes escaped HTML characters.

Use String.prototype.replace() with a regex that matches the characters that need to be unescaped, using a callback function to replace each escaped character instance with its associated unescaped character using a dictionary (object).

const unescapeHTML = str =>
  str.replace(
    /&amp;|&lt;|&gt;|&#39;|&quot;/g,
    tag =>
      ({
        '&amp;': '&',
        '&lt;': '<',
        '&gt;': '>',
        '&#39;': "'",
        '&quot;': '"'
      }[tag] || tag)
  );

Examples

unescapeHTML('&lt;a href=&quot;#&quot;&gt;Me &amp; you&lt;/a&gt;'); // '<a href="#">Me & you</a>'

⬆ Back to top

URLJoin

Joins all given URL segments together, then normalizes the resulting URL.

Use String.prototype.join('/') to combine URL segments, then a series of String.prototype.replace() calls with various regexps to normalize the resulting URL (remove double slashes, add proper slashes for protocol, remove slashes before parameters, combine parameters with '&' and normalize first parameter delimiter).

const URLJoin = (...args) =>
  args
    .join('/')
    .replace(/[\/]+/g, '/')
    .replace(/^(.+):\//, '$1://')
    .replace(/^file:/, 'file:/')
    .replace(/\/(\?|&|#[^!])/g, '$1')
    .replace(/\?/g, '&')
    .replace('&', '?');

Examples

URLJoin('http://www.google.com', 'a', '/b/cd', '?foo=123', '?bar=foo'); // 'http://www.google.com/a/b/cd?foo=123&bar=foo'

⬆ Back to top

words

Converts a given string into an array of words.

Use String.prototype.split() with a supplied pattern (defaults to non-alpha as a regexp) to convert to an array of strings. Use Array.prototype.filter() to remove any empty strings. Omit the second argument to use the default regexp.

const words = (str, pattern = /[^a-zA-Z-]+/) => str.split(pattern).filter(Boolean);

Examples

words('I love javaScript!!'); // ["I", "love", "javaScript"]
words('python, javaScript & coffee'); // ["python", "javaScript", "coffee"]

⬆ Back to top

📃 Type

getType

Returns the native type of a value.

Returns lowercased constructor name of value, "undefined" or "null" if value is undefined or null.

const getType = v =>
  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();

Examples

getType(new Set([1, 2, 3])); // 'set'

⬆ Back to top

is

Checks if the provided value is of the specified type.

Ensure the value is not undefined or null using Array.prototype.includes(), and compare the constructor property on the value with type to check if the provided value is of the specified type.

const is = (type, val) => ![, null].includes(val) && val.constructor === type;

Examples

is(Array, [1]); // true
is(ArrayBuffer, new ArrayBuffer()); // true
is(Map, new Map()); // true
is(RegExp, /./g); // true
is(Set, new Set()); // true
is(WeakMap, new WeakMap()); // true
is(WeakSet, new WeakSet()); // true
is(String, ''); // true
is(String, new String('')); // true
is(Number, 1); // true
is(Number, new Number(1)); // true
is(Boolean, true); // true
is(Boolean, new Boolean(true)); // true

⬆ Back to top

isArrayLike

Checks if the provided argument is array-like (i.e. is iterable).

Check if the provided argument is not null and that its Symbol.iterator property is a function.

const isArrayLike = obj => obj != null && typeof obj[Symbol.iterator] === 'function';

Examples

isArrayLike(document.querySelectorAll('.className')); // true
isArrayLike('abc'); // true
isArrayLike(null); // false

⬆ Back to top

isBoolean

Checks if the given argument is a native boolean element.

Use typeof to check if a value is classified as a boolean primitive.

const isBoolean = val => typeof val === 'boolean';

Examples

isBoolean(null); // false
isBoolean(false); // true

⬆ Back to top

isEmpty

Returns true if the a value is an empty object, collection, map or set, has no enumerable properties or is any type that is not considered a collection.

Check if the provided value is null or if its length is equal to 0.

const isEmpty = val => val == null || !(Object.keys(val) || val).length;

Examples

isEmpty(new Map()); // true
isEmpty(new Set()); // true
isEmpty([]); // true
isEmpty({}); // true
isEmpty(''); // true
isEmpty([1, 2]); // false
isEmpty({ a: 1, b: 2 }); // false
isEmpty('text'); // false
isEmpty(123); // true - type is not considered a collection
isEmpty(true); // true - type is not considered a collection

⬆ Back to top

isFunction

Checks if the given argument is a function.

Use typeof to check if a value is classified as a function primitive.

const isFunction = val => typeof val === 'function';

Examples

isFunction('x'); // false
isFunction(x => x); // true

⬆ Back to top

isNil

Returns true if the specified value is null or undefined, false otherwise.

Use the strict equality operator to check if the value and of val are equal to null or undefined.

const isNil = val => val === undefined || val === null;

Examples

isNil(null); // true
isNil(undefined); // true

⬆ Back to top

isNull

Returns true if the specified value is null, false otherwise.

Use the strict equality operator to check if the value and of val are equal to null.

const isNull = val => val === null;

Examples

isNull(null); // true

⬆ Back to top

isNumber

Checks if the given argument is a number.

Use typeof to check if a value is classified as a number primitive.

const isNumber = val => typeof val === 'number';

Examples

isNumber('1'); // false
isNumber(1); // true

⬆ Back to top

isObject

Returns a boolean determining if the passed value is an object or not.

Uses the Object constructor to create an object wrapper for the given value. If the value is null or undefined, create and return an empty object. Οtherwise, return an object of a type that corresponds to the given value.

const isObject = obj => obj === Object(obj);

Examples

isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false

⬆ Back to top

isObjectLike

Checks if a value is object-like.

Check if the provided value is not null and its typeof is equal to 'object'.

const isObjectLike = val => val !== null && typeof val === 'object';

Examples

isObjectLike({}); // true
isObjectLike([1, 2, 3]); // true
isObjectLike(x => x); // false
isObjectLike(null); // false

⬆ Back to top

isPlainObject

Checks if the provided value is an object created by the Object constructor.

Check if the provided value is truthy, use typeof to check if it is an object and Object.constructor to make sure the constructor is equal to Object.

const isPlainObject = val => !!val && typeof val === 'object' && val.constructor === Object;

Examples

isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false

⬆ Back to top

isPrimitive

Returns a boolean determining if the passed value is primitive or not.

Use Array.prototype.includes() on an array of type strings which are not primitive, supplying the type using typeof. Since typeof null evaluates to 'object', it needs to be directly compared.

const isPrimitive = val => !['object', 'function'].includes(typeof val) || val === null;

Examples

isPrimitive(null); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false

⬆ Back to top

isPromiseLike

Returns true if an object looks like a Promise, false otherwise.

Check if the object is not null, its typeof matches either object or function and if it has a .then property, which is also a function.

const isPromiseLike = obj =>
  obj !== null &&
  (typeof obj === 'object' || typeof obj === 'function') &&
  typeof obj.then === 'function';

Examples

isPromiseLike({
  then: function() {
    return '';
  }
}); // true
isPromiseLike(null); // false
isPromiseLike({}); // false

⬆ Back to top

isString

Checks if the given argument is a string. Only works for string primitives.

Use typeof to check if a value is classified as a string primitive.

const isString = val => typeof val === 'string';

Examples

isString('10'); // true

⬆ Back to top

isSymbol

Checks if the given argument is a symbol.

Use typeof to check if a value is classified as a symbol primitive.

const isSymbol = val => typeof val === 'symbol';

Examples

isSymbol(Symbol('x')); // true

⬆ Back to top

isUndefined

Returns true if the specified value is undefined, false otherwise.

Use the strict equality operator to check if the value and of val are equal to undefined.

const isUndefined = val => val === undefined;

Examples

isUndefined(undefined); // true

⬆ Back to top

isValidJSON

Checks if the provided argument is a valid JSON.

Use JSON.parse() and a try... catch block to check if the provided argument is a valid JSON.

const isValidJSON = obj => {
  try {
    JSON.parse(obj);
    return true;
  } catch (e) {
    return false;
  }
};

Examples

isValidJSON('{"name":"Adam","age":20}'); // true
isValidJSON('{"name":"Adam",age:"20"}'); // false
isValidJSON(null); // true

⬆ Back to top

🔧 Utility

castArray

Casts the provided value as an array if it's not one.

Use Array.prototype.isArray() to determine if val is an array and return it as-is or encapsulated in an array accordingly.

const castArray = val => (Array.isArray(val) ? val : [val]);

Examples

castArray('foo'); // ['foo']
castArray([1]); // [1]

⬆ Back to top

cloneRegExp

Clones a regular expression.

Use new RegExp(), RegExp.source and RegExp.flags to clone the given regular expression.

const cloneRegExp = regExp => new RegExp(regExp.source, regExp.flags);

Examples

const regExp = /lorem ipsum/gi;
const regExp2 = cloneRegExp(regExp); // /lorem ipsum/gi

⬆ Back to top

coalesce

Returns the first non-null/undefined argument.

Use Array.prototype.find() to return the first non null/undefined argument.

const coalesce = (...args) => args.find(_ => ![undefined, null].includes(_));

Examples

coalesce(null, undefined, '', NaN, 'Waldo'); // ""

⬆ Back to top

coalesceFactory

Returns a customized coalesce function that returns the first argument that returns true from the provided argument validation function.

Use Array.prototype.find() to return the first argument that returns true from the provided argument validation function.

const coalesceFactory = valid => (...args) => args.find(valid);

Examples

const customCoalesce = coalesceFactory(_ => ![null, undefined, '', NaN].includes(_));
customCoalesce(undefined, null, NaN, '', 'Waldo'); // "Waldo"

⬆ Back to top

extendHex

Extends a 3-digit color code to a 6-digit color code.

Use Array.prototype.map(), String.prototype.split() and Array.prototype.join() to join the mapped array for converting a 3-digit RGB notated hexadecimal color-code to the 6-digit form. Array.prototype.slice() is used to remove # from string start since it's added once.

const extendHex = shortHex =>
  '#' +
  shortHex
    .slice(shortHex.startsWith('#') ? 1 : 0)
    .split('')
    .map(x => x + x)
    .join('');

Examples

extendHex('#03f'); // '#0033ff'
extendHex('05a'); // '#0055aa'

⬆ Back to top

getURLParameters

Returns an object containing the parameters of the current URL.

Use String.match() with an appropriate regular expression to get all key-value pairs, Array.prototype.reduce() to map and combine them into a single object. Pass location.search as the argument to apply to the current url.

const getURLParameters = url =>
  (url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
    (a, v) => ((a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a),
    {}
  );

Examples

getURLParameters('http://url.com/page?name=Adam&surname=Smith'); // {name: 'Adam', surname: 'Smith'}
getURLParameters('google.com'); // {}

⬆ Back to top

hexToRGB

Converts a color code to a rgb() or rgba() string if alpha value is provided.

Use bitwise right-shift operator and mask bits with & (and) operator to convert a hexadecimal color code (with or without prefixed with #) to a string with the RGB values. If it's 3-digit color code, first convert to 6-digit version. If an alpha value is provided alongside 6-digit hex, give rgba() string in return.

const hexToRGB = hex => {
  let alpha = false,
    h = hex.slice(hex.startsWith('#') ? 1 : 0);
  if (h.length === 3) h = [...h].map(x => x + x).join('');
  else if (h.length === 8) alpha = true;
  h = parseInt(h, 16);
  return (
    'rgb' +
    (alpha ? 'a' : '') +
    '(' +
    (h >>> (alpha ? 24 : 16)) +
    ', ' +
    ((h & (alpha ? 0x00ff0000 : 0x00ff00)) >>> (alpha ? 16 : 8)) +
    ', ' +
    ((h & (alpha ? 0x0000ff00 : 0x0000ff)) >>> (alpha ? 8 : 0)) +
    (alpha ? `, ${h & 0x000000ff}` : '') +
    ')'
  );
};

Examples

hexToRGB('#27ae60ff'); // 'rgba(39, 174, 96, 255)'
hexToRGB('27ae60'); // 'rgb(39, 174, 96)'
hexToRGB('#fff'); // 'rgb(255, 255, 255)'

⬆ Back to top

httpGet

Makes a GET request to the passed URL.

Use XMLHttpRequest web api to make a get request to the given url. Handle the onload event, by calling the given callback the responseText. Handle the onerror event, by running the provided err function. Omit the third argument, err, to log errors to the console's error stream by default.

const httpGet = (url, callback, err = console.error) => {
  const request = new XMLHttpRequest();
  request.open('GET', url, true);
  request.onload = () => callback(request.responseText);
  request.onerror = () => err(request);
  request.send();
};

Examples

httpGet(
  'https://jsonplaceholder.typicode.com/posts/1',
  console.log
); /* 
Logs: {
  "userId": 1,
  "id": 1,
  "title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
  "body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
}
*/

⬆ Back to top

httpPost

Makes a POST request to the passed URL.

Use XMLHttpRequest web api to make a post request to the given url. Set the value of an HTTP request header with setRequestHeader method. Handle the onload event, by calling the given callback the responseText. Handle the onerror event, by running the provided err function. Omit the third argument, data, to send no data to the provided url. Omit the fourth argument, err, to log errors to the console's error stream by default.

const httpPost = (url, data, callback, err = console.error) => {
  const request = new XMLHttpRequest();
  request.open('POST', url, true);
  request.setRequestHeader('Content-type', 'application/json; charset=utf-8');
  request.onload = () => callback(request.responseText);
  request.onerror = () => err(request);
  request.send(data);
};

Examples

const newPost = {
  userId: 1,
  id: 1337,
  title: 'Foo',
  body: 'bar bar bar'
};
const data = JSON.stringify(newPost);
httpPost(
  'https://jsonplaceholder.typicode.com/posts',
  data,
  console.log
); /*
Logs: {
  "userId": 1,
  "id": 1337,
  "title": "Foo",
  "body": "bar bar bar"
}
*/
httpPost(
  'https://jsonplaceholder.typicode.com/posts',
  null, //does not send a body
  console.log
); /*
Logs: {
  "id": 101
}
*/

⬆ Back to top

isBrowser

Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.

Use Array.prototype.includes() on the typeof values of both window and document (globals usually only available in a browser environment unless they were explicitly defined), which will return true if one of them is undefined. typeof allows globals to be checked for existence without throwing a ReferenceError. If both of them are not undefined, then the current environment is assumed to be a browser.

const isBrowser = () => ![typeof window, typeof document].includes('undefined');

Examples

isBrowser(); // true (browser)
isBrowser(); // false (Node)

⬆ Back to top

mostPerformant

Returns the index of the function in an array of functions which executed the fastest.

Use Array.prototype.map() to generate an array where each value is the total time taken to execute the function after iterations times. Use the difference in performance.now() values before and after to get the total time in milliseconds to a high degree of accuracy. Use Math.min() to find the minimum execution time, and return the index of that shortest time which corresponds to the index of the most performant function. Omit the second argument, iterations, to use a default of 10,000 iterations. The more iterations, the more reliable the result but the longer it will take.

const mostPerformant = (fns, iterations = 10000) => {
  const times = fns.map(fn => {
    const before = performance.now();
    for (let i = 0; i < iterations; i++) fn();
    return performance.now() - before;
  });
  return times.indexOf(Math.min(...times));
};

Examples

mostPerformant([
  () => {
    // Loops through the entire array before returning `false`
    [1, 2, 3, 4, 5, 6, 7, 8, 9, '10'].every(el => typeof el === 'number');
  },
  () => {
    // Only needs to reach index `1` before returning false
    [1, '2', 3, 4, 5, 6, 7, 8, 9, 10].every(el => typeof el === 'number');
  }
]); // 1

⬆ Back to top

nthArg

Creates a function that gets the argument at index n. If n is negative, the nth argument from the end is returned.

Use Array.prototype.slice() to get the desired argument at index n.

const nthArg = n => (...args) => args.slice(n)[0];

Examples

const third = nthArg(2);
third(1, 2, 3); // 3
third(1, 2); // undefined
const last = nthArg(-1);
last(1, 2, 3, 4, 5); // 5

⬆ Back to top

parseCookie

Parse an HTTP Cookie header string and return an object of all cookie name-value pairs.

Use String.prototype.split(';') to separate key-value pairs from each other. Use Array.prototype.map() and String.prototype.split('=') to separate keys from values in each pair. Use Array.prototype.reduce() and decodeURIComponent() to create an object with all key-value pairs.

const parseCookie = str =>
  str
    .split(';')
    .map(v => v.split('='))
    .reduce((acc, v) => {
      acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
      return acc;
    }, {});

Examples

parseCookie('foo=bar; equation=E%3Dmc%5E2'); // { foo: 'bar', equation: 'E=mc^2' }

⬆ Back to top

prettyBytes

Converts a number in bytes to a human-readable string.

Use an array dictionary of units to be accessed based on the exponent. Use Number.toPrecision() to truncate the number to a certain number of digits. Return the prettified string by building it up, taking into account the supplied options and whether it is negative or not. Omit the second argument, precision, to use a default precision of 3 digits. Omit the third argument, addSpace, to add space between the number and unit by default.

const prettyBytes = (num, precision = 3, addSpace = true) => {
  const UNITS = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
  if (Math.abs(num) < 1) return num + (addSpace ? ' ' : '') + UNITS[0];
  const exponent = Math.min(Math.floor(Math.log10(num < 0 ? -num : num) / 3), UNITS.length - 1);
  const n = Number(((num < 0 ? -num : num) / 1000 ** exponent).toPrecision(precision));
  return (num < 0 ? '-' : '') + n + (addSpace ? ' ' : '') + UNITS[exponent];
};

Examples

prettyBytes(1000); // "1 KB"
prettyBytes(-27145424323.5821, 5); // "-27.145 GB"
prettyBytes(123456789, 3, false); // "123MB"

⬆ Back to top

randomHexColorCode

Generates a random hexadecimal color code.

Use Math.random to generate a random 24-bit(6x4bits) hexadecimal number. Use bit shifting and then convert it to an hexadecimal String using toString(16).

const randomHexColorCode = () => {
  let n = (Math.random() * 0xfffff * 1000000).toString(16);
  return '#' + n.slice(0, 6);
};

Examples

randomHexColorCode(); // "#e34155"

⬆ Back to top

RGBToHex

Converts the values of RGB components to a color code.

Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (<<) and toString(16), then String.padStart(6,'0') to get a 6-digit hexadecimal value.

const RGBToHex = (r, g, b) => ((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');

Examples

RGBToHex(255, 165, 1); // 'ffa501'

⬆ Back to top

serializeCookie

Serialize a cookie name-value pair into a Set-Cookie header string.

Use template literals and encodeURIComponent() to create the appropriate string.

const serializeCookie = (name, val) => `${encodeURIComponent(name)}=${encodeURIComponent(val)}`;

Examples

serializeCookie('foo', 'bar'); // 'foo=bar'

⬆ Back to top

timeTaken

Measures the time taken by a function to execute.

Use console.time() and console.timeEnd() to measure the difference between the start and end times to determine how long the callback took to execute.

const timeTaken = callback => {
  console.time('timeTaken');
  const r = callback();
  console.timeEnd('timeTaken');
  return r;
};

Examples

timeTaken(() => Math.pow(2, 10)); // 1024, (logged): timeTaken: 0.02099609375ms

⬆ Back to top

toCurrency

Take a number and return specified currency formatting.

Use Intl.NumberFormat to enable country / currency sensitive formatting.

const toCurrency = (n, curr, LanguageFormat = undefined) =>
  Intl.NumberFormat(LanguageFormat, { style: 'currency', currency: curr }).format(n);

Examples

toCurrency(123456.789, 'EUR'); // €123,456.79  | currency: Euro | currencyLangFormat: Local
toCurrency(123456.789, 'USD', 'en-us'); // $123,456.79  | currency: US Dollar | currencyLangFormat: English (United States)
toCurrency(123456.789, 'USD', 'fa'); // ۱۲۳٬۴۵۶٫۷۹ ؜$ | currency: US Dollar | currencyLangFormat: Farsi
toCurrency(322342436423.2435, 'JPY'); // ¥322,342,436,423 | currency: Japanese Yen | currencyLangFormat: Local
toCurrency(322342436423.2435, 'JPY', 'fi'); // 322 342 436 423 ¥ | currency: Japanese Yen | currencyLangFormat: Finnish

⬆ Back to top

toDecimalMark

Use toLocaleString() to convert a float-point arithmetic to the Decimal mark form. It makes a comma separated string from a number.

const toDecimalMark = num => num.toLocaleString('en-US');

Examples

toDecimalMark(12305030388.9087); // "12,305,030,388.909"

⬆ Back to top

toOrdinalSuffix

Adds an ordinal suffix to a number.

Use the modulo operator (%) to find values of single and tens digits. Find which ordinal pattern digits match. If digit is found in teens pattern, use teens ordinal.

const toOrdinalSuffix = num => {
  const int = parseInt(num),
    digits = [int % 10, int % 100],
    ordinals = ['st', 'nd', 'rd', 'th'],
    oPattern = [1, 2, 3, 4],
    tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
  return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
    ? int + ordinals[digits[0] - 1]
    : int + ordinals[3];
};

Examples

toOrdinalSuffix('123'); // "123rd"

⬆ Back to top

validateNumber

Returns true if the given value is a number, false otherwise.

Use !isNaN() in combination with parseFloat() to check if the argument is a number. Use isFinite() to check if the number is finite. Use Number() to check if the coercion holds.

const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;

Examples

validateNumber('10'); // true

⬆ Back to top

yesNo

Returns true if the string is y/yes or false if the string is n/no.

Use RegExp.test() to check if the string evaluates to y/yes or n/no. Omit the second argument, def to set the default answer as no.

const yesNo = (val, def = false) =>
  /^(y|yes)$/i.test(val) ? true : /^(n|no)$/i.test(val) ? false : def;

Examples

yesNo('Y'); // true
yesNo('yes'); // true
yesNo('No'); // false
yesNo('Foo', true); // true

⬆ Back to top

Collaborators

_{Angelos Chalaris}	_{Felix Wu}	_{Stefan Feješ}	_{King David Martins}	_{Soorena Soleimani}
_{Elder Henrique Souza}	_{Robert Mennell}	_atomiks

Credits

Icons made by Smashicons from www.flaticon.com is licensed by CC 3.0 BY.

License

Ma2iio/30-seconds-of-code

About

Resources

License

Code of conduct

Stars

Watchers

Forks

Releases

Packages 0

Languages

Packages