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A collection of F(unctional) Util(ities). Resistance is futile.

Mostly, these are generic utilities that could conceivably be part of a library like lodash/fp, but for one reason or another are not.


Version History/Changelog

See our changelog


npm i -S futil or npm i -S futil-js

This package requires lodash/fp, so make sure that's available in your app.


import * as F from 'futil' or import F from 'futil' or import {x,y,z} from 'futil'




(fn, a, b) -> fn(a, b) If fn is a function, call the function with the passed-in arguments. Otherwise, return false.


(fn, a, b) -> fn(a, b) If fn is a function, call the function with the passed-in arguments. Otherwise, return fn.


(a, Monoid f) -> f[a] :: f a Binds a function of an object to it's object.


(f, [g1, g2,]) -> a -> f([g1(a), g2(a), ...]) Note that f is called on the array of the return values of [g1, g2,] rather than applied to it.

comply (alias: composeApply)

(f, g) -> x -> f(g(x))(x) A combinator that combines compose and apply. f should be a 2 place curried function. Useful for applying comparisons to pairs defined by some one place function, e.g. var isShorterThanFather = F.comply(isTallerThan, fatherOf)


Implement defer, ported from bluebird docs and used by debounceAsync


A _.debounce for async functions that ensure the returned promise is resolved with the result of the execution of the actual call. Using _.debounce with await or .then would result in the earlier calls never returning because they're not executed - the unit tests demonstate it failing with _.debounce.


(f1, f2, ...fn) -> f1Arg1 -> f1Arg2 -> ...f1ArgN -> fn(f2(f1)) Flurry is combo of flow + curry, preserving the arity of the initial function. See lodash/lodash#3612.


(mapper, fn) -> (...args) -> fn( Returns a function that applies the mapping operation to all of the arguments of a function. Very similar to _.overArgs, but runs a single mapper on all of the args args.



handleItem -> handleLastItem -> iterator Creates an iterator that handles the last item differently for use in any function that passes (value, index, list) (e.g. mapIndexed, eachIndexed, etc). Both the two handlers and the result are iterator functions that take (value, index, list).



([f1, f2, ...fn]) -> !f1(x) && !f2(x) && ...!fn(x) Creates a function that checks if none of the array of predicates passed in returns truthy for x


(condition, onTrue, onFalse, x) -> (T(condition)(x) ? onTrue(x) : onFalse(x)) The transform function T supports passing a boolean for condition as well as any valid argument of _.iteratee, e.g. myBool = applyTest(x); F.ifElse(myBool, doSomething, doSomethingElse);


(condition, onTrue, x) -> (T(condition)(x) ? onTrue(x) : _.identity(x)) T extends _.iteratee as above.


(condition, onFalse, x) -> (T(condition)(x) ? _.identity(x) : onFalse(x)) T extends _.iteratee as above.


when curried with Boolean


when curried with exists



[f1, f2, ...fn] -> Maps a flow of f1, f2, ...fn over a collection.


f -> x -> f(find(f, x)) A version of find that also applies the predicate function to the result. Useful when you have an existing function that you want to apply to a member of a collection that you can best find by applying the same function.


(a -> b) -> [a] -> [b] Maps a function over an iterable. Works by default for Arrays and Plain Objects.


(a -> b) -> [a] -> [b] Maps a function over a recursive iterable. Works by default for nested Arrays, nested Plain Objects and mixed nested Arrays and Plain Objects. Also works for any other iterable data type as long as two other values are sent: a mapping function, and a type checker (See the unit tests for deepMap).


(index, val, array|string) -> array|string Inserts value into an array or string at index


(1, '123', 'hi') -> 'h123i'


(fn, collection) -> collection Maps fn over the input collection and compacts the result.


lodash/fp is great, but sometimes the curry order isn't exactly what you want.

These methods provide alternative orderings that are sometimes more convenient.

The idea of In methods is to name them by convention, so when ever you need a method that actually takes the collection first (e.g. a get where the data is static but the field is dynamic), you can just add In to the end (such as getIn which takes the object first)


Just like _.get, but with {rearg: false} so the argument order is unchanged from non fp lodash.


Just like _.has, but with {rearg: false} so the argument order is unchanged from non fp lodash.


Just like _.pick, but with {rearg: false} so the argument order is unchanged from non fp lodash.


Just like _.includes, but with {rearg: false} so the argument order is unchanged from non fp lodash.


lodash/fp likes to keep things pure, but sometimes JS can get pretty dirty.

These methods are alternatives for working with data that--for whatever the use case is--needs to be mutable

Any methods that interact with mutable data will use the On convention (as it is some action occuring On some data)


Just like _.extend, but with {mutable: true} so it mutates.


Just like _.defaults, but with {mutable: true} so it mutates.


Just like _.merge, but with {mutable: true} so it mutates.


Just like _.set, but with {mutable: true} so it mutates.


Just like _.unset, but with {mutable: true} so it mutates.


Just like _.pull, but with {mutable: true} so it mutates.


Just like _.update, but with {mutable: true} so it mutates.


lodash/fp caps iteratees to one argument by default, but sometimes you need the index.

These methods are uncapped versions of lodash's methods.

Any method with uncapped iteratee arguments will use the Indexed convention.


Just like, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).


Just like _.find, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).


Just like _.each, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).


Just like _.reduce, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).


Just like _.pickBy, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).


Just like _.mapValues, but with {cap: false} so iteratees are not capped (e.g. indexes are passed).



joinString -> [string1, string2, ...stringN] -> string1 + joinString + string2 + joinString ... + stringN Joins an array after compacting. Note that due to the underlying behavior of _.curry no default join value is supported -- you must pass in some string with which to perform the join.


[string1, string2, ...stringN] -> string1 + '.' + string2 + '.' ... + stringN Compacts and joins an array with .


filterFunction -> [string1, string2, ...stringN] -> string1 + '.' + string2 + '.' ... + stringN Compacts an array by the provided function, then joins it with .


[a] -> [a] Returns an array of elements that are repeated in the array.


(val, array) -> array Return array with val pushed.


(from, to, array) -> array Moves a value from one index to another


([[], [], []]) -> [[], []] Takes any number of ranges and return the result of merging them all.


[[0,7], [3,9], [11,15]] -> [[0,9], [11,15]]


[a, b...] -> a -> b Creates a function that takes an element of the original array as argument and returns the next element in the array (with wrapping). Note that (1) This will return the first element of the array for any argument not in the array and (2) due to the behavior of _.curry the created function will return a function equivalent to itself if called with no argument.


(k, v, [a]) -> { k(a): v(a) } Creates an object from an array by generating a key/value pair for each element in the array using the key and value mapper functions.


A version of _.zipObjectDeep that supports passing a function to determine values intead of an array, which will be invoked for each key.


[a, b] -> {a:true, b:true} Converts an array of strings into an object mapping to true. Useful for optimizing includes.


['a', 'b', 'c'] -> [['a'], ['a', 'b'], ['a', 'b', 'c']] Returns a list of all prefixes. Works on strings, too. Implementations must guarantee that the orginal argument has a length property.


string -> {encode: array -> string, decode: string -> array} Creates an object with encode and decode functions for encoding arrays as strings. The input string is used as input for join/split.


{ encode: ['a', 'b'] -> 'a.b', decode: 'a.b' -> ['a', 'b'] } An encoder using . as the separator


{ encode: ['a', 'b'] -> 'a/b', decode: 'a/b' -> ['a', 'b'] } An encoder using / as the separator


(([a], a) -> Boolean) -> [a] -> [[a]] Takes a predicate function and an array, and returns an array of arrays where each element has one or more elements of the original array. Similar to Haskell's groupBy.

The predicate is called with two arguments: the current group, and the current element. If it returns truthy, the element is appended to the current group; otherwise, it's used as the first element in a new group.


bool -> value -> list -> newList Just like toggleElement, but takes an iteratee to determine if it should remove or add. This is useful for example in situations where you might have a checkbox that you want to represent membership of a value in a set instead of an implicit toggle. Used by includeLens.


(any, array) -> array Removes an element from an array if it's included in the array, or pushes it in if it doesn't. Immutable (so it's a clone of the array).


f -> array -> [array[0], f(), array[n], ....) Puts the result of calling f in between each element of the array. f is a standard lodash iterator taking the value, index, and list. If f is not a function, it will treat f as the value to intersperse. See


// Example with words (toSentence is basically this flowed into a `_.join('')`):
    () => "or",
    () => "or perhaps"
  ["first", "second", "third"]
// ['first', 'or', 'second', 'or perhaps', 'third']

// Example with React and JSX:
let results = [1, 2, 3]
return (
    <br />
    {_.flow( => <b>{x}</b>),
          () => ", ",
          () => " and "
// Output:
// **Results:**
// **1**, **2** and **3**.

Note: This works great with the differentLast iterator. Also, intersperse can be used with JSX components!


(fn(array_element), value, array) -> array Replaces an element in an array with value based on the boolean result of a function fn.


(target, value, array) -> array Replaces all elements equal to target in an array with value.



(k, v) -> {k: v} Creates an object with a key and value.


(v, k) -> {k: v} Flipped version of singleObject.


({a, b}) -> [{a}, {b}] Breaks an object into an array of objects with one key each.


Remove properties with falsey values.


({ a: 1, b: null, c: false }) -> {a:1}


Check if the variable is an empty object ({}).


Check if the variable is not an empty object ({}).


Omit properties whose values are empty objects.


{ a:1, b:{}, c:2 } -> {a:1, c:2}

Note: (TODO rename to omitEmptyObjects)




sourcePropertyName -> targetPropertyName -> sourceObject -> sourceObject Rename a property on an object.


renameProperty('a', 'b', { a: 1 }) -> { b: 1 }


k -> { k: [a, b] } -> [{ k: a }, { k: b }] Just like mongo's $unwind: produces an array of objects from an object and one of its array-valued properties. Each object is constructed from the original object with the array value replaced by its elements. Unwinding on a nonexistent property or a property whose value is not an array returns an empty array.


F.unwind("b", [{ a: true, b: [1, 2] }])
//=> [{ a: true, b: 1 }, { a: true, b: 2 }]


k -> [{ k: [a, b] }] -> [{ k: a }, { k: b }] Unwinds an array of objects instead of a single object, as you might expect if you're used to mongo's $unwind. Alias for (key, data) => _.flatMap(F.unwind(key), data)


F.unwindArray("b", [
  { a: true, b: [1, 2] },
  { a: false, b: [3, 4] },
//=> [
//=>  { a: true, b: 1 },
//=>  { a: true, b: 2 },
//=>  { a: false, b: 3 },
//=>  { a: false, b: 4 },
//=> ]


Flatten an object with the paths for keys.


{ a: { b: { c: 1 } } } => { 'a.b.c' : 1 }


Unlatten an object with the paths for keys.


{ 'a.b.c' : 1 } => { a: { b: { c: 1 } } }


Returns true if object keys are only elements from signature list. (but does not require all signature keys to be present)


Similar to _.matches, except it returns true if 1 or more object properties match instead of all of them. See lodash/lodash#3713.


Checks if an object's property is equal to a value.


Deprecated in favor of lodash update Applies a map function at a specific path


mapProp(double, 'a', {a: 2, b: 1}) -> {a: 4, b: 1}


_.get that returns the target object if lookup fails


_.get that returns the prop if lookup fails


Flipped alias


A _.get that takes an array of paths (or functions to return values) and returns the value at the first path that matches. Similar to _.overSome, but returns the first result that matches instead of just truthy (and supports a default value)


Flipped cascade


A _.get that takes an array of paths and returns the first path that matched


A _.get that takes an array of paths and returns the first path that exists


A _.get that takes an array of paths and returns the first value that has an existing path


newKey -> {a:x, b:y} -> [{...x, newKey: a}, {...y, newKey: b}] Opposite of _.keyBy. Creates an array from an object where the key is merged into the values keyed by newKey.


F.unkeyBy('_key')({ a: { status: true}, b: { status: false }) -> [{ status: true, _key: 'a' }, { status: false, _key: 'b' }]

Note: Passing a falsy value other than undefined for newKay will result in each object key being pushed into its corresponding return array member with itself as value, e.g. F.unkeyBy('')({ a: { status: true}, b: { status: false }) -> [{ status: true, a: 'a' }, { status: false, b: 'b' }]. Passing undefined will return another instance of F.unkeyBy.


(from, to) -> simpleDiff Produces a simple flattened (see flattenObject) diff between two objects. For each (flattened) key, it produced a from and a to value. Note that this will omit any values that are not present in the deltas object.


(from, to) -> [simpleDiffChanges] Same as simpleDiff, but produces an array of { field, from, to } objects instead of { field: { from, to } }


(from, to) -> diff Same as simpleDiff, but also takes in count deleted properties. Note: We're considering not maintaining this in the long term, so you might probably have more success with any existing library for this purpose.


(from, to) -> [diffChanges] Same as simpleDiffArray, but also takes in count deleted properties. Note: We're considering not maintaining this in the long term, so you might probably have more success with any existing library for this purpose.


A _.pick that mutates the object


Like _.mergeAll, but concats arrays instead of replacing. This is basically the example from the lodash mergeAllWith docs.


{ a: [x, y, z], b: [x] } -> { x: [a, b], y: [a], z: [a] } Similar to _.invert, but expands arrays instead of converting them to strings before making them keys.


key -> { a: { x: 1 }, b: { y: 2 } } -> { a: { x: 1, key: 'a' }, b: { y: 2, key: 'b' } } Iterates over object properties and stamps their keys on the values in the field name provided.


_.omitBy using _.isNil as function argument.


_.omitBy using _.isNull as function argument.


_.omitBy using F.isBlank as function argument.


_.omitBy using _.isEmpty as function argument.


([f, g], ...args) -> {...f(...args), ...g(...args)} Composition of _.over and _.mergeAll. Takes an array of functions and an arbitrary number of arguments, calls each function with those arguments, and merges the results. Can be called with mergeOverAll([f, g], x, y) or mergeOverAll([f, g])(x, y). Note: For functions that do not return objects, _.merge's behavior is followed: for strings and arrays, the indices will be converted to keys and the result will be merged, and for all other primitives, nothing will be merged.


(customizer, [f, g], ...args) -> {...f(...args), ...g(...args)} A customizable mergeOverAll that takes a function of the form (objValue, srcValue) -> newValue as its first argument; see _.mergeWith. Both the customizer and array of functions can be partially applied.


([f, g], ...args) -> {...f(...args), ...g(...args)} A customized mergeOverAll that applies the array-merging behavior of mergeAllArrays.


(x -> y) -> k -> {k: x} -> y Like _.get, but accepts a customizer function which is called on the value to transform it before it is returned. Argument order is (customizer, path, object).


(transform: obj -> newObj) -> obj -> { ...obj, ...newObj } Accepts a transform function and an object. Returns the result of applying the transform function to the object, merged onto the original object. expandObject(f, obj) is equivalent to mergeOverAll([_.identity, f], obj).


key -> (transform: x -> newObj) -> (obj: { key: x }) -> { ...obj, ...newObj } Expands an object by transforming the value at a single key into a new object, and merging the result with the original object. Similar to expandObject, but the argument order is (key, transform, object), and the transform function is called on the value at that key instead of on the whole object.


(x, y) -> [keys] Takes two objects and returns the keys they have in common


(x, y) -> key Takes two objects and returns the first key in y that x also has



'asdf' -> '(asdf)' Wraps a string in parenthesis.


Maps _.trim through all the strings of a given object or array.


string -> string Converts strings like variable names to labels (generally) suitable for GUIs, including support for acronyms and numbers. It's basically _.startCase with acronym and number support.


string -> {value:string, label:string} Creates a {value, label} which applies autoLabel the string parameter on puts it on the label property, with the original on the value property. You can also pass in an object with value or with both value and label.


[string] -> [{value:string, label:string}] Applies autoLabelOption to a collection. Useful for working with option lists like generating select tag options from an array of strings.


(separator, lastSeparator, array) => string Just like toSentence, but with the ability to override the separator and lastSeparator


(' - ', ' or ', ['a', 'b', 'c']) -> 'a - b or c'


array => string Joins an array into a human readable string. See


['a', 'b', 'c'] -> 'a, b and c'


(fn, array) -> string -> string Allows passing a "cachizer" function (array -> object) to override the way uniqueString's initial array is converted into a cache object. Can be curried to create a custom uniqueString function, eg: let myUniqueString = uniqueStringWith(myFunc)

Like uniqueString, the resulting deduplication function exposes cache and clear() properties.


let uniqueStringStripDigits = uniqueStringWith(
  _.countBy(_.replace(/(\d+)$/, ""))
let dedupe = uniqueStringStripDigits(["foo", "foo42", "foo3000"])
dedupe("foo") //-> 'foo3'
uniqueStringWith(_.identity, dedupe.cache)("foo") //-> 'foo4'


array -> string -> string Returns a function that takes a string and de-duplicates it against an internal cache. Each time this function is called, the resulting deduplicated string is added to the cache. Exposes cache and clear() properties to read and clear the cache, respectively.


let dedupe = uniqueString(), ["foo", "foo", "foo"]) //-> ['foo', 'foo1', 'foo2']
dedupe.cache //-> { foo: 3, foo1: 1, foo2: 1 }
dedupe.cache //-> {}
dedupe("foo") //-> 'foo'



regex -> string -> bool Just like ramda test, creates a function to test a regex on a string.


options:string -> string -> regex A curried implementation of RegExp construction.


options:string -> string -> (string -> bool) Makes and tests a RegExp with makeRegex and testRegex.


string -> string -> bool Returns true if the second string matches all of the words in the first string.


string -> string -> bool Returns true if the second string matches any of the words in the first string.


regex -> string -> [{text: string, start: number, end: number}] Returns an array of matches with start/end data


F.allMatches(/a/g, 'vuhfaof') -> [ { text: 'a', start: 4, end: 5 } ]


regex -> string -> [[number, number]] Returns an array of postings (position ranges) for a regex and string to test, e.g. F.postings(/a/g, 'vuhfaof') -> [[4, 5]]


words -> string -> [[[number, number]]] Takes a string of words and a string to test, and returns an array of arrays of postings for each word.


F.postingsForWords("she lls", "she sells sea shells")
// [
//   [[0, 3], [14, 17]]
//   [[6, 9], [17, 20]]
// ]


start -> end -> pattern -> input -> highlightedInput Wraps the matches for pattern found in input with the strings start and end. The pattern argument can either be a string of words to match, or a regular expression.


let braceHighlight = F.highlight("{", "}")
braceHighlight("l o", "hello world") //-> "he{llo} w{o}r{l}d"
braceHighlight(/l+\w/, "hello world") //-> "he{llo} wor{ld}"



number -> bool Returns true if number is greater than one.


Language level utilities


x -> bool A utility that checks if the argument passed in is of type promise


Just throws whatever it is passed.


Tap error will run the provided function and then throw the first argument. It's like _.tap for rethrowing errors.

exists (alias: isNotNil)

Negated _.isNil


(Array<T> | string | {length}) -> bool Returns true if the input has a length property > 1, such as arrays, strings, or custom objects with a lenth property


(a, b) => b + a A curried, flipped _.add. The flipping matters for strings, e.g. F.append('a')('b') -> 'ba'


x -> bool Designed to determine if something has a meaningful value, like a ux version of truthiness. It's false for everything except null, undefined, '', [], and {}. Another way of describing it is that it's the same as falsiness except 0 and false are truthy and {} is falsey. Useful for implementing "required" validation rules.


x -> bool Opposite of isBlank


f -> x -> bool Recurses through an object's leaf properties and passes an array of booleans to the combinator, such as _.some, _.every, and F.none


A lens is a getter and setter pair. You use them to write code that needs to read and write a value (like a method to flip a boolean switch, or a React component that reads and writes some state) without worrying about the implementation.

Functions that operate on lenses can handle a few different "shorthand" structures. This is similar to lodash's _.iteratee (which allows their methods to treat strings, objects, or functions as shorthand predicates)

A lens can be any of these formats:

({ get, set }) An object with a get function and set function. Found in: MobX "boxed observables" Example Usage: F.flip({ get, set })

([value, setter]) An array of the value and a setter function to change it. Found in: React's useState hook Example Usage: F.flip([value, setter])

(lookup, object) A lookup path and object pair e.g. ('key', object). The lookup path is anything you can pass to _.get (so nested paths with . or as an array are supported) Found in: MobX observable objects, native JS objects Example Usage: F.flip(lookup, object)

(x => {}) A function which returns the value when called with no arguments and sets it when called with one. Found in: Knockout observables, jQuery plugin APIs Example Usage: F.flip(x => {})

(getter, setter) A getter and setter pair. Found in: Anywhere you have a getter and setter function Example Usage: F.flip(getter, setter)

Note: Setter methods are generally mutable (unlike Ramda's lenses, for example).

We've included a few example "bindings" on F.domLens. These take a lens and return an object that's useful in a DOM context (like React or raw JS). In React terms, they're methods that generate the props you'd use to do two way binding to a lens. lens meme


Takes a value and returns a function lens for that value. Mostly used for testing and mocking purposes.


Takes a value and returns a object lens for that value. Mostly used for testing and mocking purposes.


propertyValue -> Lens -> object.propertyName Sets the value of the lens, regardless of its format


Converts a function lens an object lens. Mostly used for testing and mocking purposes.


Converts an object lens to a function lens. Mostly used for testing and mocking purposes.


propertyName -> object -> { get: () -> object.propertyName, set: propertyValue -> object.propertyName } Creates an object lens for a given property on an object. .get returns the value at that path and set places a new value at that path. Supports deep paths like lodash get/set. You typically won't use this directly since it is supported implicitly.


Takes an object and returns an object with lenses at the values of each path. Basically mapValues(lensProp). Typically you would use the implicit (key, object) format instead.


value -> arrayLens -> includeLens An include lens represents membership of a value in a set. It takes a value and lens and returns a new lens - kind of like a "writeable computed" from MobX or Knockout. The view and set functions allow you to read and write a boolean value for whether or not a value is in an array. If you change to true or false, it will set the underlying array lens with a new array either without the value or with it pushed at the end.


Lens -> object.propertyName Gets the value of the lens, regardless of its format


Lens -> (() -> object.propertyName) Returns a function that gets the value of the lens, regardless of its format


Creates a function that will set a lens with the provided value


Takes an iteratee and lens and creates a function that will set a lens with the result of calling the iteratee with the provided value


Takes a lens and negates its value


Returns a function that will set a lens to true


Returns a function that will set a lens to false


lens -> {value, onChange} Takes a lens and returns a value/onChange pair that views/sets the lens appropriately. onChange sets with (or e if that path isn't present).


let Component = () => {
  let state = React.useState('')
  return <input {...F.domLens.value(state)}>


(value, lens) -> {checked, onChange} Creates an includeLens and maps view to checked and set to onChange (set with or e if that path isn't present)


lens -> { onMouseEnter, onMouseLeave } Takes a lens and returns on onMouseEnter which calls on on the lens and onMouseLeave which calls off. Models a mapping of "hovering" to a boolean.


lens -> { onFocus, onBlur } Takes a lens and returns on onFocus which calls on on the lens and onBlur which calls off. Models a mapping of "focusing" to a boolean.


field -> lens -> {[field], onChange} Utility for building lens consumers like value and checkboxValues


(field, getValue) -> lens -> {[field], onChange} Even more generic utility than targetBinding which uses getEventValue to as the function for a setsWith which is mapped to onChange.


([value, setValue]) -> lens Given the popularity of React, we decided to include this little helper that converts a useState hook call to a lens. Ex: let lens = stateLens(useState(false)). You generally won't use this directly since you can pass the [value, setter] pair directly to lens functions


Aspects provide a functional oriented implementation of Aspect Oriented Programming. An aspect wraps a function and allows you run code at various points like before and after execution. Notably, aspects in this library allow you to have a shared state object between aspects and are very useful for automating things like status indicators, etc on functions.

There is a lot of prior art in the javascript world, but most of it assumes a vaguely object oriented context. The implementation in futil-js is done in just 20 lines of code and seems to capture all of the use cases of AOP.

Note: To do OO style AOP with this these aspects, just use lodash's _.update method and optionally boundMethod from futil if this matters

Caveat: While you can and should compose (or _.flow) aspects together, don't put non aspects in the middle of the composition. Aspects rely on a .state property on the wrapped function that they propagate through, but the chain will break if a non-aspect is mixed in between. Additionally, if you need external access to the state, make sure the aspects are the outer most part of the composition so the .state property will be available on the result of the composition.

There are a few basic aspects included on F.aspects (E.g. var loggedFunc = F.aspect(F.aspects.logs)(func)) because they seem to be universally useful. All of the provided aspects take an extend function to allow customizing the state mutation method (e.g. in mobx, you'd use extendObservable). If null, they default to defaultsOn from futil-js - check the unit tests for example usage.


{options} -> f -> aspectWrapped(f) The aspect api takes an options object and returns a function which takes a function to wrap. The wrapped function will be decorated with a state object and is equivalent to the original function for all arguments.

Options supports the following parameters:

Name Description
init: (state) -> () A function for setting any inital state requirements. Should mutate the shared state object.
after: (result, state, params) -> () Runs after the wrapped function executes and recieves the shared state and the result of the function. Can be async.
before: (params, state) -> () Runs before the wrapped function executes and receves the shared state and the params passed to the wrapped function. Can be async.
onError: (error, state, params) -> () Runs if the wrapped function throws an error. If you don't throw inside this, it will swallow any errors that happen.
always: (state, params) -> () Runs after the wrapped function whether it throws an error or not, similar to a Promise.catch


let exampleAspect = aspect({
  before: () => console.log("pre run"),
  after: () => console.log("post run"),
let f = () => console.log("run")
let wrapped = exampleAspect(f)
// Logs to the console:
// pre run
// run
// post run


This is a synchronous version of aspect, for situations when it's not desirable to await a method you're adding aspects to. The API is the same, but things like onError won't work if you pass an async function to the aspect.


Logs adds a logs array to the function state and just pushes in results on each run


Captures any exceptions thrown and set it on an error error it puts on state


Captures any exceptions thrown and pushes them sequentially into an errors array it puts on state


Adds a status property that is set to processing before the wrapped function runs and succeeded when it's done or failed if it threw an exception. Also adds shortcuts on state for processing, succeeded, and failed, which are booleans which are based on the value of status. Also adds a setStatus method which is used internally to update these properties.


Utility for marking functions as deprecated - it's just a before with a console.warn. Takes the name of thing being deprecated, optionally deprecation version, and optionally an alternative and returns a higher order function which you can wrap deprecated methods in. This is what's used internally to mark deprecations. Includes a partial stack trace as part of the deprecation warning.


Sets status to null after provided timeout (default is 500ms) elapses. If a null timeout is passed, it will never set status to null.


Prevents a function from running if it's state has processing set to true at the time of invocation


Flows together status, clearStatus, concurrency, and error, taking extend and timeout as optional parameters to construct the aspect


All tree functions take a traversal function so that you can customize how to traverse arbitrary nested structures.

Note: Be careful about cyclic structures that can result in infinite loops, such as objects with references to itself. There are cases where you'd intentionally want to visit the same node multiple times, such as traversing a directed acyclic graph (which would work just fine and eventually terminate, but would visit a node once for each parent it has connected to it) - but it's up to the user to be sure you don't create infinite loops.


node -> bool A default check if something can be traversed - currently it is arrays and plain objects.


node -> [...childNodes] The default traversal function used in other tree methods if you don't supply one. It returns false if it's not traversable or empty, and returns the object if it is.


traverse -> (pre, post=_.noop) -> tree -> x A depth first search which visits every node returned by traverse recursively. Both pre-order and post-order traversals are supported (and can be mixed freely). walk also supports exiting iteration early by returning a truthy value from either the pre or post functions. The returned value is also the return value of walk. The pre, post, and traversal functions are passed the current node as well as the parent stack (where parents[0] is the direct parent).


traverse -> (pre, post=_.noop) -> async tree -> x A version of walk which supports async traversals.


traverse -> _iteratee -> tree -> newTree Structure preserving pre-order depth first traversal which clones, mutates, and then returns a tree. Basically walk with a _.cloneDeep first (similar to a tree map because it preserves structure). _iteratee can be any suitable argument to _.iteratee


traverse -> (accumulator, initialValue, tree) -> x Just like _.reduce, but traverses over the tree with the traversal function in pre-order.


(traverse, writeNode) -> f -> tree -> newTree Structure preserving tree map! writeNode informs how to write a single node, but the default will generally work for most cases. The iteratee is passed the standard node, index, parents, parentIndexes args and is expected to return a transformed node.


(traverse, writeNode) -> f -> tree -> newTree Like mapTree, but only operates on lead nodes. It is a convenience method for mapTree(next, writeNode)(F.unless(next, mapper), tree)


traverse -> f -> tree -> [f(treeNode), f(treeNode), ...] Like treeToArray, but accepts a customizer to process the tree nodes before putting them in an array. The customizer is passed the standard node, index, parents, parentIndexes args and is expected to return a transformed node. It's for trees - but it's not called treeMap because it does not preserve the structure as you might expect map to do. See mapTree for that behavior.


traverse -> tree -> [treeNode, treeNode, ...] Flattens the tree nodes into an array, simply recording the node values in pre-order traversal.


traverse -> f -> tree -> [f(treeNode), f(treeNode), ...] Like leaves, but accepts a customizer to process the leaves before putting them in an array.


traverse -> tree -> [treeNodes] Returns an array of the tree nodes that can't be traversed into in pre-order.


(traverse, buildIteratee) -> ([_iteratee], tree) -> treeNode Looks up a node matching a path, which defaults to lodash iteratee but can be customized with buildIteratee. The _iteratee members of the array can be any suitable arguments for _.iteratee


traverse -> transformer -> _iteratee -> tree -> result Similar to a keyBy (aka groupBy) for trees, but also transforms the grouped values (instead of filtering out tree nodes). The transformer takes three args, the current node, a boolean of if the node matches the current group, and what group is being evaluated for this iteratee. The transformer is called on each node for each grouping. _iteratee is any suitable argument to _.iteratee, as above.


(x, i, xs, is) => [i,] A utility tree iteratee that returns the stack of node indexes


(x, i, xs) => [x, ...xs] A utility tree iteratee that returns the stack of node values


(build, encoder) -> treePathBuilderFunction Creates a path builder for use in flattenTree. By default, the builder will look use child indexes and a dotEncoder. Encoder can be an encoding function or a futil encoder (an object with encode and decode functions)


prop -> treePathBuilderFunction Creates a path builder for use in flattenTree, using a slashEncoder and using the specified prop function as an iteratee on each node to determine the keys.


traverse -> buildPath -> tree -> result Creates a flat object with a property for each node, using buildPath to determine the keys. buildPath takes the same arguments as a tree walking iteratee. It will default to a dot tree path.


(traverse, buildIteratee, writeNode) -> {walk, reduce, transform, toArray, toArrayBy, leaves, leavesBy, map, mapLeaves, lookup, keyByWith, traverse, flatten, flatLeaves } Takes a traversal function and returns an object with all of the tree methods pre-applied with the traversal. This is useful if you want to use a few of the tree methods with a custom traversal and can provides a slightly nicer api. Exposes provided traverse function as traverse