JavaScript Cheatsheet

Last updated 07/14/2016

Primitives

Primitives are the JavaScript building blocks.

• Boolean - true or false
• Numbers - integers or floats
• Strings - text
• Undefined - value has not been defined
• Null - intentional absence of an object value
• Symbol (es6)

Variables

Container to store data (values) Shortcuts for storing data

• Declared using var
• Naming - can't start with a number, can't have spaces, avoid symbols except _, $, -, the name should be descriptive, should be camelCase

Data Structures

• Arrays: container for primitives (and other data structures) accessed via index, ordered
• Objects: key/value pairs, accessed via bracket or dot notation, not ordered

Programming Paradigms

Procedural vs functional

• Procedural: JS renders commands in ordered
• Functional: break apart code/problem logically, generally separate state from behavior, reusable

Flow Control

Flow control - decides which things run based on specific conditions (order) or loops

Conditionals control the flow of the program

• if, if-else if-else
• can be nested

remember to fail fast!

Switch statements:

• We haven't covered these yet, but they can be used to execute multiple blocks of code.

Loops

• you can keep running a piece of code for a certain amount of iterations

• infinite loop - loop that runs forever, stack overflow, no more memory :(

• for

• while

• for in

• for of

• do while

Operators

Math - +, -, *, /, %, can combine (++, +=)

Logical - &&, ||

Comparison - === (strict), == (loose), >, !, <, >=, <=

functions

Set of instructions that achieve a specific task defined by code block. Can be defined and then invoked to return some output, can contain parameters (when defined) and arguments (passed in when called/invoked)

• methods vs functions: method is a function that is contained within an object
• functions are first class citizens and can be used as arguments (just like primitives and data structures) in JS
• can have optional
• functions always return something; if not explicitly set, then it returns undefined
• expression vs declaration

variable scope

• range in which a variable can be accessed
• global and local (function/lexical)
• global - accesible everywhere; local - accessible only when the function is invoked

function foo() {
    var test = "some string"
    function bar() {
      console.log(test);
    }
  }

hoisting

The process by which the computer moves all variable declarations to the top of the applicable scope, so that it never encounters a variable it is unaware of. It's important to note that it does not move the variable assignment to the top of the page.

var foo = "bar"

moves declarations (var: foo) but NOT assignment (var foo = bar)

callbacks/higher order functions

function can take functions; the function that's passed into the HOF is the callback

asynchronous - different code can run at different times

function testing() {
  return ing
}

function test() {
  return "test" + arg
}

var x = testing()
test(x)

string concatenation

Used to join 2 or more strings

"test" + "ing"

"test".concat("ing") // => "testing"

you can also use +=

computer science

BigO notation: how developers discuss the complexity of an algorithm as a way to understand how fast a program will run given it's input. Big-O notation deals with the worst case scenario for the algorithm. In other words, if the program may run quickly, but there is a chance it could take a long time given some input, then the Big-O runtime will deal with the longer case

• O(n) - Linear Time
  • Directly related to input size (longer input, longer runtime)
• O(1) - Constant Time
  • Bound by a constant number. Regardless of input size, the runtime stays the statements
• O(n^2) - Quadratic Time
  • A loop that iterates over all items nested within a loop that also iterates over all items

The exponent grows based on the number of nested loops.

A "flogarithm" of a positive number (n) represented by (log(n)) is the number of digits it has.

Factorials - Way worse than quadratix.

Big O Cheatsheet