JS Vocab Cheatsheet


Named places to store data for later. var x = 2;

Data Types

Different formats for storing data.


Includes any kind of number (Integers and Decimals).

Also includes negative numbers and zero.

Also, Infinity and -Infinity for the adventuresome.


Words (collections of individual characters).

To create strings surround stuff in quotes (single'Stuff', double "Stuff", or back ticks `Stuff`)


Can be either true or false statements.

null and all it's friends

Different states of nothingness.

A slight change from just false.

  • null - Empty Value
  • undefined - Variable has not been defined
  • NaN - Not a Number (when JS goes "WAT" when trying to do mathy things...)

More about undefined

Ways to get undefined:

  • A variable that is never set.
  • Asking for object properties that have not been assigned.
  • Asking for array positions that have not been assigned.
  • Function that has an argument that is not passed in.
  • When a function doesn't have a return statement.


Set of code to be run later (usually multiple times).

Configurable via arguments.

Can return a value after doing work.

var a = function() {
  // Code to run goes here

function b() {
  // Code to run goes here

var c = () => {
  // Code to run goes here


Ordered set of values (usually should be similar in format). Created with []. Commas separate values: ['a', 'x']. Start counting positions at 0. Get positions with [position] so: ['a', 'x'][0] will give you 'a'.


Collections of values, referenced by "property" names. Created with {}. Each property has a "key" and a "value" separated by a colon: { firstName: 'Jon' }. Commas separate properties: { firstName: 'Jon', lastName: 'Snow' }. Get properties with either .propertyName or ['propertyName']: so { firstName: 'Jon', lastName: 'Snow' }.firstName or { firstName: 'Jon', lastName: 'Snow' }['firstName'] returns 'Jon'

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