3-types.md

Types

JavaScript is loosely typed (a dynamic language) and has seven data types.

typeof is safe to call on anything, including undefined variables, but it does not return the types you expect.

Boolean, Number, and String

const a = true	// boolean values are true or false
const b = 3.14	// number values are 64-bit floating point (IEEE 754)
const c = 'hi'	// string values are array-like lists of strings
'h'[0] === 'h'	// beware infinite string recursion

Special number NaN

Some mathematical operations will return the special number NaN. All comparisons with NaN are false because it is the impossible value, so use the global function isNaN() to test instead of equality.

const d = 1/0;
isNaN(d); // true

Keywords null and undefined

Unlike NaN which is evaluated at a hardware level because it's part of the floating point specification, null and undefined are references that you may more freely use in your code.

let e;    // e === undefined
e = null; // e === null

Objects

Everything else is an object, a very generic dictionary of string keys and any values.

const obj = {}; // new Object()
obj.one = 1;
obj['two'] = 2;
console.log(obj); // { "one": 1, "two": 2 }

Arrays

Arrays can be easily created with literals like [1, 2, 3]. They are dynamically sized, so you can add to or remove elements from them as much as you want after creating them. Arrays can contain mixed types so [true, 0, 'thing'] is valid.