JavaScript Types

Primitive types:

• number
• bool
• string
• undefined - is an absence of definition.
• null
• Symbol (ES6)

Non-primitive types:

• {}
• []
• function

typeof

The typeof operator returns string of type. The typeof null returns 'object' This is an actual error, which exists for years.

Arrays and Functions are Objects actually.

typeof [] returns 'object'. typeof function() {} returns 'function'.

There are standard build-in objects, like NaN, Infinity, Date etc. But there are the following types: Boolean, Number etc. When we use something like this true.toString(), JS create some kind of wrapper: Boolean(true).toString().

Arrays

Arrays are object. When we have something like this: var a = [1, 2, 3], we actually have:

var a = {
  0: 1,
  1: 2, 
  2: 3
}

That’s why typeof [] is equal to 'object'.

Pass by Value || Reference

Pass by value:

var a = 10;
var b = a;
a = 5;
console.log(b); // returns 10

Clone objects

let obj = { a: 'a', b: 'b' };
let clonedObj = Object.assign({}, obj);

Another way is a using of spread operator:

let obj = { a: 'a', b: 'b' };
let clonedObj = { ...obj };

Type coercion

1 == '1' => true

Object.is(-0, +0) returns false.