Variable

Declarations

Keyword Scope Reassignable Mutable Temporal Dead Zone
const Block No Yes* Yes
let Block Yes Yes Yes
var Function Yes Yes No

Other languages

C:

int number = 10;

Python:

number = 10

Keyword

var number = 10;
console.log(number); //=> 10

let number = 10;
console.log(number); //=> 10

const number = 10;
console.log(number); //=> 10

Reassignable

var number;
number = 10;
number = 100;
console.log(number); //=> 100

let number;
number = 10;
number = 100;
console.log(number); //=> 100

const number; // SyntaxError
const number = 10;
number = 100; // TypeError

const numbers = [];
numbers.push(1);
console.log(numbers); //=> [ 1 ]

Reference:

Automatic semicolon insertion (ASI)

const number = 10
console.log(number) //=> 10

Dynamic Type

let variable = 10;
console.log(variable); //=> 10
console.log(typeof variable); //=> number

variable = 'fulano';
console.log(variable); //=> 'fulano'
console.log(typeof variable); //=> string

Weak Typing

const x = 10;
const y = '5';
console.log(x - y); //=> 5

Case Sensitive

const number = 8;
const Number = 80;
const NUMBER = 800;

Identifier Pattern

const 0number  //=> SyntaxError (^\W)
const %number  //=> SyntaxError (^\W)
const var      //=> SyntaxError (keyword)
const function //=> SyntaxError (keyword)
const number
const _number
const $number

Reference:

Multiple Declaration

let x, y, z;
console.log(x); //=> undefined
console.log(y); //=> undefined
console.log(z); //=> undefined

const a = 1,
  b = 2,
  c = 3;
console.log(a); //=> 1
console.log(b); //=> 2
console.log(c); //=> 3

Hoisting

name = 'fulano';
console.log(name); //=> 'fulano'

console.log(name); //=> ReferenceError: name is not defined
name = 10;

console.log(name); //=> undefined
var name = 10;

console.log(name); //=> ReferenceError: name is not defined
let name = 10;

Reference: