But, what is the difference between var, let, and const? 🤔

Some interviews in the UK asked about the difference between var, let, and const in JavaScript. And I decided to share what I think about them and what itself with few words, this way I fixed the conception in my mind as well. :)

Photo credit: https://www.flickr.com/photos/ramireziblog/48462879966

Var

In this code JavaScript, var is the older option for declaration news variable for JS. It is a lot used before the specification 2015 (ES6). We can declare a variable globally, or locally. For both cases, it is a consideration of the block scope.

redeclaration or update

We can declare the variable and redeclaration that in the same or another scope.

var test = "hello world zero"
var test = "hello world one"

or just

test = "hello world three"

var hoisting

Initialization of the var is optional, but if the var was not the initial value, for default value is undefined before the code run, and it call

According to Mabishi Waki in

Understanding Hoisting in JavaScript | DigitalOcean

Hoisting is

a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution.

The sample below is possible, without error, because the hoisting mechanism.

console.log (test);
var test = “hello world”
// undefined

Let

It was to add on specification ECMAScript 2015 — ES6. introduced two new keywords let and const for declaring variables.

redeclaration or update

The declaration is limited to the scope of a block statement, such as a function. We can not do the declaration twice, if you do that, will happen an error.

let x = 1; let x = 2 
// Uncaught SyntaxError: Identifier ‘x’ has already been declared

But we can update the variable as many times as you want.

let x = 1; x = 2; x = 3; x = 4;

The world let came from Mathematics, this word is a lot of in math prepositions, like at the beginning of the Theorem.

var hoisting

It is not applying. if you do that, will happen an error as well.

console.log (test);
let test = “hello world”
// Uncaught ReferenceError: test is not defined

Const

It cannot be redeclared or updated. It needs to be initialized, if it is not initialized we will have a compile error.

const SHOULD_BE_INITIALIZED = "Hello World";

reinitialized

const TEST; TEST = "Hello World"
// Uncaught SyntaxError: Missing initializer in const declaration

redeclaration

const TEST = "Hello World"; TEST="Hello World 2"
// Uncaught SyntaxError: Identifier ‘test’ has already been declared

References:

ES6 In Depth: let and const - Mozilla Hacks - the Web developer blog

let

Var, Let, and Const - What's the Difference?