Const VS Let VS Var

Background

• var happened first , let and const occurs in es6

• They are not types, but are keywords, since javaScript is weak and dynamic(check errors after running the program) typed

Const

• the value of variable cannot be changed

• cannot re-declared

• cannot be undefined

Let

• Cannot re-declared the variable

• Example:

let test = 2;
let test = 3; // result :failed

• block-scoped

• Example:

 function(){
   if(true){
     let i = 1;
    }
    console.log(i);// error
  }

Var

• Function-Scoped

• Can be re-declared

• Hoisting

• Example:

var test = 2;
var test = 3; // result : test will be 3

console.log(x);
// undefined
var x = 10;