Understanding the Differences Between JavaScript's var, let, and const

	var	let	const
global scoped	yes	no	no
function scoped	yes	yes	yes
block scoped	no	yes	yes
can reassignable	yes	yes	no
can be redeclarable	yes	no	no
can be hoisted	yes	no	no

Global Scoped :

Variables declared Globally (outside any function).

//var
a = 4;
var a;
console.log(a);

//let
a = 4;
let a;
console.log(a); // Uncaught ReferenceError: Cannot access 'a' before initialization

//const
a = 4;
const a;
console.log(a); // Uncaught SyntaxError: Missing initializer in const declaration

Function Scoped :

Variables defined inside a function are not accessible (visible) from outside the function.

//var
function myFunction() {
    var a = 10;
}
myFunction()
console.log(a);

//let
function myFunction() {
    let a = 10;
}
myFunction()
console.log(a);

//const
function myFunction() {
    const a = 10;
}
myFunction()
console.log(a);

Block Scoped :

This scope restricts the variable that is declared inside a specific block, from access by the outside of the block.

//var
function myFunction () {
    if(true) {
        var a = 10; // it exists in function scope
    }
    console.log(a);
}
myFunction();

//let
function myFunction () {
    if(true) {
        let a = 10; // it exists in block scope
    }
    console.log(a);
}
myFunction();

//const
function myFunction () {
    if(true) {
        const a = 10; // it exists in block scope
    }
    console.log(a);
}
myFunction();

Can be reassignable :

//var
var a = 1;
a = 30;
console.log(a); // 30

//let
let b = 1;
b = 30;
console.log(b); // 30

//const
const c = 1;
c = 30;
console.log(c); // Uncaught TypeError: Assignment to constant variable.

Can be redeclarable :

//var
var a = 10;
console.log(a); // 10
var a = 12;
console.log(a); // 12

//let
let b = 10;
console.log(b); // 10
let b = 12;
console.log(b); // Uncaught SyntaxError: Identifier 'b' has already been declared

//const
const c = 10;
console.log(c); // 10
const c = 12;
console.log(c); // Uncaught SyntaxError: Identifier 'c' has already been declared

Can be hoisted :

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

//let
console.log(b);
let b = 10; // Uncaught ReferenceError: b is not defined

//const
console.log(c);
const c = 10; // Uncaught ReferenceError: c is not defined