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Let and Const

let: mutable variables, means we can reassign its value. let can be declared without being initialized.

const: immutable variables, means we cannot reassign value to const variables. const must be initialized during declaration.

Both of them are block-scope variables, which means it is only available within the block where the variable is declared.

Takeaways

It is recommended to use let and const than using var, because when we use let and const will be hoisted to the top but they are not initialized (unlike var that has been initialized with the value of undefined) and this helps us to get better error message. Furthermore, we can't iniatialized those variables before we declare them. This is useful to make sure that our variables has been declared before we can use them and this is a good practice in programming.

Code Examples:

// traditional
var x = 5;
y = 8;
var y;

console.log(x); // 5
console.log(y); // 8

// es6
let i = 10;
j = 15;
let j;
console.log(i); //10
console.log(j); // Error: Cannot access 'j' before initialization

const k = 29;
k = 39;
console.log(k); // Error: Assignment to constant variable.

let m;
m = 'hello';

const n;
n = 'goodbye';
console.log(m); // hello
console.log(n); // Error: Missing initializer in const declaration