let and const

Introduction

In this subunit, we'll dive deep into the let and const, new ES2015 keywords used to declare variables. They both introduce a new kind of scope into JavaScript, and these keywords set rules on whether we can reassign or redeclare variables. This subunit will teach you all you need to know about what makes these keywords so powerful and what sets them apart.

Reviewing var

var

We use the var keyword to declare variables
When defined in a function, the var keyword scopes a variable to that function
var will hoist to the top of the scope it is defined in
You can redeclare and reassign values with var

var instructor = 'Colt'; // accessible everywhereI
function greet() {
  var message = 'Hello!'; // scoped to the greet function
}

let

The let keyword creates a block-scoped variable: a variable that only exists inside a code block.

What Is A Code Block

Essentially any pair of curly braces (outside of object syntax).

{
  // this is a code block
  let x = 5;
  var y = 10;
}

console.log(x);
// ReferenceError: x is not defined
console.log(y);
// 10

Where Are Code Blocks Commonly Used

You’ll mostly use code blocks in for loops and if statements.

if (x > 10) {
  let happy = true; // happy lives ONLY in this code block
}

// can't use it outside the block
console.log(happy); // ReferenceError: happy is not defined

An Example

for (var i = 1; i < 4; i++) {
  console.log(i);
}
// 1
// 2
// 3
console.log(i);
// 4

for (let i = 1; i < 4; i++) { console.log(i);
// 1
// 3
// 3
console.log(i); // ReferenceError: i is not defined

More About let

It can be reassigned but not redeclared (unlike var).

let z = 5;
z = 25;
let z = 10;
// SyntaxError: Identifier rzr has already been declared

const

The const keyword prevents a variable from ever being reassigned or redeclared.

const PI = 3.14;
PI = 15; // TypeError: Assignment to constant variable
const PI = 5; // SyntaxError

const is also block-scoped, like let.

{
  const x = 10;
}
console.log(x); // ReferenceError: x is not defined

Comparison of Variable Declaration Keywords

Keyword	Can Reassign	Can Redeclare	Can Mutate	Scope Rules
var	yes	yes	yes	function scope
let	yes	no	yes	block scope
const	no	no	yes	block scope

What about var

There's really no need to use it
Just be careful of block scoping with let

Last modified: 10 March 2024