Variables: let, const and var

JavaScript basics · lesson 2 of 10

In this lesson: Choose correctly between const and let, and avoid var.

A variable is a name for a value.

let age = 24;
const country = 'Rwanda';

const — the name cannot be reassigned. Use this by default.
let — use only when the value genuinely needs to change (a counter, a loop variable).
var — the old way. Its scoping rules cause bugs. Do not use it in new code.

const city = 'Kigali';
city = 'Huye';        // TypeError: Assignment to constant variable

let score = 0;
score = score + 10;   // fine

const does not mean frozen. It stops the name being pointed at something else. The contents of an object or array can still change:

const list = [1, 2];
list.push(3);      // fine — list is still the same array
list = [9];        // error — reassigning the name

let and const exist only inside the { } they were declared in:

if (true) {
  const secret = 'hidden';
  console.log(secret);   // 'hidden'
}
console.log(secret);     // ReferenceError — it does not exist here

This is exactly what you want. var ignores block scope and leaks out of the braces, which is why it causes trouble.

// Poor
let x = 34.99;
let d = true;

// Good
let priceInUsd = 34.99;
let isPublished = true;

You will read code far more often than you write it. Names are documentation.

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