JavaScript FoundationsLesson 1.2

var vs let vs const — which one to use and when

var hoisting, let block scope, const immutability, temporal dead zone, variable declaration best practices, re-assignment vs re-declaration

Three Ways to Declare Variables

JavaScript has three variable declaration keywords. Choosing correctly prevents entire categories of bugs.

var — Avoid in Modern Code

var is function-scoped and hoisted to the top of its enclosing function. This means you can reference it before the line it is declared — it just evaluates to undefined.

console.log(x); // undefined — NOT an error
var x = 5;
console.log(x); // 5

let — Default for Mutable Values

let is block-scoped. It exists only inside the nearest {} block and cannot be accessed before declaration (temporal dead zone).

let count = 0;
count = count + 1; // re-assignment is fine
// let count = 1;  // SyntaxError: re-declaration not allowed

const — Default for Everything Else

const prevents re-assignment. It does not make objects or arrays immutable — it only locks the binding.

const PI = 3.14159;
// PI = 3; // TypeError

const user = { name: "Ana" };
user.name = "Bob"; // fine — object properties can change
// user = {};       // TypeError — binding is locked

Rule of Thumb

Default to const. Use let when you know the value will change. Never use var in new code.