Let & Const

let vs const vs var

Modern JavaScript gives you three ways to declare variables. Each serves a different purpose. Getting this right means fewer bugs and cleaner code.

const — value cannot be reassigned. Use this by default.
let — can be reassigned. Use when you need to update the value.
var — old way. Avoid in modern code.

let vs const

let score = 10;
score = 20; // Works fine

const maxScore = 100;
maxScore = 200; // Error: Assignment to constant variable

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const Does Not Mean Immutable

const person = { name: "Alice" };
person.name = "Bob"; // Works — const prevents reassignment, not mutation
console.log(person);
// person = {}; // Error!

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Block Scope vs Function Scope

if (true) {
  var a = "visible everywhere";
  let b = "only in this block";
}
console.log(a); // Works
console.log(b); // ReferenceError

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Temporal Dead Zone (TDZ)

console.log(x); // undefined (var is hoisted)
var x = 5;

console.log(y); // ReferenceError: Cannot access before initialization
let y = 5;

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let and const exist in a "temporal dead zone" from the start of the block until they're declared.

🧪 Quick Quiz

What is the difference between let and const?

← Variables

Data Types →