What is the difference between var, let and const?

Question

Accepted Answer

They differ in three ways: **scope**, **hoisting behavior**, and **reassignment**.

```js
function demo() {
  if (true) {
    var a = 1;    // function-scoped — leaks out of the if-block
    let b = 2;    // block-scoped — only exists inside { }
    const c = 3;  // block-scoped, can't be reassigned
  }
  console.log(a); // 1  (var escaped the block)
  console.log(b); // ReferenceError (let stayed in the block)
}
```

## The key differences

- **`var`** is **function-scoped**, hoisted and initialized to `undefined`, and can be redeclared. This causes surprising leaks — avoid it.
- **`let`** is **block-scoped** (`{ }`), reassignable, and lives in the *temporal dead zone* until its declaration line (accessing it early throws).
- **`const`** is block-scoped and **cannot be reassigned** — but note the binding is constant, *not* the value:

```js
const user = { name: "Ann" };
user.name = "Bob"; // ✅ allowed — mutating the object, not reassigning
user = {};          // ❌ TypeError — can't rebind `user`
```

## Rule of thumb

Default to **`const`**; switch to **`let`** only when you must reassign; **never use `var`** in new code. This makes intent clear ("this won't change") and avoids the scope leaks and hoisting confusion `var` brings.