var vs let vs const: When to Use Each in JavaScript

var vs let vs const: When to Use Each in JavaScript

JavaScript has three variable declarations. Each behaves differently. Here is how to choose.

Quick Comparison

When to Use const

Default to const. Most variables should not be reassigned:

const API_URL = "https://api.example.com";
const user = { name: "Kunal" };
const handleClick = () => { ... };
``
`const` signals intent: "this binding will not change." It prevents accidental reassignment bugs.

### When to Use `let`

Use `let` when you genuinely need to reassign:

```js
let count = 0;
count++;

let total = 0;
for (const item of items) {
  total += item.price;
}

let result;
if (condition) {
  result = computeA();
} else {
  result = computeB();
}
``
### When to Use `var`

Almost never in modern code. The only legitimate case is when you specifically need function scoping or hoisting, which is rare. In old codebases (pre-ES6), `var` is all you have.

### A Common Misconception About `const`

`const` does not mean "immutable value." It means "the binding cannot be reassigned." Objects and arrays declared with `const` can still be mutated:

```js
const list = [];
list.push(1); // OK (mutating the array)
list = [2];   // TypeError (reassigning the variable)
``
For immutability, use `Object.freeze` or libraries like Immutable.js.

### Loop Variables

```js
for (let i = 0; i < 3; i++) { ... } // i changes, use let
for (const item of items) { ... }   // item does not change, use const
for (const key in obj) { ... }      // key does not change, use const
``
Use `let` for the loop counter (it increments). Use `const` for `for...of` and `for...in` (each iteration is a fresh binding).

### The Takeaway

Use `const` by default (most variables should not be reassigned). Use `let` when you need to reassign. Avoid `var` in modern code. `const` does not freeze objects; it only prevents reassignment of the binding.