What is the difference between var, let, and const?

Question

Explain the differences between var, let, and const in JavaScript. When would you use each one?

ModernJS · Accepted Answer

## var
- **Function scoped** (or global if declared outside)
- **Hoisted** and initialized as `undefined`
- Can be redeclared

```javascript
console.log(x); // undefined (hoisted)
var x = 5;
var x = 10; // OK - redeclaration allowed
```

## let
- **Block scoped** (inside `{}`)
- **Hoisted** but NOT initialized (temporal dead zone)
- Cannot be redeclared in same scope

```javascript
console.log(y); // ReferenceError!
let y = 5;
let y = 10; // SyntaxError!
```

## const
- Same as `let`, but **cannot be reassigned**
- Must be initialized at declaration
- Object properties CAN still be modified

```javascript
const z = 5;
z = 10; // TypeError!

const obj = { a: 1 };
obj.a = 2; // OK - modifying property
obj = {}; // TypeError - reassigning
```

## Best Practice
Use `const` by default, `let` when you need to reassign, avoid `var`.

1 Answer