What are the differences between var, let, and const in JavaScript?-JS Tutorial-php.cn

Table of Contents

2. Reassignment

3. Hoisting and Temporal Dead Zone (TDZ)

4. Window Object Attachment

Summary Table

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What are the differences between var, let, and const in JavaScript?

Margaret Anne Kelly

Aug 02, 2025 pm 01:30 PM

variable declaration

var is function-scoped, can be reassigned, hoisted with undefined, and attached to the global window object; 2. let and const are block-scoped, with let allowing reassignment and const not allowing it, though const objects can have mutable properties; 3. let and const are hoisted but not initialized, existing in the Temporal Dead Zone until declared, and are not added to the global window object; 4. Modern best practices recommend using const by default, let when reassignment is needed, and avoiding var due to its broader scope and hoisting behavior, making const > let > var in current standards.

What are the differences between var, let, and const in JavaScript?

The main differences between var, let, and const in JavaScript come down to scope, hoisting, and whether the variable can be reassigned. Here's a clear breakdown:

1. Scope

var: Function-scoped
Variables declared with var are accessible within the function they're defined in, or globally if outside any function.
```
function example() {
  if (true) {
    var x = 10;
  }
  console.log(x); // 10 — accessible outside the 'if' block
}
```
let and const: Block-scoped
These are only accessible within the block {} they're defined in (like inside an if, for, or any pair of braces).
```
function example() {
  if (true) {
    let y = 20;
  }
  console.log(y); // ReferenceError: y is not defined
}
```

2. Reassignment

var and let: Can be reassigned
You can change the value after declaration.
```
let name = "Alice";
name = "Bob"; // ✅ Allowed
```

const: Cannot be reassigned
Must be assigned at declaration, and the variable cannot point to a new value.

const age = 25;
age = 30; // ❌ TypeError: Assignment to constant variable

⚠️ Note: const doesn't make objects or arrays immutable — their contents can still change.

const user = { name: "Alice" };
user.name = "Bob"; // ✅ Allowed — object property changed
user = {}; // ❌ Not allowed — reassigning the variable

3. Hoisting and Temporal Dead Zone (TDZ)

var: Hoisted and initialized with undefined
You can access it before declaration (but it returns undefined).
```
console.log(value); // undefined, not an error
var value = 5;
```
let and const: Hoisted but not initialized
They exist in the "Temporal Dead Zone" before declaration — accessing them causes a ReferenceError.
```
console.log(count); // ReferenceError
let count = 1;
```
So you must declare them before use.

4. Window Object Attachment

var: Adds to the global window object (in browsers) when declared globally.
```
var apiKey = "123";
console.log(window.apiKey); // "123"
```

let and const: Do not add to the window object.

let apiKey = "123";
console.log(window.apiKey); // undefined

Summary Table

Feature	`var`	`let`	`const`
Scope	Function	Block	Block
Can reassign	Yes	Yes	No
Hoisted	Yes (as `undefined`)	Yes (TDZ)	Yes (TDZ)
Global to `window`	Yes	No	No
Must initialize	No	No	Yes

In modern JavaScript, use const by default, and let only when you know the variable will change. Avoid var unless working with legacy code. It's clearer and less error-prone.

Basically, const > let > var in today's best practices.

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