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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.
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 withvarare 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 } letandconst: Block-scoped
These are only accessible within the block{}they're defined in (like inside anif,for, or any pair of braces).
function example() { if (true) { let y = 20; } console.log(y); // ReferenceError: y is not defined }
2. Reassignment
varandlet: 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:
constdoesn'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 withundefined
You can access it before declaration (but it returnsundefined).console.log(value); // undefined, not an error var value = 5;
letandconst: Hoisted but not initialized
They exist in the "Temporal Dead Zone" before declaration — accessing them causes aReferenceError.console.log(count); // ReferenceError let count = 1;
So you must declare them before use.
4. Window Object Attachment
var: Adds to the globalwindowobject (in browsers) when declared globally.var apiKey = "123"; console.log(window.apiKey); // "123"
letandconst: Do not add to thewindowobject.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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