JavaScript Declaration Variable

In JavaScript, variables are declared using one of three keywords: var, let, or const. These keywords differ in their scope, hoisting behavior, and re-assignment rules.

1. var – the old way (avoid using it)

• Function-scoped → only respects function boundaries.
• Can be re-declared.
• Can be updated.
• Hoisted → moved to the top of the scope, but initialized as undefined.

It can cause weird bugs because it ignores block scope.

var x = 1;
if (true) {
  var x = 2;  // same variable!
}
console.log(x); // 2

2. let – recommended for variables that change

• Block-scoped → respects { } blocks.
• Can be updated, but not re-declared in the same scope.
• Also hoisted, but not usable before declared (“temporal dead zone”).

Use let when the value needs to change.

let count = 1;
count = 2; // OK
// let count = 3; // ❌ Error (same scope)

3. const – recommended default

• Block-scoped
• Cannot be updated or re-declared
• Best for values that should not change

Objects and arrays can still be modified, but the variable reference cannot change.

const PI = 3.14;
// PI = 4; // ❌ Error

const user = { name: "Khaf" };
user.name = "Fido"; // ✔ OK
// user = {} // ❌ Not allowed

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Summary

Feature	var	let	const
Scope	Function	Block	Block
Re-declare	✔ Yes	❌ No	❌ No
Update value	✔ Yes	✔ Yes	❌ No
Hoisted	Yes (weird)	Yes (safer)	Yes (safer)
When to use	Avoid	Value changes	Value doesn't change