Explain the difference between var, let, and const.

Explain the difference between var, let, and const.

September 04, 2025

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1. var

Scope: Function-scoped (visible throughout the function where declared).
Hoisting: Hoisted and initialized with undefined. You can use it before declaration (not recommended).
Re-declaration: Allowed in the same scope.
Re-assignment: Allowed.

Example:


var x = 10;
var x = 20; // valid
x = 30;     // valid

2. let

Scope: Block-scoped (only inside {} where declared).
Hoisting: Hoisted but not initialized → using before declaration causes ReferenceError.
Re-declaration: ❌ Not allowed in the same scope.
Re-assignment: ✅ Allowed.

Example:


let y = 10;
// let y = 20; ❌ Error
y = 30; // ✅ valid

3. const

Scope: Block-scoped (like let).
Hoisting: Hoisted but not initialized (same as let).
Re-declaration: ❌ Not allowed.
Re-assignment: ❌ Not allowed. Must be initialized at declaration.
Mutable objects/arrays: You can change object properties or array elements, but not reassign the variable itself.

Example:


const z = 10;
// z = 20; ❌ Error

const arr = [1,2,3];
arr.push(4); // ✅ allowed
// arr = [5,6]; ❌ Error

✅ Summary

var → function-scoped, hoisted, re-declarable.
let → block-scoped, hoisted (but TDZ), re-assignable but not re-declarable.
const → block-scoped, must be initialized, can’t be reassigned, but object/array contents can mutate

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