✅ 1. var – The Old Way
- Scope: Function-scoped
If you declare a variable using
var inside a function, it's only accessible inside that function. If it's inside a block (like an if or for), it’s still accessible outside that block.
1function example() {
2 if (true) {
3 var x = 10;
4 }
5 console.log(x); // 10 – accessible outside the block!
6}
1function example() {
2 if (true) {
3 var x = 10;
4 }
5 console.log(x); // 10 – accessible outside the block!
6}
- Hoisting: Variables declared with
var are hoisted to the top of their scope and initialized as undefined.
1console.log(a); // undefined
2var a = 5;
1console.log(a); // undefined
2var a = 5;
- Re-declaration: You can re-declare the same variable in the same scope without error.
1var name = 'Alice';
2var name = 'Bob'; // Allowed
1var name = 'Alice';
2var name = 'Bob'; // Allowed
✅ 2. let – Block Scoped
- Scope: Block-scoped
Meaning it's only accessible within the
{} block it’s declared in.
1if (true) {
2 let y = 20;
3}
4console.log(y); // ReferenceError
1if (true) {
2 let y = 20;
3}
4console.log(y); // ReferenceError
- Hoisting: Yes, but not initialized. Accessing before declaration gives a
ReferenceError (Temporal Dead Zone).
1console.log(b); // ReferenceError
2let b = 10;
1console.log(b); // ReferenceError
2let b = 10;
- Re-declaration: Not allowed in the same scope.
1let name = 'Alice';
2let name = 'Bob'; // SyntaxError
3
1let name = 'Alice';
2let name = 'Bob'; // SyntaxError
3
✅ 3. const – Constant Variables
- Scope: Block-scoped (same as
let)
- Must be initialized when declared.
- Cannot be re-assigned after the initial assignment.
1const z = 30;
2z = 40; // ❌ TypeError
1const z = 30;
2z = 40; // ❌ TypeError
- Mutability: If the value is an object or array, its contents can still be modified — only the binding is constant.
1const arr = [1, 2, 3];
2arr.push(4); // Allowed
3arr = [5, 6]; // Not allowed
1const arr = [1, 2, 3];
2arr.push(4); // Allowed
3arr = [5, 6]; // Not allowed
