JavaScript Variables

Variables are containers used to store data values. In JavaScript, you declare variables using let, const, or var.

1. Declaring Variables

a) let (Block-scoped, mutable)

let name = "Alice";name = "Bob";  // You can change the value

• Scope: limited to the block ({ }) it is defined in.

• Use for variables whose values may change.

b) const (Block-scoped, immutable)

const PI = 3.14;

• Value cannot be reassigned after initialization.

• Use for constants or variables that shouldn't change.

• Note: For objects and arrays declared with const, the reference is constant, but object properties or array elements can still be modified.

const arr = [1, 2, 3];arr.push(4);  // Allowedarr = [5];    // Error

c) var (Function-scoped, mutable) — Avoid using in modern JS

var age = 25;age = 26;

• Has function scope, not block scope.

• Hoisted differently compared to let and const.

• Generally avoided now due to confusing scoping behavior.

2. Variable Naming Rules

• Must start with a letter, $, or _.

• Subsequent characters can be letters, digits, $, or _.

• Case-sensitive (myVar and myvar are different).

• Cannot be a reserved keyword (e.g., for, if, class).

3. Example

let message = "Hello";const year = 2025;message = "Hi";   // OK// year = 2026;   // Error: assignment to constant variableconsole.log(message); // Hiconsole.log(year);    // 2025

4. Hoisting (Brief)

• var declarations are hoisted (moved to the top of scope), initialized as undefined.

• let and const are hoisted but not initialized — accessing them before declaration causes an error.

If you want, I can explain hoisting in detail or show examples of scoping with var, let, and const!