JS Basics

JavaScript Variables

Understand var, let, and const — when to use each, scope differences, and modern best practices.

JavaScript Variables

There are 4 ways to declare a JavaScript variable:

var — old way, function-scoped, avoid in modern JS
let — block-scoped, can be reassigned
const — block-scoped, cannot be reassigned
(no keyword) — creates global variable, always avoid this

var vs let vs const

Feature	var	let	const
Scope	Function	Block	Block
Hoisting	Yes (undefined)	Yes (TDZ)	Yes (TDZ)
Re-declare	Yes	No	No
Re-assign	Yes	Yes	No

Modern Best Practices

Use const by default
Use let when you need to reassign
Never use var

Destructuring

Destructuring allows you to unpack values from arrays or properties from objects.

Example

javascript

// const - cannot be reassigned
const PI = 3.14159;
const name = "DevForge";
// PI = 3; // Error!

// let - can be reassigned
let score = 0;
score = 100;  // OK
score++;      // OK

// Destructuring
const [first, second, ...rest] = [1, 2, 3, 4, 5];
console.log(first);  // 1
console.log(rest);   // [3, 4, 5]

const { name: userName, age, role = "user" } = {
  name: "Alice",
  age: 28
};
console.log(userName);  // Alice
console.log(role);      // user (default)

// Template literals
const greeting = `Hello, ${userName}! You are ${age} years old.`;
console.log(greeting);

// Spread operator
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5, 6];
const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 };

Try it yourself — JAVASCRIPT