Understanding Variables and Data Types in JavaScript

Variables — Containers for Storing Data

A variable is a container where you can store data. In JavaScript, there are 3 ways to declare variables:

• var — Old method, function-scoped, can be overwritten anytime

• let — Modern method, block-scoped, value can be changed

• const — Block-scoped, value CANNOT be changed after assignment

Adds element at beginning.

var score = 10; // old way
let name = "Shaaz"; // modern way
const pi = 3.14; // constant, never changes

Tip: Use 'const' by default. Use 'let' only when the value needs to change. Avoid 'var' in modern code.

✅Remember: var is function-scoped. let and const are block-scoped. This difference matters inside loops and if-blocks.

JavaScript has 7 main data types. Knowing these is very important for interviews:

1. Number — for all numeric values

let age = 25;
let price = 99.99;

2. String — for text

let name = "Shaaz";
let city = 'Delhi';

3. Boolean — true or false only

let isLoggedIn = true;
let isAdmin = false;

4. Null — intentionally empty

Use null when you want to say 'this variable exists but has no value on purpose'.

let emptyValue = null;

5. Undefined — not assigned yet

When you declare a variable but do not give it a value, it becomes undefined automatically.

let noValue;
console.log(noValue); // Output: undefined

6. Object — key-value pairs

Objects store multiple related values together.

let person = { name: "Shaaz", age: 25, city: "Delhi" };
console.log(person.name); // Output: Shaaz

7. Array — ordered list of values

Arrays store multiple values in a sequence. Each value has an index starting from 0.

Adds element at beginning.

const nums = [2, 3];

nums.unshift(1);

console.log(nums);
// [1, 2, 3]

✅Remember: null means empty on purpose. undefined means not yet assigned. They are NOT the same!

🟢 Tip: Use typeof to check the data type of any variable: console.log(typeof 'hello'); // Output: string