Variables & Data Types in JavaScript

What is a Variable?

A variable is like a container that stores data values. In JavaScript, we use keywords like var, let, and const to declare variables.

Variable Declaration Keywords

var - old way of declaring variables (function-scoped, can be re-declared and updated).
let - modern way (block-scoped, can be updated but not re-declared in same scope).
const - block-scoped, cannot be updated or re-declared (used for constants).

    // Example

    var name = "John";      // old

    let age = 25;           // modern

    const country = "India"; // constant

Data Types in JavaScript

Know more in Detail

JavaScript has different types of data that variables can hold. These are divided into two main categories:

1. Primitive Data Types

Primitive means simple data types that store a single value like a name or number. They are stored directly in memory.

String: A sequence of characters, like text.
Number: Any numeric value (integer or floating-point).
Boolean: True or false.
Undefined: A variable that has been declared but not assigned a value.
Null: Intentionally set to have no value.
BigInt: Used for very large integers.
Symbol: Unique and immutable value often used as object keys.

Primitive & Non-primitive in Detail

    let name = "Alice";     // String

    let age = 30;           // Number

    let isStudent = true;   // Boolean

    let score;              // Undefined

    let empty = null;       // Null

    let bigNumber = 1234567890123456789012345678901234567890n; // BigInt

    let uniqueId = Symbol("id"); // Symbol

2. Non-Primitive (Reference) Data Types

Non-Primitive means complex data types like arrays, objects, and functions. They are stored by reference, which means they point to a memory address.

Object: A collection of key-value pairs.
Array: A list-like object.
Function: A block of code that can be called.

    let person = { name: "Bob", age: 40 };   // Object

    let fruits = ["Apple", "Banana"];        // Array

    function greet() {                       // Function
      console.log("Hello!");
    }
  

Type Checking

You can use typeof to check the data type of a variable:

    console.log(typeof "Hello");   // "string"

    console.log(typeof 42);        // "number"

    console.log(typeof true);      // "boolean"

Summary

Use let for values that may change.
Use const for constants.
Understand the difference between primitive and reference types.
Use typeof to inspect types.