JavaScript Variables and Data Types – A Beginner’s Guide

🔹 What is a Variable?

A variable is a named container that stores data that can be used and modified later.

In modern JavaScript, you can declare variables using:

• var (old, avoid using it)

• let (block-scoped, preferred)

• const (block-scoped, for constants)

Example:

javascriptCopyEditlet name = "Abhay";  // can change later
const pi = 3.14;     // constant value

🔹 Rules for Naming Variables

• Names can contain letters, digits, $, and _

• Must begin with a letter, $, or _

• Case-sensitive (Name and name are different)

• Cannot be reserved keywords (let, if, class, etc.)

🔹 JavaScript Data Types

JavaScript has two categories of data types:

🧾 1. Primitive Data Types

These hold single values and are immutable.

javascriptCopyEditlet firstName = "Abhay";     // String
let age = 22;                // Number
let isStudent = true;        // Boolean
let address = null;          // Null
let score;                   // Undefined
let id = Symbol("unique");   // Symbol

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📦 2. Non-Primitive (Reference) Data Types

These store collections of values.

javascriptCopyEditlet person = {
  name: "Abhay",
  age: 22,
};

let skills = ["JS", "HTML", "CSS"];

function greet() {
  console.log("Hello from devsync!");
}

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🔍 typeof Operator

To check the type of a variable, use typeof:

javascriptCopyEditconsole.log(typeof "Hello");     // string
console.log(typeof 42);          // number
console.log(typeof true);        // boolean
console.log(typeof undefined);   // undefined
console.log(typeof null);        // object (quirk in JS)
console.log(typeof person);      // object

🔄 Dynamic Typing

JavaScript is dynamically typed, which means:

• You don’t need to declare a type.

• Variables can change types at runtime.

javascriptCopyEditlet value = 42;      // Number
value = "Forty-two"; // Now a String

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✅ Tips for Beginners

• Always use let or const (avoid var unless necessary).

• Use const when you don’t plan to reassign a variable.

• Learn how types interact with each other (type coercion can be tricky!).

• Use typeof to check a variable's type:

javascriptCopyEditconsole.log(typeof 123);       // "number"
console.log(typeof "hello");   // "string"

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🎓 Summary

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