Primitive Data Types in JavaScript

Primitive data types are the basic building blocks of JavaScript. They represent single, immutable values (not objects) and are stored directly in memory.

When you assign a primitive value to a variable, that variable holds the actual data — not a reference.

What Are Primitive Data Types?

JavaScript has 7 primitive data types:

Type	Description	Example
String	Textual data enclosed in quotes	"Ajay"
Number	Numeric values (integer or float)	42, 3.14
Boolean	Logical values	true, false
Undefined	Declared but not assigned any value	let x;
Null	Intentional absence of any value	let y = null;
BigInt	Large integers beyond Number’s limit	12345678901234567890n
Symbol	Unique and immutable identifiers	Symbol("id")

Key Characteristics

1. Immutable:
   Once created, their value cannot be changed.

   let name = "Ajay";
   name[0] = "a"; // ❌ Doesn't change the original string
   console.log(name); // "Ajay"

2. Stored by Value: When assigned or passed, a copy of the value is made.

   let a = 10;
   let b = a;
   b = 20;
   console.log(a); // 10 (not affected)

3. Fixed Memory Size: Each primitive type has a known, fixed amount of memory.

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Primitive Data Types in Action

1. String

Used for representing text.

let message = "Hello, World!";
let greeting = 'Hi!';
let template = `Welcome, ${message}`;

2. Number

Represents both integer and floating-point numbers.

let count = 42;
let price = 9.99;
let infinity = Infinity;
let notANumber = NaN;

3. Boolean

Represents logical truth values.

let isOnline = true;
let hasAccess = false;

4. Undefined

A variable declared but not initialized.

let x;
console.log(x); // undefined

5. Null

Represents the intentional absence of any object value.

let user = null;
console.log(user); // null

6. BigInt

Used for numbers larger than the safe integer limit (2^53 - 1).

let bigNumber = 123456789012345678901234567890n;

7. Symbol

Used for creating unique, immutable identifiers (often used in objects).

let id = Symbol("id");
let anotherId = Symbol("id");
console.log(id === anotherId); // false

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Type Checking with typeof

You can check a variable’s type using the typeof operator:

console.log(typeof "Hello"); // string
console.log(typeof 42);      // number
console.log(typeof true);    // boolean
console.log(typeof undefined); // undefined
console.log(typeof null);    // object (a historical JS bug)
console.log(typeof 123n);    // bigint
console.log(typeof Symbol()); // symbol

🧠 Summary

Feature	Primitive Types
Stored As	Value
Mutable	❌ No
Copied By	Value
Examples	String, Number, Boolean, Undefined, Null, BigInt, Symbol