What are the primitive data types in JavaScript?

Question

IT Interview Practice · Accepted Answer

JavaScript has **7 primitive types**, and everything else (arrays, functions, objects) is an **object**.

The primitives: `string`, `number`, `boolean`, `null`, `undefined`, `bigint`, `symbol`.

```js
typeof "hi";       // "string"
typeof 42;         // "number"  (both ints and floats)
typeof true;       // "boolean"
typeof undefined;  // "undefined"
typeof 10n;        // "bigint"
typeof Symbol();   // "symbol"
typeof null;       // "object"  ← famous historical bug!
typeof {};         // "object"
typeof [];         // "object"  (arrays are objects)
typeof function(){};// "function"
```

## Two things that matter

**1. Primitives are immutable and copied by value; objects are copied by reference.**

```js
let a = 5; let b = a; b++;          // a is still 5 (independent copies)
let x = {n:1}; let y = x; y.n = 2;  // x.n is now 2 (same object!)
```

**2. `typeof null === "object"`** is a long-standing language bug you must remember — to check for null, compare directly (`x === null`).

## Why it matters

Understanding value-vs-reference explains why mutating a shared object affects "both" variables, why `{} === {}` is `false`, and why you copy objects (spread) before changing them in state-driven UIs like React.

Two things that matter

Why it matters

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