What data types does JavaScript have?

Seven primitives and one object type.

Primitives — immutable, compared by value:

string, number, boolean, undefined, null, symbol, bigint

Objects — mutable, compared by reference: plain objects, arrays, functions, Date, Map, Set, RegExp and everything else.

typeof "hi";       // "string"
typeof 42;         // "number"
typeof 10n;        // "bigint"
typeof Symbol();   // "symbol"
typeof undefined;  // "undefined"
typeof null;       // "object"  ← a long-standing bug, kept for compatibility
typeof [];         // "object"  ← use Array.isArray()
typeof (() => {}); // "function" ← functions are objects with a special typeof

null vs undefined

undefined means "no value has been assigned" — the engine produces it. null means "deliberately empty" — you produce it. A missing function argument is undefined; a cleared field is usually null.

Numbers

There is one number type, a 64-bit float, so 1 and 1.0 are identical and 0.1 + 0.2 === 0.3 is false. Integers are exact only up to Number.MAX_SAFE_INTEGER (about 9×10¹⁵); beyond that use bigint. NaN is a number, and it is the only value not equal to itself — test with Number.isNaN.

Value vs reference

let a = { n: 1 };
let b = a;
b.n = 2;
a.n;                 // 2 — same object

let x = 1, y = x;
y = 2;
x;                   // 1 — copied

This one distinction explains most confusion about copying, equality and why [] === [] is false.

Type coercion is the other half of the story: == converts before comparing, === does not. Use === unless you specifically want == null to catch both null and undefined.