JavaScript Data Types

Primitive types

JavaScript has seven primitive types:

Everything else — arrays, functions, dates, plain {} objects — is an object.

typeof

The typeof operator tells you the type of a value:

console.log(typeof "Ada");
console.log(typeof 42);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof null);
console.log(typeof [1, 2]);
console.log(typeof { a: 1 });
console.log(typeof function () {});
console.log(Array.isArray([1, 2]));

string
number
boolean
undefined
object
object
object
function
true

Note the famous quirk: typeof null is "object" — a bug from 1995 that can never be fixed without breaking the web. Use value === null to test for null, and Array.isArray(value) to test for arrays.

Dynamic typing

A variable can hold any type, and can change type:

let value = 5;
console.log(value, typeof value);
value = "five";
console.log(value, typeof value);

5 number
five string