Basics

Data Types

Primitives (string, number, boolean, null, undefined, bigint, symbol) and Objects.

1. Explanation

JavaScript has 7 primitive types — string, number, boolean, null, undefined, bigint, symbol — and one non-primitive type: object (which includes arrays and functions). Use typeof to inspect a value's type. Primitives are immutable and compared by value; objects are compared by reference.

2. Example

Try it yourself

console.log(typeof "hi");      // "string"
console.log(typeof 42);        // "number"
console.log(typeof true);      // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null);      // "object"  ← historic bug
console.log(typeof {});        // "object"
console.log(typeof []);        // "object"
console.log(typeof function(){}); // "function"

Console

Click Run to see output.

3. Expected output

output

string
number
boolean
undefined
object
object
object
function

4. Common mistakes

✗Assuming typeof null returns 'null' — it returns 'object'
✗Using == which performs type coercion: 0 == '' is true
✗Mutating an object you thought was const — const only locks the binding, not the contents

5. Practice questions

Print the typeof for: NaN, [], {}, () => {}, 1n
Convert the string '42' into a number two different ways
Check if a value is exactly null without confusing it with undefined

→ Try more practice problems