JavaScript - Data Types

Overview

Estimated time: 20–30 minutes

JavaScript has seven primitive types and objects. typeof helps inspect types—with a few quirks.

Learning Objectives

• List JS primitive types and objects.
• Use typeof and understand edge cases.

Prerequisites

• JavaScript - Reserved Keywords

Primitives

• string
• number
• bigint
• boolean
• undefined
• symbol
• null

typeof examples

typeof 'hi'        // 'string'
typedef = undefined; typeof typedef // 'undefined'
typeof 42           // 'number'
typeof 10n          // 'bigint'
typeof true         // 'boolean'
typeof Symbol('x')  // 'symbol'
typeof null         // 'object'  // historical quirk
typeof {}           // 'object'
typeof []           // 'object'
typeof (() => {})   // 'function'

Objects

Arrays, functions, dates, maps, sets, and user-defined objects are all objects. Use Array.isArray for arrays.

Common Pitfalls

• typeof null returns 'object' (quirk). Check value === null explicitly.