typeof and instanceof

Syntax

typeof value
object instanceof Constructor

Examples

console.log(typeof "hello");  // "string"
console.log(typeof 42);       // "number"
console.log(typeof true);     // "boolean"
console.log(typeof undefined);// "undefined"
console.log(typeof function(){}); // "function"
console.log(typeof {});       // "object"
console.log(typeof []);       // "object" - arrays are objects too, typeof cannot distinguish them

const arr = [1, 2, 3];
const date = new Date();

console.log(arr instanceof Array);  // true
console.log(arr instanceof Object); // true - arrays are also objects
console.log(date instanceof Date);  // true
console.log(date instanceof Array); // false

class Dog {}
const rex = new Dog();
console.log(rex instanceof Dog); // true

Best practices

• Use typeof for primitives (string, number, boolean, etc.) and Array.isArray() specifically for arrays, since typeof cannot tell arrays apart from plain objects
• Use instanceof to check if an object was created by a specific class or constructor, especially useful in error handling (error instanceof TypeError)
• Remember instanceof checks the prototype chain, so it can behave unexpectedly across different execution contexts (like iframes) - it is not foolproof for all edge cases
• Combine typeof checks with early returns to validate function inputs before processing them

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation