typeof Basics

JavaScript · Reference cheat sheet

📋 Overview

typeof is an operator that returns a string describing a value’s type tag. It is useful for quick checks while learning, with a few famous quirks to memorize.

🔧 Core concepts

Expression	Typical result
`typeof "hi"`	`"string"`
`typeof 42`	`"number"`
`typeof 42n`	`"bigint"`
`typeof true`	`"boolean"`
`typeof undefined`	`"undefined"`
`typeof Symbol()`	`"symbol"`
`typeof \{\}` / `[]` / `null`	`"object"` (see pitfalls)
`typeof function () \{\}`	`"function"`

typeof never throws on an undeclared variable in older patterns like typeof x — it yields "undefined". Prefer declared variables anyway.

💡 Examples

Primitives:

console.log(typeof "hello"); // string
console.log(typeof 3.14);    // number
console.log(typeof true);    // boolean
console.log(typeof undefined); // undefined

Objects and functions:

console.log(typeof { a: 1 });     // object
console.log(typeof [1, 2]);       // object
console.log(typeof null);         // object  ← historical bug
console.log(typeof (() => 1));    // function

Guarding before use:

function double(n) {
  if (typeof n !== "number" || Number.isNaN(n)) {
    throw new TypeError("n must be a number");
  }
  return n * 2;
}

console.log(double(21));

Optional value check:

let title;
console.log(typeof title); // undefined
title = "Docs";
console.log(typeof title); // string

⚠️ Pitfalls

typeof null === "object" — check null with value === null.
Arrays are "object" — use Array.isArray(x).
typeof NaN is "number".
For class instances, prefer instanceof or brand checks over typeof.

typeof Basics

typeof Basics

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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