typeof

The typeof operator returns a string indicating the type of the unevaluated operand.

Syntax

The typeof operator is followed by its operand:

typeof operand

Parameters

operand is an expression representing the object or primitive whose type is to be returned.

Description

The following table summarizes the possible return values of typeof. For more information about types and primitives, see also the JavaScript data structure page.

Type	Result
Undefined	"undefined"
Null	"object" (see below)
Boolean	"boolean"
Number	"number"
String	"string"
Symbol (new in ECMAScript 6)	"symbol"
Host object (provided by the JS environment)	Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms)	"function"
Any other object	"object"

Examples

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Despite being "Not-A-Number"
typeof Number(1) === 'number'; // but never use this form!


// Strings
typeof "" === 'string';
typeof "bla" === 'string';
typeof (typeof 1) === 'string'; // typeof always return a string
typeof String("abc") === 'string'; // but never use this form!


// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(true) === 'boolean'; // but never use this form!


// Symbols
typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'


// Undefined
typeof undefined === 'undefined';
typeof blabla === 'undefined'; // an undefined variable


// Objects
typeof {a:1} === 'object';

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === 'object';

typeof new Date() === 'object';


// The following is confusing. Don't use!
typeof new Boolean(true) === 'object'; 
typeof new Number(1) === 'object'; 
typeof new String("abc") === 'object';


// Functions
typeof function(){} === 'function';
typeof class C {} === 'function';
typeof Math.sin === 'function';

 

null

// This stands since the beginning of JavaScript
typeof null === 'object';

 

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the bogus typeofreturn value. (reference)

A fix was proposed for ECMAScript (via an opt-in), but was rejected. It would have resulted in typeof null === 'null'.

Regular expressions

Callable regular expressions were a non-standard addition in some browsers.

typeof /s/ === 'function'; // Chrome 1-12 Non-conform to ECMAScript 5.1
typeof /s/ === 'object';   // Firefox 5+  Conform to ECMAScript 5.1

 

Specifications

Specification	Status	Comment
ECMAScript 1st Edition.	Standard	Initial definition. Implemented in JavaScript 1.1
ECMAScript 5.1 (ECMA-262) The definition of 'The typeof Operator' in that specification.	Standard
ECMAScript 6 (ECMA-262) The definition of 'The typeof Operator' in that specification.	Release Candidate

Browser compatibility

• Desktop
 
• Mobile

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)

IE host objects are objects not functions

On IE 6, 7, and 8 a lot of host objects are objects and not functions. For example:

typeof alert === 'object'