typeof Redirect 1

Summary

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

Operator
Implemented in: JavaScript 1.1
ECMA Version: ECMA-262 (and ECMA-357 for E4X objects)

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

This table summarizes the possible return values of typeof:

Type Result
Undefined "undefined"
Null "object"
Boolean "boolean"
Number "number"
String "string"
Host object (provided by the JS environment) Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms) "function"
E4X XML object "xml"
E4X XMLList object "xml"
Any other object "object"

Examples

Normal cases

// 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!

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

// Objects
typeof {a:1} === 'object';
typeof [1, 2, 4] === 'object'; // use Array.isArray or Object.prototype.toString.call to differentiate regular objects from arrays
typeof new Date() === 'object';

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

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

null

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

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 typeof return 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 (need reference to say which).

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

Other quirks

alert in old Internet Explorer versions

On IE 6, 7, and 8, typeof alert === 'object'

Note: Not a quirk. It's telling the truth. Lots of old IE host objects are objects not functions

Specification

ECMA-262 section 11.4.3

See also

Document Tags and Contributors

Contributors to this page: Sheppy
Last updated by: Sheppy,