typeof

typeof 연산자는 피연산자의 타입을 가리키는 문자열을 반환합니다.

문법

typeof 연산자는 다음에 피연산자가 옵니다 : 

typeof피연산자

파라미터

피연산자는 타입을 반환할 오브젝트나 기본형(primitive)을 나타내는 표현식입니다.

설명

다음 표는 반환 가능한 타입 값들을 요약하고 있습니다. 타입과 기본형에 대한 더 많은 정보에 대해서는 자바스크립트 데이터 구조(JavaScript data structure) 페이지를 보세요.

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 returns 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 declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined'; 


// 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 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.

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

Specifications

Browser compatibility

IE-specific notes

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

typeof alert === 'object'

See also

• instanceof