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.

Examples

// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof(42) === '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 '1' === 'string'; // note that a number within a string is still typeof 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'.

Using new operator

// All constructor functions while instantiated with 'new' keyword will always be typeof 'object'
var str = new String('String');
var num = new Number(100);

typeof str; // It will return 'object'
typeof num; // It will return 'object'

// But there is a exception in case of Function constructor of Javascript

var func = new Function();

typeof func; // It will return 'function'

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

Temporal Dead Zone errors

Before ECMAScript 2015, typeof was always guaranteed to return a string for any operand it was supplied with. But with the addition of non-hoisted, block-scoped let and const, using typeof on let and const variables in a block before they are declared will throw a ReferenceError. This is in contrast with undeclared variables, for which typeof will return 'undefined'. Block scoped variables are in a "temporal dead zone" from the start of the block until the initialization is processed, during which, it will throw an error if accessed.

typeof undeclaredVariable === 'undefined';
typeof newLetVariable; let newLetVariable; // ReferenceError
typeof newConstVariable; const newConstVariable = 'hello'; // ReferenceError

Exceptions

All current browsers expose a non-standard host object document.all with type Undefined.

typeof document.all === 'undefined';

Although the specification allows custom type tags for non-standard exotic objects, it requires those type tags to be different from the predefined ones. The case of document.all having type tag 'undefined' must be classified as an exceptional violation of the rules.

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
• Why typeof is no longer "safe"