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    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 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

    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

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (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'

    See also

    Document Tags and Contributors

    Last updated by: fscholz,
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