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
typeof(operand)

Parameters

operand

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"
BigInt (new in ECMAScript 2020) "bigint"
String "string"
Symbol (new in ECMAScript 2015) "symbol"
Function object (implements [[Call]] in ECMA-262 terms) "function"
Any other object "object"

Note: ECMAScript 2019 and older permitted implementations to have typeof return any implementation-defined string value for non-callable non-standard exotic objects.

The only known browser to have actually taken advantage of this is old Internet Explorer (see below).

Examples

Basic usage

// 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';      // Number tries to parse things into numbers
typeof Number('shoe') === 'number';   // including values that cannot be type coerced to a number

typeof 42n === 'bigint';

// Strings
typeof '' === 'string';
typeof 'bla' === 'string';
typeof `template literal` === '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(1) === 'string'; // String converts anything into a string, safer than toString

// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(1) === 'boolean'; // Boolean() will convert values based on if they're truthy or falsy
typeof !!(1) === 'boolean'; // two calls of the ! (logical NOT) operator are equivalent to Boolean()

// 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';
typeof /regex/ === 'object'; // See Regular expressions section for historical results

// The following are confusing, dangerous, and wasteful. Avoid them.
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';

typeof 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 typeof return value "object". (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, with the exception of the Function constructor, will always be typeof 'object'
let str = new String('String');
let num = new Number(100);

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

let func = new Function();

typeof func; // It will return 'function'

Need for parentheses in Syntax

// Parentheses can be used for determining the data type of expressions.
let iData = 99;

typeof iData + ' Wisen'; // 'number Wisen'
typeof (iData + ' Wisen'); // 'string'

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

Errors

Before ECMAScript 2015, typeof was always guaranteed to return a string for any operand it was supplied with. Even with undeclared identifiers, typeof will return 'undefined'. Using typeof could never generate an error.

However, with the addition of block-scoped let and const, using typeof on let and const variables (or using typeof on a class) in a block before they are declared will throw a ReferenceError. 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; // ReferenceError
typeof newConstVariable; // ReferenceError
typeof newClass; // ReferenceError

let newLetVariable;
const newConstVariable = 'hello';
class newClass{};

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 'undefined' is classified in the web standards as a "willful violation" of the original ECMA JavaScript standard.

Real-world usage

typeof is very useful, but it's not as versatile as might be required. For example, typeof([]) , is 'object', as well as typeof(new Date()), typeof(/abc/), etc.

For greater specificity in checking types, a typeof wrapper for usage in production-level code would be as follows (provided obj exists): 

  function type(obj, showFullClass) {

    // get toPrototypeString() of obj (handles all types)
    if (showFullClass && typeof obj === "object") {
        return Object.prototype.toString.call(obj);
    }
    if (obj == null) { return (obj + '').toLowerCase(); } // implicit toString() conversion

    var deepType = Object.prototype.toString.call(obj).slice(8,-1).toLowerCase();
    if (deepType === 'generatorfunction') { return 'function' }

    // Prevent overspecificity (for example, [object HTMLDivElement], etc).
    // Account for functionish Regexp (Android <=2.3), functionish <object> element (Chrome <=57, Firefox <=52), etc.
    // String.prototype.match is universally supported.

    return deepType.match(/^(array|bigint|date|error|function|generator|regexp|symbol)$/) ? deepType :
       (typeof obj === 'object' || typeof obj === 'function') ? 'object' : typeof obj;
  }

For checking non-existent variables that would otherwise throw a ReferenceError, use typeof nonExistentVar === 'undefined'.

Specifications

Specification
ECMAScript Language Specification
# sec-typeof-operator

Browser compatibility

BCD tables only load in the browser

IE-specific notes

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

typeof alert === 'object'

Some non-standard IE properties return other values (tc39/ecma262#1440 (comment)): 

typeof window.external.AddSearchProvider === "unknown";
typeof window.external.IsSearchProviderInstalled === "unknown";

See also