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";