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The toString() method returns a string representing the source code of the function.



Return value

A string representing the source code of the function.


The Function object overrides the toString method inherited from Object; it does not inherit Object.prototype.toString. For user-defined Function objects, the toString method returns a string containing the source text segment which was used to define the function.

JavaScript calls the toString method automatically when a Function is to be represented as a text value, e.g. when a function is concatenated with a string.

The toString() method will throw a TypeError exception ("Function.prototype.toString called on incompatible object"), if its this value object is not a Function object.'foo'); // TypeError

If the toString() method is called on built-in function objects or a function created by Function.prototype.bind, toString() returns a native function string which looks like

"function () {\n    [native code]\n}"

If the toString() method is called on a function created by the Function constructor, toString() returns the source code of a synthesized function declaration named "anonymous" using the provided parameters and function body.


Function Function.prototype.toString result
function f(){}
"function f(){}"
class A { a(){} }
"class A { a(){} }"
function* g(){}
"function* g(){}"
a => a
"a => a"
({ a(){} }.a)
({ *a(){} }.a)
({ [0](){} }[0])
    get a(){}
}, "a").get
"get a(){}"
    set a(x){}
}, "a").set
"set a(x){}"
"function toString() { [native code] }"
(function f(){}.bind(0))
"function () { [native code] }"
Function("a", "b")
"function anonymous(a\n) {\nb\n}"


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function.prototype.toString' in that specification.
Standard Added more specific requirements for the string representation.
Function.prototype.toString revisions proposal Draft Standardizes native function string, line endings.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function.prototype.toString' in that specification.

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
toStringChrome Full support YesEdge Full support 12Firefox Full support 1IE Full support 5Opera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
Support of toString revisionChrome No support NoEdge No support NoFirefox Full support 54IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 54Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support Nonodejs No support No


Full support  
Full support
No support  
No support

Firefox-specific notes

  • Since Firefox 17, Function.prototype.toString() has been implemented by saving the function's source. The decompiler was removed, so that the indentation parameter is not needed any more. See bug 761723 for more details.
  • From with Firefox 38 to 63, Function.prototype.toString() was throwing for Proxy objects (bug 1100936 and bug 1440468).

See also