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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. It will also throw for Proxy objects, for example.

Function.prototype.toString.call('foo'); // TypeError

var proxy = new Proxy(function() {}, {});
Function.prototype.toString.call(proxy); // 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 revision Draft Standardizes native function string, line endings.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Function.prototype.toString' in that specification.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes15 Yes Yes
Support of toString revision No No54 No No No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes Yes
Support of toString revision No No No54 No No No

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.
  • Starting with Firefox 38, Function.prototype.toString() throws for Proxy objects (bug 1100936).

See also

Document Tags and Contributors

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