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 Function objects, the toString method returns a string representation of the object in the form of a function declaration. That is, toString decompiles the function, and the string returned includes the function keyword, the argument list, curly braces, and the source of the function body.

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.

JavaScript"foo"); // TypeError

var proxy = new Proxy(function() {}, {});; // TypeError


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)
The definition of 'Function.prototype.toString' in that specification.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function.prototype.toString' in that specification.
Standard Added more specific requirements for the string representation.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Function.prototype.toString' in that specification.

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)

Gecko-specific notes

  • Since Gecko 17.0 (Firefox 17 / Thunderbird 17 / SeaMonkey 2.14), 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 Gecko 38 (Firefox 38 / Thunderbird 38 / SeaMonkey 2.35), Function.prototype.toString() throws for Proxy objects (bug 1100936).

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

Function JavaScript Method Prototype