function.toString

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

Syntax

function.toString()

Return value

A string representing the source code of the function.

Description

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.

Examples

function f(){}

"function f(){}"

class A { a(){} }

"class A { a(){} }"

function* g(){}

"function* g(){}"

a => a

"a => a"

({ a(){} }.a)

"a(){}"

({ *a(){} }.a)

"*a(){}"

({ [0](){} }[0])

"[0](){}"

Object.getOwnPropertyDescriptor({
    get a(){}
}, "a").get

"get a(){}"

Object.getOwnPropertyDescriptor({
    set a(x){}
}, "a").set

"set a(x){}"

Function.prototype.toString

"function toString() { [native code] }"

(function f(){}.bind(0))

"function () { [native code] }"

Function("a", "b")

"function anonymous(a\n) {\nb\n}"

Specifications

Browser compatibilityUpdate compatibility data on GitHub

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

• Object.prototype.toString()