Function.prototype.toString()

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

* Some parts of this feature may have varying levels of support.

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

Try it

Syntax

js
toString()

Parameters

None.

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.

js
Function.prototype.toString.call("foo"); // throws TypeError

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

function someName() { [native code] }

For intrinsic object methods and functions, someName is the initial name of the function; otherwise its content may be implementation-defined, but will always be in property name syntax, like [1 + 1], someName, or 1.

Note: This means using eval() on native function strings is a guaranteed syntax error.

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. For example, Function("a", "b", "return a + b").toString() will return:

function anonymous(a,b
) {
return a + b
}

Since ES2018, the spec requires the return value of toString() to be the exact same source code as it was declared, including any whitespace and/or comments — or, if the host doesn't have the source code available for some reason, requires returning a native function string. Support for this revised behavior can be found in the compatibility table.

Examples

Comparing actual source code and toString results

js
function test(fn) {
  console.log(fn.toString());
}

function f() {}
class A {
  a() {}
}
function* g() {}

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

Note that after the Function.prototype.toString() revision, when toString() is called, implementations are never allowed to synthesize a function's source that is not a native function string. The method always returns the exact source code used to create the function — including the getter and setter examples above. The Function constructor itself has the capability of synthesizing the source code for the function (and is therefore a form of implicit eval()).

Getting source text of a function

It is possible to get the source text of a function by coercing it to a string — for example, by wrapping it in a template literal:

js
function foo() {
  return "bar";
}
console.log(`${foo}`);
// function foo() {
//   return "bar";
// }

This source text is exact, including any interspersed comments (which won't be stored by the engine's internal representation otherwise).

js
function foo /* a comment */() {
  return "bar";
}
console.log(foo.toString());
// function foo /* a comment */() {
//   return "bar";
// }

Specifications

Specification
ECMAScript Language Specification
# sec-function.prototype.tostring

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
toString
Implements Function.prototype.toString revision

Legend

Tip: you can click/tap on a cell for more information.

Full support
Full support
No support
No support

See also