None.

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.

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.

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.

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()).

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:

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).

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

• Object.prototype.toString()