Function.prototype.toString()
The toString()
method returns a string representing the source code of the specified Function
.
Try it
Syntax
js
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.
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
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