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

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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 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.'foo'); // 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.

It's also possible to explicitly get the string representation of a function using the + operator:

function foo() { return 'bar' }
console.log(foo + ''); // "function foo() { return 'bar' }"


Comparing actual source code and toString results

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


ECMAScript Language Specification
# sec-function.prototype.tostring

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See also