Question

Given a function, I’d like to know whether it’s a developer-defined function or a built-in function provided by JavaScript engine. Is it possible?

For a developer-defined function, I’d like to trace its execution.

Answer 1

The valueOf method you mention in your own answer will not work as you mention it.

The Function.prototype doesn’t have a valueOf method, it is inherited from Object.prototype and this method will simply return the same function object where you call it:

Function.valueOf() === Function; // true

I think you are confusing it with the toString method (or you are alerting the valueOf method call which causes on most browsers an implicit ToString conversion).

However, you can use the toString method directly on function objects, and in almost all implementations, will return you a string representation containing "[native code]" in its function body, I wouldn’t recommend it too much because, the Function.prototype.toString method is implementation dependent…

function isNative(fn) {
  return /native code/.test(fn.toString);
}

isNative(Function); // true
isNative(function () {}); // false

Again I advise you that there are some browsers that will return different results when using the toString method on functions, for example, some Mobile browsers will return the same string for any function object.