Question

In JavaScript, let’s say you have:

function doSomething(callback) {
    if (callback instanceof Function) callback();
}

doSomething(function() {
    alert('hello world');
});

Is there a way to check what is inside ‘callback’ (like, the fact that alert() is called) from doSomething()? Something like:

function doSomething(callback) {
    alert(callback.innards().indexOf('alert('));
}

I’m just curious

Answer 1

Function.prototype.toString() gives an implementation-dependent representation of the function. However built-in functions will return something like:

function Array() {
   /* [native code] */
}

and host methods can return anything, even throw an error. So the strict answer is yes, maybe. But in a practical sense, it is not reliable.