Question

Possible Duplicate:
If Javascript has first-class functions, why doesn’t this work?

When I try to make an alias function for document.getElementById as below:

f = document.getElementById;

But, when I try to call:

var e_fullname = f(“fullname”);

It was rised an error: Could not convert JavaScript argument

And below is OK:

var e_fullname = f.call(document, “funname”);

Can you tell me why?

Answer 1

There are four ways of calling a function:

1. Function invocation: f(p1, p2)
2. Method invocation: obj.f(p1, p2)
3. Apply or Call invocation: f.apply(obj, [p1, p2]), f.call(obj, p1, p2)
4. Constructor invocation: new f(p1, p2)

In all these cases, f is just a reference (pointer) to a function object (an object with a [[Call]] internal property). What makes it behave different in all these cases is the way the function is called, and that matters a lot.

So, f is just a reference to the getElementById object, there’s no difference between document.getElementById and someOtherHTMLElement.getElementById; the function doesn’t hold back a reference to the object that references it.

If you want to bind a certain “owner” object, use the bind method:

var f = document.getElementById.bind(document);