JavaScript function aliasing doesn't seem to work

I was just reading this question and wanted to try the alias method rather than the function-wrapper method, but I couldn't seem to get it to work in either Firefox 3 or 3.5beta4, or Google Chrome, both in their debug windows and in a test web page.

Firebug:

>>> window.myAlias = document.getElementById
function()
>>> myAlias('item1')
>>> window.myAlias('item1')
>>> document.getElementById('item1')
<div id="item1">

If I put it in a web page, the call to myAlias gives me this error:

uncaught exception: [Exception... "Illegal operation on WrappedNative prototype object" nsresult: "0x8057000c (NS_ERROR_XPC_BAD_OP_ON_WN_PROTO)" location: "JS frame :: file:///[...snip...]/test.html :: <TOP_LEVEL> :: line 7" data: no]

Chrome (with >>>'s inserted for clarity):

>>> window.myAlias = document.getElementById
function getElementById() { [native code] }
>>> window.myAlias('item1')
TypeError: Illegal invocation
>>> document.getElementById('item1')
<div id=?"item1">?

And in the test page, I get the same "Illegal invocation".

Am I doing something wrong? Can anyone else reproduce this?

Also, oddly enough, I just tried and it works in IE8.

Solution

You have to bind that method to the document object. Look:

>>> $ = document.getElementById
getElementById()
>>> $('bn_home')
[Exception... "Cannot modify properties of a WrappedNative" ... anonymous :: line 72 data: no]
>>> $.call(document, 'bn_home')
<body id="bn_home" onload="init();">

When you’re doing a simple alias, the function is called on the global object, not on the document object. Use a technique called closures to fix this:

function makeAlias(object, name) {
    var fn = object ? object[name] : null;
    if (typeof fn == 'undefined') return function () {}
    return function () {
        return fn.apply(object, arguments)
    }
}
$ = makeAlias(document, 'getElementById');

>>> $('bn_home')
<body id="bn_home" onload="init();">

This way you don’t loose the reference to the original object.

In 2012, there is the new bind method from ES5 that allows us to do this in a fancier way:

>>> $ = document.getElementById.bind(document)
>>> $('bn_home')
<body id="bn_home" onload="init();">