[thelist] Extending Javascript Objects

[Keith Gaughan]

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Martyn Haigh wrote:

> I'#m trying to make my life a bit easier, and kinda failing.  I've
> written a few JavaScript prototype functions and attached them to the
> Object :
> 
> Object.prototype.getStyle = function(strProperty) {
> ...
> }
> Object.prototype.setStyle = function(strPoperty, strValue) {
> ...
> }
> Object.prototype.addClass = function(strClass) {
> ...
> }
> Object.prototype.removeClass = function(strClass) {
> ...
> }
> Object.prototype.replaceClass = function(strOldClass, strNewClass) {
> ...
> }
> Object.prototype.classExists = function(strClass) {
> ...
> }

Everybody else has covered the other issues, but there's one more that I
ought to mention.

<tip title="Never extend the Object prototype in JavaScript">
It's tempting to extend the Object prototype so that you'll have all
these extra methods on all the objects derived from Object. But this
is a *really* bad idea.

You see, doing so makes objects unusable as hashtables. Lets say that
you've done this:

    Object.prototype.foo = function() {};

And later on in some completely unrelated code you've got something
that does something like:

    var bar = { "Fred": "Wilma", "Barney": "Betty" };
    if ("foo" in bar)
    {
        alert("Whoa! This shouldn't happen!");
    }
    else
    {
        alert("All is well: objects behaving as proper hashes.");
    }

You'll get a pop-up window saying "Whoa! This shouldn't happen!",
because there's a phantom element in 'bar' called 'foo' from your
extending Object like that.

Erik explains things even better here:
    http://erik.eae.net/archives/2005/06/06/22.13.54/
</tip>

K.
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