[Javascript] Check if an element exists in the DOM

[John Deighan]

At 08:49 AM 10/18/2006, you wrote:
>Hello.
>
>How do I check if an element with id="XYZ" is present in my page/DOM ?
>
>I've tried with:
>
>if ($('XYZ') == undefined) {
>  // Do something
>} else {
>  // Do something else
>}
>
>but, of course, javascript give me an error of undefined object.

The function named '$', I think, is part of a popular library named 
prototype.js. You'd have to at a minimum be sure that it's loaded. 
The standard DOM way would be:

if (document.getElementById('XYZ')===undef) {

Note that it's better to use '===' to compare to undef since '==' 
will to type conversion on it's arguments so that, 
e.g.   0==undef  and ''==undef   will return true, but the same 
comparisons using '===' will return false.