Before I get into the details of this problem, I'd like to make the situation clear. Our web analytics company works as a consultant for large sites, and (other than adding a single SCRIPT tag) we have no control over the pages themselves.
Our existing script installs handlers using "old" way (a fancy version of element.onclick = blah; that also executes the original handler) which is completely unaware of "new" (addEventListener or attachEvent) handlers on the page. We'd like to fix this to make our script able to run on more sites without requiring as much custom development.
The initial thought here was to have our own script use addEventListener/attachEvent, but this presents a problem: of the client's site sets a handler using the "old" way, it would wipe out the handler we installed the "new" way. Quick and dirty testing shows this happens in both IE7 and FF3, although I didn't test the whole range of browsers. There's also a risk that if we use the "new" way after the page's event handlers are already set, we could erase their handlers.
So my question is: what safe technique can I use to add an event handler in Javascript using addEventListener/attachEvent that works regardless of how other event handlers on the page are installed?
Please remember: we have no way of modifying the site that our script is installed on. (I have to emphasize that because the default answer to questions like this is always, "just rewrite the page to do everything the same way.")
Can you try your quick-and-dirty testing again? This doesn't happen for me in FF3.
elem.onclick = function() { alert("foo"); };
elem.addEventListener("click", function() { alert("bar"); }, false);
Both handlers fire for me when I click on the element.
I'm guessing you forgot the final boolean argument in addEventListener (whether to use the capture phase). I'm also guessing you forgot that IE's attachEvent needs onclick, not click.