What is the behavior of addEventListener when you try and add a non-existent event?

Question

For example:

someElement.addEventListener("blahblah", alert("hello!"));

when typed into both the Chrome and Firefox development consoles seems to fire the alert("hello!") call once and then return ' undefined '.

If I embed that same call into the page, then nothing seems to happen - no error is fired, no interesting value is returned, etc.

Answer 1

It is just a custom event, which can be also be fired .

However, the problem you got in your test is that you need to pass a function as a handler, not undefined which the invoked alert() returns. Try these:

el.addEventListener("foo", alert.bind(window, "hello!"));

or

el.addEventListener("bar", function(e){ alert("hello "+e.name); });