EventTarget.addEventListener lacks a proper definition on how to handle invalid callback

[denschub]

EventTarget.addEventListener specifies that browsers should terminate and ignore the call if the callback is null:

If callback is null, terminate these steps.

However, it's unclear about what should happen if the callback is not a function/not a callable.

Take the following source, which is clearly a misuse:

document.addEventListener("click", 42);

The callback is not null, so according to the spec, there is no reason to ignore the addEventListener call here. However, browsers behave different. In Chrome, the call simply gets ignored, acting as if the callback would be null. In Firefox, however, we throw an exception since a Number is not callable.

To avoid further issues and to make it easier for vendors to adjust their implementations, a note should be added to the spec, saying that the call should be terminated if the callback not callable.

[domenic]

That is governed by the Web IDL specification which is used to define the method.

[annevk]

Yeah, that would be a bug in Chrome. Could you please file one and leave a link here?

[denschub]

Fill file a Chrome bug. Also, Safari Technology Preview also throws, so this is the way to go. I'll throw a link here to the Chrome report, but I'll close this issue nonetheless, since it is actually covered in the WebIDL (sorry, did miss that).

[denschub]

Was already reported, see Chromium bug 676845.

[annevk]

Thanks @denschub!