Nullish event handler produces TypeError

[vkurko]

Describe the bug

Some event handlers, such as onmouseenter or onmouseleave, throw an error when set to null/undefined instead of not registering the event handler.

Reproduction

https://svelte.dev/playground/fa643cc4e0ed4434bb6ba9bc5a5a8ec0?version=5.19.1

Logs

TypeError
TypeError: Cannot read properties of undefined (reading 'call')
    at eval (playground:output:2036:21)
    at without_reactive_context (playground:output:2004:11)
    at HTMLButtonElement.target_handler (playground:output:2035:12)

System Info

Svelte 5.19.1 (Playground)

Severity

blocking an upgrade

[Ocean-OS]

The reason for that is if you look at the compiled output, you'll notice that for onclick, it assigns a property of the button to the event handler, while with the other event, it calls a function. In the function, it runs Element.addEventListener, which throws if the provided handler is not a function. You shouldn't have a nullish event handler (partly because there's no reason to).

[vkurko]

partly because there's no reason to

Well, just because you can't think of a use case doesn't mean there's no reason.

You shouldn't have a nullish event handler

This worked fine in Svelte 4 (see this feature request). And I didn't find in the documentation that there should be any changes in this regard in Svelte 5. On the contrary, the documentation clearly says that events are just attributes, the same rules as for attributes apply. And for attributes it says that attributes are included unless their value is nullish (null or undefined).

Based on this, I believe that either this is a regression in Svelte 5 and needs to be fixed, or the fact that some events are now registered even with nullish handler needs to be documented.

[paoloricciuti]

I agree that this needs to be fixed both for consistency and because it might be good to remove a listener once not needed.

Gonna fix this later.

[vkurko]

@paoloricciuti Thank you for the quick fix of the bug.