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).