[Feature Request Svelte4] Use labels for mount and destroy callbacks instead of functions.

[TheCymaera]

Describe the problem

Lifecycle callbacks are currently registered using functions that are imported from the svelte runtime.

import { onDestroy } from "svelte";
onDestroy(()=>{
    // do clean up here...
});

Describe the proposed solution

It would be more concise to use labels, similar to reactive statements. It may also more performant since the callbacks are known at compile-time.

onDestroy: {
    // do clean up here...
}

Alternatives considered

If the Explicit Resource Management API is accepted, they could be used for cleanup as well.

use timer = createTimer(callback);

Importance

nice to have

[jrmoynihan]

I see a couple thumbs down, but haven't seen any reasoning why this wouldn't be beneficial. I'm all for more terse syntax for the same functionality.

Is there anything potentially lost by using a labeled statement (that gets compiled to a function) instead of using the lecture funding we have now?

[brunnerh]

• The Svelte team does not want different syntax to achieve the same thing and replacing the existing syntax is probably not worth it (the gain is not significant enough)
• You cannot return a value from a labeled statement (and the callback returned from onMount is different from registering it with onDestroy - the latter runs on the server)
• There are alternatives:
  • If you don't like adding imports for onMount/onDestroy, there are build system plugins that can automatically add those
  • This does not need to be supported by Svelte itself, one can probably implement this fairly easily via a Svelte pre-processor

[baseballyama]

AFAIK we can not declare types for label statements at least without a hack so it may be weak for types. And if we support the label statement for that, we definitely need to drop exist onDestroy function because we should have 1 way to do this. But it is more painful than benefits for users because users need to do migration tasks.

Regarding performance, I think Svelte is compiler, so we can do what we want to do.