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fix: update onMount type definition to prevent async function return #8136

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merged 9 commits into from
Apr 18, 2023
+120 −59
Merged

fix: update onMount type definition to prevent async function return #8136

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7ea02c8
update onmount typing to prevent async function return
chrskerr Dec 22, 2022
5522a35
Merge remote-tracking branch 'upstream/master' into onMount-type-prev…
baseballyama Feb 23, 2023
eb97b53
enhance type
baseballyama Feb 23, 2023
1dec6e4
simplify
baseballyama Feb 23, 2023
f7c2a66
force conflict so I can merge version-4 into this
dummdidumm Apr 14, 2023
c941525
Merge branch 'version-4' into onMount-type-prevent-async-function-return
dummdidumm Apr 14, 2023
cda63cf
docs, test
dummdidumm Apr 14, 2023
5c15082
lint
dummdidumm Apr 14, 2023
1246260
Update src/runtime/internal/lifecycle.ts
dummdidumm Apr 18, 2023
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2 changes: 1 addition & 1 deletion src/runtime/internal/lifecycle.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,7 @@ export function beforeUpdate(fn: () => any) {
*
* https://svelte.dev/docs#run-time-svelte-onmount
*/
export function onMount(fn: () => any) {
export function onMount(fn: (() => () => void) | (() => void | Promise<void>)) {
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Suggested change
export function onMount(fn: (() => () => void) | (() => void | Promise<void>)) {
export function onMount(fn: (() => () => any) | (() => any | Promise<any>)) {

In this case, this will be a type error, right?
But fundamentally, there is no problem.

// api.js
export const fetch = () => Promise.resolve({foo: 'bar'});

// Component.svelte
<script>
  import { onMount } from 'svelte';
  import { fetch } from './api';
  onMount(() => fetch())
</script>

So IMO, it's better to use any type instead of void.

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Thanks for the suggestion but I think that this won't work to prevent a function return from an async onMount.

I am attempting to cause a typescript failure on arguments of type () => Promise<() => any>, but the introduction of () => any means that this remains a valid argument.

Given I only actually want to disallow this single type, perhaps the following is more appropriate:

type AnyExcept<Type, Disallowed> = Type & (Type extends Disallowed ? never : Type);
export declare function onMount<T>(fn: () => AnyExcept<T, Promise<() => any>>) {

Testing locally, this allows your example above and any other return from onMount except for the following:

onMount(async() => { // fails 
   return () => {
      /// anything
   }
})

get_current_component().$$.on_mount.push(fn);
}

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