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fix: update onMount type to allow async to return any #8714

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Merged
merged 2 commits into from
Jun 19, 2023
Merged

fix: update onMount type to allow async to return any #8714

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5ee4cc8
fix: update onMount type to allow async to return any
GrygrFlzr Jun 8, 2023
e397d65
make type private
dummdidumm Jun 19, 2023
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10 changes: 7 additions & 3 deletions packages/svelte/src/runtime/internal/lifecycle.js
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,12 @@ export function beforeUpdate(fn) {
get_current_component().$$.before_update.push(fn);
}

/**
* Anything except a function
* @template T
* @typedef {T extends Function ? never : T} NotFunction
*/

/**
* The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM.
* It must be called during the component's initialisation (but doesn't need to live *inside* the component;
Expand All @@ -36,9 +42,7 @@ export function beforeUpdate(fn) {
*
* https://svelte.dev/docs#run-time-svelte-onmount
* @template T
* @param {() => T extends Promise<() => any>
* ? "Returning a function asynchronously from onMount won't call that function on destroy"
* : T} fn
* @param {() => NotFunction<T> | Promise<NotFunction<T>> | (() => any)} fn
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@GrygrFlzr GrygrFlzr Jun 8, 2023 •
edited
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I'm actually surprised this works as expected.

Consider that NotFunction<T> currently only negates Function. In theory, this should not exclude Promise<T>. Indeed, if we change the onMount type definition to:

function onMount<T>(
    fn: () => NotFunction<T> | (() => any)
): void

fn can now be anything (including a Promise!), because the union of (not a function) and (a function) should logically be any. For some reason, a further union with Promise<NotFunction<T>> successfully narrows the type to exclude promises which return functions, and yet a standalone Promise<NotFunction<T>> type would block any non-async callback functions.

Is this behavior with TypeScript expected or a bug?
Even if it turns out to be a bug, we can instead narrow the conditional types:

type NotFunction<T> = T extends Function ? never : T;
type NotFunctionOrPromise<T> = T extends (Function | Promise<T>) ? never : T;
function onMount<T>(
    fn: () => NotFunctionOrPromise<T> | Promise<NotFunction<T>> | (() => any)
): void

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@baseballyama baseballyama Jun 9, 2023 •
edited
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I think this is because the return type of the below code is Promise<Fuinction>.

onMount(async () => {
	return () => {}
});

It means below code doesn't have a type error.

type NotFunction<T> = T extends Function ? never : T;
type A<T> = () => NotFunction<T>

function onMountA<T>(param: A<T>) {
    return param()
}

onMountA(async () => {
	return () => {}
});

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Was trying to wrap my head around this, and from this playground and hovering over the infered types it makes sense why this works - T is infered to be a certain type, and that is put into each of those, and if there's something left, it works. It works because through Promise<NotAFunction<T>> the T is narrowed differently to () => void instead of Promise<() => void>

* @returns {void}
*/
export function onMount(fn) {
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9 changes: 8 additions & 1 deletion packages/svelte/test/types/on-mount.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -51,8 +51,15 @@ onMount(async () => {
};
});

// @ts-expect-error async and return any
// async and return any
onMount(async () => {
const a: any = null as any;
return a;
});

// async and return function casted to any
// can't really catch this without also catching above
onMount(async () => {
const a: any = (() => {}) as any;
return a;
});