The value returned from computed(() => ...) shouldn't have a set(...) method

[Nokel81]

Intended outcome:

Have strict and complete type safety to help with refactoring.

Actual outcome:

The return type of computed() and observable.box() both have set() and get() methods, but the return type of computed() doesn't have a set() method so a type error is thrown at run time.

How to reproduce the issue:

const myObservableValue = observable.box(10);
const myComputedValue = computed(() => myObservableValue.get() + 1);

/**
 * Uncaught:
 * Error: [MobX] [ComputedValue 'ComputedValue@2'] It is not possible to assign a new value to a computed value.
 *    at die (/node_modules/mobx/dist/mobx.cjs.development.js:89:11)
 *    at ComputedValue.set (/node_modules/mobx/dist/mobx.cjs.development.js:1448:7)
 */
myComputedValue.set(12);

Versions

mobx -> 6.10.9

[mweststrate]

Please follow the issue template when filing a bug.

…

On Tue, 26 Sept 2023, 09:52 Sebastian Malton, ***@***.***> wrote: Current Behaviour: computed(...) currently returns the IComputedValue<T> type which has the following signature: interface IComputedValue<T> { get(): T; set(value: T): void;} However, actually using that .set(...) results in an error being thrown TypeError: cannot call function undefined. Expected Behaviour: computed(...) should return a type that only has the get() field as valid. Notes: I tried providing a set impl through the options for computed but the same was still the case versions: mobx -> 6.10.9 — Reply to this email directly, view it on GitHub <#3767>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAN4NBH7PPUFWTIWSMNNIGTX4LMYZANCNFSM6AAAAAA5HYOYPY> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[Nokel81]

Would you like me to recreate this filling then?

[mweststrate]

yes, or copy in the template.

[Nokel81]

done

[mweststrate]

The above example is working as expected if no custom setter is defined. There is a .set method, and you are able to call it successfully, it just throws because the preconditions aren't met, not because the method doesn't exist.

N.b. note that in general in an issue report a reproduction is something "runnable". For further follow up questions make sure to attach a codesandbox or something.

[Nokel81]

Sure, but won't it be better for computed(() => someObservable.get()) to not even allow .set() to be called via typescript?

[mweststrate]

Well, '.set' is a valid API of computed, that the runtime requirements aren't met isn't of typescripts concern. (could do some complicated juggling to make that detectable in TS in cases, but doubt that'd be worth the effort)

[Nokel81]

It is actually rather easy, see my playground here