TypeScript 3.6.4 incorrectly infers type from observable method

[nvigne]

I have a:

2. Issue:

• Provide a minimal sample reproduction. Create a reproduction based on this sandbox
• Did you check this issue wasn't filed before?
• Elaborate on your issue. What behavior did you expect?
• State the versions of MobX and relevant libraries. Which browser / node / ... version?

3. Idea:

• Do you think others will benefit from this change as well and it should in core package (see also mobx-utils)?
• Are you willing to (attempt) a PR yourself?

Hi,

I'm working with MobX and TypeScript and I notice that TypeScript incorrectly infers types from observable.

TypeScript version : 3.6.4 (LKG)

import { observable } from 'mobx'

const myObservable = observable({ id: 'myId' });
myObservable.id = 'newId'

The compilation error we get from TypeScript when trying to compile is the following:

error TS2339: Property 'id' does not exist on type 'ObservableSet<any>'.
myObservable.id = 'newId'

If we take a closer look at the issue, we can see that TypeScript infers that myObservable is typeof ObservableSet when it should be simply the type of the parameter pass to observable.

I can have a look to try to fix the issue on mobx side, the only condition is too being able to upgrade the TypeScript version up to 3.6.4

[nvigne]

In fact, it seems to repro on older version of TypeScript as well, I will try to investigate a bit more.

[nvigne]

The issue seems due to the fact that I was missing the lib es2016 in my tsconfig causing the TypeScript engine to consider the Set type as an object type. Closing for now. Maybe it may require to add some information when building a new project.

[lock]

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