Fix type inference of the action callback arguments

[kubk]

Fixes: #2211

• Added typescript typings
• Verified that there is no significant performance drop (npm run test:performance)

I prepared the links to TS playground with multiple test cases so you can verify that type inference works now.

Example 1 - No changes were made from my side. I copy-pasted types of the IActionFactory from here:

mobx/src/api/action.ts

Line 12 in db4b3e7

export interface IActionFactory {

As you can see, the type inference doesn't work here, the first argument of the action callback has type unknown. Moreover, TypeScript fails to compile actions with 2 more arguments when --noImplicityAny is enabled (default in Create React App).

Example 2 - All function overloads were removed. Now TypeScript is able to infer types of function arguments. There is only one problem left: type inference doesn't work with promises.

Example 3 - <T extends Function> is replaced with <T extends Function | null | undefined>. This change fixes the problem with promises. It looks like type ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null is not a subtype of type Function.

[danielkcz]

That's really interesting! I always thought this overload hell is a necessary evil. It's nice to see there are some ways around. Although now it depends if it doesn't break some other edge cases we cannot think of.

[kubk]

To prevent typing regressions in the future we can cover TS types with tests. It looks like Mobx already includes tests for the Flow typings: https://github.com/mobxjs/mobx/blob/master/test/flow/test.js__FIXTHIS
There are multiple packages that can help us:

• https://github.com/cartant/ts-snippet
• https://github.com/SamVerschueren/tsd

Will be happy to help with this task.

[danielkcz]

I definitely support that idea, go ahead, please :) Probably in different PR first and then we can continue here.

Not sure which package to choose, have no experience with either.