Update TypeScript typings

[aikoven]

This PR continues work from #2563 and fixes #1648, #2602 and #2740.

Tests are rewritten to cover different supported approaches. All TS strict checks are enabled in tests.

Reducers

• Reducers support simple definition where action shape is not type-checked to aid the transition from JS to TS.
• Discriminated union for actions is still supported as well.
• A test is added for writing reducers with type guards.
• Typings for combineReducers are updated to allow strict combination using discriminated unions, as well as combining custom and third-party reducers.

See tests.

Dispatch

• Default Dispatch type only accepts plain actions. It also supports restricting action types using discriminated unions.
• Extra signatures are added using intersection type e.g. Dispatch & PromiseDispatch.

See tests and tests for injected dispatch e.g. connect from react-redux.

Middleware

• Middleware type supports declaring extra Dispatch signatures which are added to the Store type by applyMiddleware enhancer.
• Middleware type supports restricting the state type.

See tests.

Store Enhancers

• Store enhancer type supports extending Store type. Particularly, Dispatch signature is extending using this mechanism. It also supports for adding custom methods.
• Support for enhancers extending the state type.

See tests.

cc. @pelotom @KingHenne

[ikokostya]

Actually, this is not needed, because any interface with additional properties extends Action interface.

[aikoven]

The default type for A being AnyAction is needed so that any reducer acting on the state State would be assignable to Reducer<State> (with omitted action type parameter).

You may want to change it back to Action and check out the tests that break.

[ikokostya]

Ah, I see now. Thanks.

[timdorr]

Since no one has come in and yelled "Holy balls, this is terrible!", I'm going to assume this is at least somewhat of an improvement and merge it in.

I swear, I'll learn TS at some point...

[jayphelps]

Edited because it unintentionally could have been read as hostile. Not intended! 🙂

---

Hey folks 👋 I'm curious if someone could elaborate on the decision to change the order of the generic params MiddlewareAPI<State, Dispatch> was reversed in v4 to MiddlewareAPI<Dispatch, State>? It means that everyone who wants to type their State must provide a Dispatch type so they can get to the second type argument. It seems like it would have been a breaking change for the average TS person upgrading.

[aikoven]

@jayphelps Old MiddlewareAPI of Redux v3 didn't have Dispatch parameter:

export interface MiddlewareAPI<S> {
  dispatch: Dispatch<S>;
  getState(): S;
}

The new one has a default for it:

export interface MiddlewareAPI<D extends Dispatch = Dispatch, S = any> {
  dispatch: D;
  getState(): S;
}

If you don't need a custom dispatch signature, you can just use the built-in Dispatch type: MiddlewareAPI<Dispatch, MyState>, or even have it inferred for you, see the test for middleware that expects exact state type.

It seems like it would have been a breaking change for the average TS person upgrading.

That's why we have bumped a major version 🙂