Issue with void Param after Typescript 3.4

[jasonvasquez]

Hello,

I've been using typescript-fsa-redux-thunk combined with typescript-fsa-reducers for quite some time now very happily - thankyou!

Today, I upgraded several of my projects to Typescript 3.4, at which point many of my thunk-based actions suddenly were not working properly in my reducers. Specifically, I have simple async actions defined somewhat like:

const doThing = createAsyncAction<void, number>("DO_THING",
  async () => {
    return 1;
  },
);

with reducers structured like:

...
.case(doThing.async.done, (state, {result}) => ({
  ...state,
  newValue: result,
}))
...

This worked fine in the past, I was able to use them simply in calls like dispatch(doThing.action()), reducers worked as expected.

Today, I can define the actions fine, but in the reducers, I receive this typescript compilation error:

Type '(action: AnyAction) => action is Action<Failure<void, Error>>' is not assignable to type '(action: AnyAction) => action is Action<{ params: void; } & { error: Error; }>'.
  Type predicate 'action is Action<Failure<void, Error>>' is not assignable to 'action is Action<{ params: void; } & { error: Error; }>'.
    Type 'Action<Failure<void, Error>>' is not assignable to type 'Action<{ params: void; } & { error: Error; }>'.
      Type 'Failure<void, Error>' is not assignable to type '{ params: void; } & { error: Error; }'.
        Type '{ params?: void | undefined; } & { error: Error; }' is not assignable to type '{ params: void; } & { error: Error; }'.
          Type '{ params?: void | undefined; } & { error: Error; }' is not assignable to type '{ params: void; }'.
            Property 'params' is optional in type '{ params?: void | undefined; } & { error: Error; }' but required in type '{ params: void; }'.

It looks like it is due to this rule in typescript-fsa:

export type Success<Params, Result> =
  ({params: Params} | (Params extends void ? {params?: Params} : never)) &
  ({result: Result} | (Result extends void ? {result?: Result} : never));

Basically, Params extends void fires here, meaning params should be defined as params?, but I think they're being passed in as params: void.

I hope I interpreted that issue correctly. If I have a type other than void for the thunk param, then everything compiles fine. This code worked prior to Typescript 3.4, so I'm presuming a more thorough type checking has occurred here to catch this particular one.

Relevant packages:

"typescript": "^3.4.5",
"typescript-fsa": "^3.0.0-beta-2",
"typescript-fsa-reducers": "^1.2.1",
"typescript-fsa-redux-thunk": "^2.0.0-beta.11",

Let me know if I can provide any other details!

[xdave]

@jasonvasquez this is a known problem with how the typings of typescript-fsa work with a feature for this project that allows optional parameters. I've offered some insight upstream, but there's been little to no activity (aikoven/typescript-fsa#56).

For this, there's a couple of different solutions for what to change on the doThing thunk:

1. Leave out the type parameters on createAsyncAction (everything is inferred):

const doThing = createAsyncAction("DO_THING",
  async () => {
    return 1;
  },
);

2. Or, use {} instead of void:

const doThing = createAsyncAction<{}, number>("DO_THING",
  async () => {
    return 1;
  },
);

Let me know if this works for you

[jasonvasquez]

Alright, cool - thanks!

[septs]

@xdave in TypeScript 3.6 use unknown replace void?

createAsync<unknown, number>("example")

[xdave]

I just implemented a fix for the return type, I could probably do the same for the params...

[septs]

Maybe this is the right way

const doThing = createAsyncAction("DO_THING",
  async (): Promise<number> => {
    return 1;
  },
);

[xdave]

Perhaps. New eslint seems to like to enforce doing that.