如何定义一个在打字稿中无法定义的变量？

Question

I am using typesafe-actions and I have the following declaration:

function getAction<Payload>():PayloadAC<string, Payload>  {
    const p = createStandardAction('FETCH')<Payload>();
    return p; // <------- Here is get typescript error
}

(This method is only an example that clarify my question, it is not a real function in my code)
This code produce typescript error:

Type '[Payload] extends [undefined] ? unknown extends Payload ? PayloadAC<"FETCH", Payload> : EmptyAC<"FETCH"> : PayloadAC<"FETCH", Payload>' is not assignable to type 'PayloadAC<string, Payload>'.
  Type '(unknown extends Payload ? PayloadAC<"FETCH", Payload> : EmptyAC<"FETCH">) | PayloadAC<"FETCH", Payload>' is not assignable to type 'PayloadAC<string, Payload>'.
    Type 'unknown extends Payload ? PayloadAC<"FETCH", Payload> : EmptyAC<"FETCH">' is not assignable to type 'PayloadAC<string, Payload>'.
      Type 'PayloadAC<"FETCH", Payload> | EmptyAC<"FETCH">' is not assignable to type 'PayloadAC<string, Payload>'.
        Type 'EmptyAC<"FETCH">' is not assignable to type 'PayloadAC<string, Payload>'.
          Property 'payload' is missing in type 'EmptyAction<"FETCH">' but required in type 'PayloadAction<string, Payload>'.ts(2322)
type-helpers.d.ts(45, 5): 'payload' is declared here.

Forget the fact that the error is very difficult to understand. Eventually I managed to understand that createStandardAction('FETCH')<Payload>() returns PayloadAC<"FETCH", Payload> or EmptyAC<"FETCH"> according to the checks [Payload] extends [undefined] ? unknown extends Payload ... [Payload] extends [undefined] ? unknown extends Payload ... .
The thing is that I don't really care wether Payload is undefined or not. I am fine with setting payload as undefined: {type: 'FETCH', payload: undefined } . But I don't know how to declare that in typescript.
This forces me to define the return value of the function as PayloadAC<string, Payload> | EmptyAC<string> PayloadAC<string, Payload> | EmptyAC<string> which causes me other problems, for example:

const action = getAction<MyType>();
const { payload } = action; // <--- Here I get an exception that payload may not exists

In this example I don't really mind to get payload as undefined but typescript is yelling at me.

How do I fix this mess?

Answer 1

Since you don't care if payload is undefined it sounds like the return type is indeed PayloadAC<string, Payload> | EmptyAC<string> PayloadAC<string, Payload> | EmptyAC<string> . Yes, you will get errors if you then try and access the payload. This makes it sound like you actually DO care if payload is undefined.

Also, there is no reason const { payload } = action; should give you an error if payload is undefined. Example here . So maybe you are doing something else with payload where a value is expected?