Type 'string' is not assignable to type '"foo"'

Question

I have this code:

const baseUpstreamImpl = (
  client: IClient,
): ProtocolExtension<
  {
    foobar: (buff: Buffer) => Promise<string>;
  }
> => ({
  name: 'base upstream impl',
  type: 'upstream',
  messageCreators: {
    foobar: (buff: Buffer)  =>
      Promise.resolve({
        type: 'foobar',
        payload: buff,
      }),
  },
});

This . is the error I get for the foobar function:

[ts]
Type '(buff: Buffer) => Promise<{ type: string; payload: Buffer; }>' is not assignable to type '(...args: any[]) => Promise<IMessage<{ foobar: (buff: Buffer) => Promise<string>; }, "foobar">>'.
  Type 'Promise<{ type: string; payload: Buffer; }>' is not assignable to type 'Promise<IMessage<{ foobar: (buff: Buffer) => Promise<string>; }, "foobar">>'.
    Type '{ type: string; payload: Buffer; }' is not assignable to type 'IMessage<{ foobar: (buff: Buffer) => Promise<string>; }, "foobar">'.
      Types of property 'type' are incompatible.
        Type 'string' is not assignable to type '"foobar"'.
baseUpstreamImpl.ts(11, 5): The expected type comes from property 'foobar' which is declared here on type '{ foobar: (...args: any[]) => Promise<IMessage<{ foobar: (buff: Buffer) => Promise<string>; }, "foobar">>; }'
(property) foobar: (...args: any[]) => Promise<IMessage<{
    foobar: (buff: Buffer) => Promise<string>;
}, "foobar">>

And here are my types:

type EnforcePromise<T, P = any> = T extends Promise<P> ? T : Promise<T>;

type Promised<T> = {
  [P in keyof T]: (...args: InferArgs<T[P]>) => EnforcePromise<InferType<T[P]>>
};

type Filter<T, Cond, U extends keyof T = keyof T> = {
  [K in U]: T[K] extends Cond ? K : never
}[U];

type EventKey<T> = Filter<T, (...args: any[]) => any>;


interface IMessage<T, K extends EventKey<T> = EventKey<T>> {
  type: K;
  payload: InferArgs<T[K]>;
}


export interface ProtocolExtension<T, U, R = {}> {
  name: string;
  type: 'upstream' | 'downstream';
  handlers: Promised<T>;
  messageCreators: {
    [K in keyof U]: (
      ...args: any[]
    ) => Promise<IMessage<U>>
  };
}

I added the missing types, still can't get your error, since I get Generic type 'ProtocolExtension<T, U, R>' requires between 2 and 3 type arguments. (which is true) could you update your question with a snippet that reproduces your issue ? — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Oct 7, 2018 at 16:09

Titian Cernicova-Dragomir · Accepted Answer · 2018-10-07 18:56:38Z

1

There are two problems in the code both related to the way typescript infers types for literals.

The first one (and the error message you have) is the fact that type: 'foobar' will infer type to be string (in the absence of an imediate reason to infer it to the string literal type 'foobar'). The solution to this is to use a type assertion to the string literal type manually.

After we fix this issue we have a new error on the payload. payload should be InferArgs<T[K]> so it should be tuple with the parameter of the function. You only assign a single value to it. If we were to write [buf] we would have a similar problem to the string literal type issue above. Namely array literal would be typed as an array not a tuple (again in the absence of an imediate reason to type it as a tuple type).

There is also a third issue, the absence of the handelrs property, but taht is probably just an oversight.

const baseUpstreamImpl = (
    client: {},
): ProtocolExtension<
{},
{
    foobar: (buff: {}) => Promise<string>;
}
> => ({
    name: 'base upstream impl',
    type: 'upstream',
    handlers: null as any,
    messageCreators: {
        foobar: (buff: {}) =>
            Promise.resolve({
                type: 'foobar' as 'foobar',
                payload: [buff] as [{}],
            }),
    },
});

answered Oct 7, 2018 at 18:56

Titian Cernicova-Dragomir

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2 Comments

Vinz243 Over a year ago

So the as foobar was what I was missing ^^ typescript can be tricky Thanks !

Titian Cernicova-Dragomir Over a year ago

@Vinz243 well, I listed all the things I had to do to get it to compile, that is why a snippet that reproduces the problem and is close to your actual code helps :)

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