Unable to fulfill the return type requirement in a typed TypeScript function

In this scenario, let's consider the following type:

  export type Transformer = <T extends any[], U>(
    data: T,
  ) => U;

Now, let's examine a function that needs to adhere to this type:

export const transform: Transformer = (
  data: Result[]
): { data:Result[] } => {
  if (!data) {
    return { data: [] };
  }


  data.forEach((record:Result) => {
    record.extraStuff = {
      foo: 'bar'
    };
  });

  return { data: data };
};

The error message from the compiler states:

Type '(data: Result[]) => { data: RecordMatchingSearchResult[]; }' is not assignable to type 'Transformer'.
  Type '{ data: Result[]; }' is not assignable to type 'U'.

Should a generic constraint be added to U to allow for proper inference?

Another observation is the absence of explicitly defining the type, leading to confusion regarding the acceptance of generic arguments exclusively within the function.

typescript

Answer №1

Your function faces an issue of lack of generality. While the function signature is generic ( Transformer ), the assigned function does not adhere to this generic nature.

The main problem lies in the return type requirement, as it must be specified by the caller. If your current assignment were allowed, a runtime error would occur when trying to access a property that was not part of the expected return type:

let result = transform<Result[], { other: number }>([]);
result.other // this would cause an error

To address this, consider fixing the type for the result and making the function implementation itself generic:

export type Transformer = <T extends { extraStuff: any }>(
    data: T[],
) => { data: T[] };

class Result {
    extraStuff: any;
}
export const transform: Transformer = <T extends { extraStuff: any }>(data: T[]): { data: T[] } => {
    if (!data) {
        return { data: [] };
    }


    data.forEach((record: T) => {
        record.extraStuff = {
            foo: 'bar'
        };
    });

    return { data: data };
};

Alternatively, you can move the generics from the function signature to the type, allowing you to specify type arguments when declaring the transform function. This way, the function itself does not need to be generic:

export type Transformer<T extends { extraStuff: any }, U> = (
    data: T[],
) => U;

export const transform: Transformer<Result, { data: Result[] }> = (data: Result[]): { data: Result[] } => {
    if (!data) {
        return { data: [] };
    }


    data.forEach((record: T) => {
        record.extraStuff = {
            foo: 'bar'
        };
    });

    return { data: data };
};

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