Creating type definitions in TypeScript for an object received from an API with an unknown data type, and attempting to enforce specific types on it

Currently in the process of learning TypeScript, so please bear with me if I am not describing this accurately.

I have a function that retrieves some data and returns a nested object. I then iterate over this object using forEach method. Although I have defined types for each value in the object, I am encountering an error:

Error: Argument of type '(value: RaceInfo) => void' is not compatible with parameter of type '(value: unknown, index: number, array: unknown[]) => void'. The types of parameters 'value' and 'value' do not match. Type 'unknown' cannot be assigned to type 'RaceInfo'.ts(2345)

Please refer to the code snippet below:

interface Selection {
    date: string;
    time: string;
    venue: string;
}
interface RaceInfo {
     codes: {
         race_sc: string;
     };
     details: {
         venue: string;
  };
}
export default async function getSC(selection: Selection) {
  let sc;

  await fetch(`URL`,{method: "GET",})
    .then((response) => response.json())
    .then((data) => {
      if (data.success) {
        Object.values(data.races).forEach((value: RaceInfo) => {
          if (
            value.codes.race_sc.slice(-4) === selection.time &&
            value.details.venue === selection.venue
          ) {
            sc = value.codes.race_sc;
          }
        });
      } else {
        console.log("Could not fetch the races");
      }
    })
    .catch((error) => {
      console.log("Error:", error);
    });

  return sc;
}

I was able to resolve this issue by modifying the loop code itself, which works perfectly fine. However, I am unsure if this approach aligns with strict TypeScript standards since I did not have to specify types/interfaces for the returned object. Is this considered an acceptable practice?

for (const key of Object.keys(data.races)) {
     const value = data.races[key];
     if (
        value.codes.race_sc.slice(-4) === selection.time && 
        value.details.venue === selection.venue
     ) {
        sc = value.codes.race_sc;
        }
     }

Answer №1

The specified value is considered to be of the type unknown. Clearly defining it as type RaceInfo would compromise the callback's type safety. In the eyes of the compiler, any value with an unknown type can be provided to the callback function. The issue you're facing prevents you from actually updating the definition to a function that anticipates a value with type RaceInfo.

If you are confident that Object.values(data.races) will always reflect a type of RaceInfo[], then employing a type assertion is necessary.

(Object.values(data.races) as RaceInfo[]).forEach((value) => /* ... */)

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