How can TypeScript be forced to output a specific data type?

I've created a generic "fetcher" function that is designed to handle different types of entities. However, I'm encountering an issue where TypeScript is inferring the return type based on all possible conditions within the function.

Is there a way to explicitly specify the return type in order to avoid this ambiguity?

(initialNotes and initialTags represent two distinct array types)

  async function fetcherFunc(entity) {
    await new Promise((resolve) => setTimeout(resolve, 2000));
    if (entity === 'notes') {
      return initialNotes;
    } else if (entity === 'tags') {
      return initialTags;
    }
  }

Answer №1

To handle the return values effectively, you have two options: either specify a union type (

TypeOfNotesArray | TypeOfTagsArray
), or consider using a function overload in case entity is known at compile time:

async function fetcherFunc(entity: "notes"): TypeOfNotesArray;
async function fetcherFunc(entity: "tags"): TypeOfTagsArray;
async function fetcherFunc(entity: "notes" | "tags"): TypeOfNotesArray | TypeOfTagsArray {
    await new Promise((resolve) => setTimeout(resolve, 2000));
    if (entity === 'notes') {
        return initialNotes;
    } else if (entity === 'tags') {
        return initialTags;
    }
    throw new Error(`entity should be either "notes" or "tags", but found "${entity}"`);
}

By calling fetcherFunc("notes"), you will get TypeOfNotesArray. And with fetcherFunc("tags"), the returned value will be TypeOfTagsArray.

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