CODEWITHSUNDEEP
typescripttypes
Code Worm
Code Worm

Reputation: 323

What does the following TypeScript expression mean?

I am wondering what does the following expression mean (<T = string>() => Promise<T>) in the following context:

type Src = string | (<T = string>() => Promise<T>);

And used in the following function:

const handleSource = async (source: Src): Promise<void> => {
    if (typeof source === 'string') {
      setState((s) => ({ ...s, src: source }));
      return;
    }

    try {
      const result = await source();
      setState((s) => ({ ...s, src: result }));
    } catch (e) {
      // eslint-disable-next-line no-console
      console.warn('[Image] handleSource', e);
      setState((s) => ({ ...s, isLoadingError: true }));
    }
  };

Upvotes: 1

Views: 490

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1073968

It means that Src can either be

  • a string, or
  • a function that accepts a generic parameter T (which defaults to string) and returns a promise using that generic parameter to say that if the promise is fulfilled, that's the type the fulfillment value will have

So for example, you could have a function that looked like this:

function example1(src: Src) {
    if (typeof src === "string") {
        return src;
    }
    return src();
}

...that returns a string or a Promise<string>, or like this:

function example2(src: Src) {
    if (typeof src === "string") {
        return src;
    }
    return src<number>();
}

...that returns a string or a Promise<number> (because of the generic type parameter on the call to src).

Upvotes: 5

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