How to write an async function with a generic type without using const

Question

Is it possible to write this code without using export const, but rather export async function?

This is a sample of the code, heavily simplified:

interface Product {
  id: string;
  name: string;
}

type Listener<Subject, Payload extends object> = (event: {
  subject: Subject;
  payload?: Partial<Payload>;
}) => Promise<void>;

export const onEvent: Listener<Product, never> = async (event) => {
  await new Promise(function (resolve) {
    
    // a simplified example of using the parameters
    console.log(event.subject.id, event.subject.name);

    setTimeout(resolve, 1000);
  });
};

I'm wondering if it's possible to write this in the form of

export async function onEvent ... {

Without breaking it down into, for example

export async function onEvent(event: { subject: Product; payload?: never }): Promise<void> {

It doesn't seem like it after reading the current Typescript docs.

Is Listener part of your codebase or is it imported from a library? — Valentin
– Valentin, Commented Feb 3, 2023 at 16:46

matt helliwell · Accepted Answer · 2023-02-03 19:47:42Z

1

Basically, no there isn't. With the 'function' keyword, the type is derived from the arguments and return type, you can't separately specify the type. You could do

export const onEvent: Listener<Product, never> = async function (event) {...}

but that's about it.

answered Feb 3, 2023 at 19:47

matt helliwell

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