Async Functions & Promise Types

An async function always returns a Promise. Declare the resolved type inside Promise<T>.

interface User { id: number; name: string }

async function fetchUser(id: number): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  return (await res.json()) as User;
}

const user = await fetchUser(1); // user: User

Promise.all keeps a type per position, and try/catch errors are unknown under strict mode, so narrow them.

const [a, b] = await Promise.all([fetchUser(1), Promise.resolve("ok")]);
// a: User, b: string

try {
  await fetchUser(2);
} catch (e) {
  if (e instanceof Error) console.log(e.message);
}

Quick check

Quick check: What is the return type of `async function f(): Promise<number>`?

• number
• Promise<number>
• void
• unknown