Typed async code in TypeScript

Promise return types and asserting fetched shapes.

interface Repo {
  name: string;
  stars: number;
}

async function fetchRepos(user: string): Promise<Repo[]> {
  const response = await fetch(`/api/users/${user}/repos`);
  if (!response.ok) throw new Error(`status ${response.status}`);
  return (await response.json()) as Repo[];
}

async function total(user: string): Promise<number> {
  const repos = await fetchRepos(user);
  return repos.reduce((sum, repo) => sum + repo.stars, 0);
}

How it works

Promise<Repo[]> types what the function resolves to.
as Repo[] asserts the shape of parsed JSON.
reduce stays type-safe over the typed array.

Keywords and builtins used here

Promise
as
async
await
const
function
if
interface
number
return
string
throw

The run, in numbers

Lines: 15
Characters to type: 425
Tokens: 123
Three-star pace: 105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 70 seconds.

Type this snippet

Step 1 of 2 in Async types, step 1 of 8 in Async & modules.