# Async Functions Typed parameters on `async` functions, and how type errors travel through promises. An `async function` in TJS is a JavaScript async function with a checked signature. The parameters are validated when the function is called, the same as for any other TJS function. The return annotation describes the value the promise _resolves_ to, not the promise itself. As everywhere in TJS, the value after a colon is an _example_, not a type. `id: 'user-1'` means "`id` is a string, for instance `'user-1'`"; it does not mean only `'user-1'` is accepted. `ids: ['user-1']` means an array of strings, of any length. See [TJS Syntax](/tjs-syntax/) for the full rules. ```tjs:inline async function fetchUser(id: 'user-1'): { name: 'User user-1', email: 'user-1@example.com' } { // Stands in for a real request await new Promise(resolve => setTimeout(resolve, 100)) return { name: 'User ' + id, email: id + '@example.com' } } async function fetchUsers(ids: ['user-1']): [{ name: 'User user-1', email: 'user-1@example.com' }] { return Promise.all(ids.map(id => fetchUser(id))) } console.log(await fetchUsers(['alice', 'bob', 'charlie'])) ``` There is no `Promise<…>` spelling. You `await` the call and get the value, so the annotation shows the value. `Promise.all` composes as it does in JavaScript: `fetchUsers` starts one `fetchUser` per id and waits for all of them, and its own `ids` parameter is checked before any of them start. The last line uses `await` outside any function. That is top-level await: it works in ES modules and in the examples on this page, which run inside an async wrapper. A classic `