14. async/await: sugar over promises, and its sharp edges

async/await doesn't change the underlying microtask machinery — it's syntax that makes promise chains read like synchronous code. An async function always returns a Promise; await pauses the function (not the whole program) until the awaited promise settles.

async function loadDashboard(userId) {
  try {
    const user = await getUser(userId);       // pauses here, yields to the event loop
    const posts = await getPosts(user.id);    // then here
    return { user, posts };
  } catch (err) {
    // catches rejections from EITHER await above — same benefit as .catch()
    throw new Error(`Dashboard load failed: ${err.message}`);
  }
}

The sharp edge: sequential awaits that don't depend on each other serialize work that could run concurrently.

// SLOW: each await blocks the next from even starting — total time = sum of both
const user = await getUser(id);
const settings = await getSettings(id);   // doesn't need `user` — but waits anyway

// FAST: start both requests immediately, await their results together
const [user, settings] = await Promise.all([getUser(id), getSettings(id)]);