How do async and await work under the hood?

async / await is syntactic sugar over Promises. It does not create new threads — it makes asynchronous code read like synchronous code while still being non-blocking.

async function loadUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`)
    if (!res.ok) throw new Error(res.statusText)
    return await res.json()
  } catch (err) {
    console.error(err)
    throw err // re-throw to reject the returned Promise
  }
}

Under the hood

1. An async function always returns a Promise — even if you return 42, callers get Promise.resolve(42).
2. await expression pauses the async function until the Promise settles.
3. The continuation after await is scheduled as a microtask, not a new thread.
4. Errors thrown inside async reject the returned Promise; use try/catch for local handling.

Parallel vs sequential

// Sequential — slower
const a = await fetchA()
const b = await fetchB()

// Parallel — start both, await together
const [a, b] = await Promise.all([fetchA(), fetchB()])

Interview tip: await only "blocks" the current async function, never the main thread or call stack.