How does `async/await` work under the hood?

Short Interview Answer

An `async` function always returns a Promise; `await` pauses the function's progress until the awaited promise settles, then resumes with the value or throws.

Detailed Explanation

`async function` wraps the return value in a resolved promise (or rejects if you throw). `await` desugars roughly to promise `.then` continuation of the rest of the function (similar to a state machine). While awaiting, the call stack unwinds so the event loop can run other work. Errors from awaited rejections become throw-able in try/catch. `await` on non-promises wraps them via `Promise.resolve`. Top-level await exists in modules. Avoid awaiting in serial loops when parallelism is safe.

Example

async function getJson(url) {
  const res = await fetch(url);
  if (!res.ok) throw new Error(String(res.status));
  return res.json();
}

Interview Tip

State 'async functions always return promises' clearly.

Common Mistake

Forgetting to `await` or return a promise inside `async`, causing race bugs.