Advanced Async Patterns — Series 3

Preview — 3 of 10 questions

What is logged?

javascript

const user = new AbortController();
const signal = AbortSignal.any([user.signal, AbortSignal.timeout(1000)]);

const p = new Promise((_, reject) => {
  signal.addEventListener('abort', () => reject(signal.reason));
});
p.catch((e) => console.log(e.name));

user.abort();

ATimeoutError, after about 1000 ms

BNothing — a composed signal only fires when every input signal aborts.

CAbortError, immediately

DTypeError: AbortSignal.any is not a function

A long computation is split into chunks so the page can render (or the server can serve I/O) between them. Which scheduling primitive actually yields?

AA macrotask — setTimeout(fn, 0), MessageChannel, or scheduler.postTask() — because the event loop only continues once the microtask queue is empty.

BqueueMicrotask(fn), because microtasks have the highest priority and therefore run in their own turn.

CPromise.resolve().then(fn), because a promise callback is asynchronous and so releases the loop.

DNothing yields on its own; only a Web Worker can, because JavaScript is single-threaded.

What is logged, and is there an unhandled rejection?

javascript

const boom = () => Promise.reject(new Error('b'));

Promise.all([Promise.reject(new Error('a')), boom()]).catch((e) => console.log('caught:', e.message));

Acaught: b, and no unhandled rejection.

Bcaught: a, plus an unhandled rejection warning for b.

Ccaught: a and caught: b.

Dcaught: a, and no unhandled rejection.

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