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Promises and async/await

Promises represent the eventual result of an operation. Know how settlement, chaining, the combinators and async/await work, and why promises can't cancel or restart the work behind them.

Key points

1
A promise settles once: later resolve/reject calls are ignored. The executor runs synchronously, and a throw inside it becomes a rejection.
2
Every then/catch/finally returns a new promise. Returned values fulfil it, thrown errors reject it, and returned promises are adopted. Adopting a returned native promise costs two extra microtasks.
3
all fails fast, allSettled reports every outcome, race copies the first settlement, and any waits for the first success or rejects with an AggregateError. None of them cancels the losers.
4
Async functions always return a promise and run synchronously until the first await. Inside try, use return await so rejections reach catch.
5
Start independent work before awaiting it, but combine the promises with Promise.all so every rejection has a handler.
6
Cancellation is cooperative: pass an AbortSignal to APIs that accept one, check signal.throwIfAborted() in your own loops, and use AbortSignal.timeout/any for deadlines.
7
To retry, call a function again; a promise is a finished result and can't be re-run.

Common traps

array.forEach(async ...) does not wait for the callbacks, and loses their rejections.
A timeout built with Promise.race leaves the original work running, and in Node.js the timer can keep the process alive.
A promise that rejects before anything awaits it is an unhandled rejection. Since Node 15 that crashes the process by default.

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