Output Quiz #8 — Promise.all, race, allSettled & any

Output:

["first", "second", "third"]

Why: Promise.all waits for every Promise in the array and returns their values in input order, not resolution order. Even if the second Promise resolved before the first, the array position is preserved. Here all three are already resolved, so the array is ["first", "second", "third"].

Output:

caught: error

Why: Promise.all is fail-fast — as soon as any Promise rejects, the whole thing rejects with that reason. The values from 1 and 3 are discarded. The .catch() (or try/catch with await) fires with "error". The other Promises are not cancelled — they continue running — but their results are ignored.

Output:

[]
0

Why: Promise.all([]) resolves immediately with an empty array — there are no Promises to wait for. This is a useful edge case when you're building the array dynamically and it might be empty. No microtask tick is needed; the resolution is synchronous within the Promise machinery.

Output:

fast

Why: Promise.race resolves (or rejects) with the first Promise that settles — regardless of whether it resolved or rejected. fast resolves after 100ms, slow after 200ms. fast wins. Once race has settled, the result of slow is permanently ignored.

Output:

catch: fail

Why: Promise.race doesn't prefer resolved Promises — it picks whichever settles first, including rejections. rejected settles at 100ms with a rejection, so the race result is a rejection. resolved settles later at 200ms but its result is ignored. Many developers assume race only picks resolved Promises — it doesn't.

Output:

3
rejected
fail

Why: Promise.allSettled never rejects. It waits for all Promises to settle and returns an array of descriptor objects — one per Promise, in input order. Fulfilled Promises give {status: "fulfilled", value: ...}. Rejected Promises give {status: "rejected", reason: ...}. This is the right tool when you want all results regardless of failures.

Output:

{"status":"fulfilled","value":42}
{"status":"rejected","reason":"oops"}

Why: The shape is fixed: fulfilled descriptors have status and value; rejected descriptors have status and reason. There is no value key on rejected descriptors and no reason key on fulfilled ones. This is important when you consume allSettled results — always check status before accessing value or reason.

Output:

any: first resolve

Why: Promise.any returns the first fulfilled Promise, ignoring rejections. The first element is a rejection — any skips it and waits for the next fulfillment. "first resolve" is the first to fulfill, so that's the result. Unlike Promise.race, a rejection does not settle any.

Output:

true
["e1", "e2", "e3"]

Why: When all Promises passed to Promise.any reject, it rejects with an AggregateError — a special Error subtype that holds all the rejection reasons in its .errors array, in input order. This is the only way Promise.any ever rejects. e.message is "All promises were rejected".

Output:

race catch: rejected first
any: resolved second

Why: This is the critical difference between the two:

• Promise.race stops on the first settlement — resolved or rejected. first is already rejected synchronously (the executor runs sync), so race immediately rejects with "rejected first".
• Promise.any stops on the first fulfillment. It skips the rejection and waits for second to resolve after 100ms, then fulfills with "resolved second".

Use race when you want the fastest result regardless. Use any when you want the fastest success.