Async JavaScript

Promise Combinators

Promise combinators coordinate multiple async operations. all, allSettled, race, and any encode different success and failure strategies.

Promise.all is all-or-fail-fast
Promise.allSettled waits for every outcome
Promise.race uses first settled result
Promise.any uses first fulfillment
Combinators do not cancel the losers automatically

Promise combinators
Combinator	Fulfillment	Rejection	Good for
Promise.all	All fulfill	First rejection	Independent required work
Promise.allSettled	Always after all settle	Never from child rejection	Batch reports
Promise.race	First settled fulfills	First settled rejects	Timeouts / first response
Promise.any	First fulfillment	All reject	Fallback mirrors

Partial success with allSettled

const results = await Promise.allSettled(urls.map(fetchJson))

const successes = []
const failures = []

for (const result of results) {
  if (result.status === "fulfilled") successes.push(result.value)
  else failures.push(result.reason)
}

Use allSettled when failure of one item should not hide the rest.