Promise Combinators: Choosing a Strategy

Four static methods combine promise lists. Choosing the right one is a design decision, not trivia.

| Combinator | Resolves when | Rejects when | Use for | | -------------------- | ------------------------- | --------------- | ------------------------ | | Promise.all | all fulfill | first rejection | everything is required | | Promise.allSettled | all settle (either way) | never | partial failure is OK | | Promise.race | first settle (either way) | — | timeouts, first response | | Promise.any | first fulfillment | all reject | fastest working source |

all vs allSettled — the partial-failure problem

A dashboard fetching user + orders + notifications: with Promise.all, one failing endpoint kills the page. With Promise.allSettled, every request finishes and you decide per-item:

const results = await Promise.allSettled([getUser(), getOrders(), getNotifs()]);
for (const r of results) {
  if (r.status === "fulfilled") render(r.value);
  else renderError(r.reason);
}

allSettled results are { status: "fulfilled", value } or { status: "rejected", reason } — always inspect status.

race for timeouts

const work = doLongThing();
const timeout = new Promise((_, rej) => setTimeout(() => rej(new Error("timeout")), 3000));
await Promise.race([work, timeout]);

Note race does not cancel the loser — the long fetch keeps running (cancellation comes two lessons ahead). Promise.any is the rarest: multiple mirrors, take whichever answers successfully first.