Promise.all / any / race / allSettled — 適切なツールを選ぶ

コンビネーターの概要

Promise.allの基本

// Small helpers for examples
function delay(ms, value, shouldFail = false) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      if (shouldFail) reject(new Error(String(value)));
      else resolve(value);
    }, ms);
  });
}

// Promise.all waits for all; rejects fast on first error
Promise.all([
  delay(10, "A"),
  delay(5, "B")
]).then(function (vals) {
  console.log("all:", vals);
});

Promise.anyの基本

// Promise.any gives the first fulfilled value
Promise.any([
  delay(8, "ok-1"),
  delay(3, "ok-2"),
  delay(6, "ok-3")
]).then(function (v) {
  console.log("any:", v);
}).catch(function (e) {
  console.log("any failed:", e.message);
});

Promise.raceの基本

// Promise.race settles with the first to finish (success or error)
Promise.race([
  delay(5, "win"),
  delay(7, "boom", true)
]).then(function (v) {
  console.log("race resolved:", v);
}).catch(function (e) {
  console.log("race rejected:", e.message);
});

Promise.allSettledの基本

// Promise.allSettled waits for all and reports each outcome
Promise.allSettled([
  delay(4, "A"),
  delay(6, "X", true)
]).then(function (results) {
  console.log("allSettled:", results.map(function (r) { return r.status; }));
});

どれを使うか

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