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Promise.all() - JavaScript | MDN

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Promise.all() 方法回傳一個 Promise 物件，當引數 iterable 中所有的 promises 都被實現（resolved），或引數 iterable 不含任何 promise 時，被實現。或以第一個被拒絕的 promise 的原因被拒絕。

iterable

一個 iterable 物件像是 Array 或 String。

• 一個**已被實現（already resolved）**的 Promise，若傳入的 iterable 為空。
• 一個非同步地被實現（asynchronously resolved）的 Promise 若傳入的 iterable 不含 promise。注意，Google Chrome 58 對此情形回傳一個已被解決的 promise。
• 一個擱置（pending）的 Promise，對所有剩餘情形。此 promise 接著被非同步地被 resolved/rejected（只要堆疊為空）當 iterable 中所有的 promises 都被實現，或其中一個被拒絕。參見下方關於"Promise.all 的非同步與同步性質"的例子。

此方法在聚集（aggregating）多個 promises 的結果時很有幫助。

實現（Fulfillment）： 若傳入空的 iterable，此方法（同步地）回傳一個已被解決的 promise。若所有傳入的 promises 都被實現，或都不是 promise，Promise.all 回傳的 promise 被非同步地實現。無論是哪個情形，回傳一個以 iterable 其內所有值（包含非 promise 值）作為引數的陣列被實現。

拒絕（Rejection）： 若任一個傳入的 promise 被拒絕，Promise.all 非同步地以其值被拒絕，無論其他 promises 是否被解決。

Promise.all 等到全部實現（或一個拒絕）。

var p1 = Promise.resolve(3);
var p2 = 1337;
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, "foo");
});

Promise.all([p1, p2, p3]).then((values) => {
  console.log(values); // [3, 1337, "foo"]
});

若 iterable 含非 promise 值，它們將被忽略，但依然會被記入回傳 promise 陣列值（若被實現）：

// this will be counted as if the iterable passed is empty, so it gets fulfilled
var p = Promise.all([1, 2, 3]);
// this will be counted as if the iterable passed contains only the resolved promise with value "444", so it gets fulfilled
var p2 = Promise.all([1, 2, 3, Promise.resolve(444)]);
// this will be counted as if the iterable passed contains only the rejected promise with value "555", so it gets rejected
var p3 = Promise.all([1, 2, 3, Promise.reject(555)]);

// using setTimeout we can execute code after the stack is empty
setTimeout(function () {
  console.log(p);
  console.log(p2);
  console.log(p3);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[3] }
// Promise { <state>: "fulfilled", <value>: Array[4] }
// Promise { <state>: "rejected", <reason>: 555 }

以下例子驗證了 Promise.all 的非同步性質（asynchronicity）（或同步性質（synchronicity），若傳入的 iterable 是空的）：

// we are passing as argument an array of promises that are already resolved,
// to trigger Promise.all as soon as possible
var resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];

var p = Promise.all(resolvedPromisesArray);
// immediately logging the value of p
console.log(p);

// using setTimeout we can execute code after the stack is empty
setTimeout(function () {
  console.log("the stack is now empty");
  console.log(p);
});

// logs, in order:
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

當 Promise.all 被拒絕時發生一樣的事情：

var mixedPromisesArray = [Promise.resolve(33), Promise.reject(44)];
var p = Promise.all(mixedPromisesArray);
console.log(p);
setTimeout(function () {
  console.log("the stack is now empty");
  console.log(p);
});

// logs
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "rejected", <reason>: 44 }

注意！Promise.all 同步地被解決若且唯若傳入的 iterable 為空：

var p = Promise.all([]); // will be immediately resolved
var p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously
console.log(p);
console.log(p2);
setTimeout(function () {
  console.log("the stack is now empty");
  console.log(p2);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[0] }
// Promise { <state>: "pending" }
// the stack is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

當任一個陣列成員被拒絕則 Promise.all 被拒絕。例如，若傳入四個將在一段時間後被解決的 promises，而其中一個立刻被拒絕，則 Promise.all 將立刻被拒絕。

var p1 = new Promise((resolve, reject) => {
  setTimeout(resolve, 1000, "one");
});
var p2 = new Promise((resolve, reject) => {
  setTimeout(resolve, 2000, "two");
});
var p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 3000, "three");
});
var p4 = new Promise((resolve, reject) => {
  setTimeout(resolve, 4000, "four");
});
var p5 = new Promise((resolve, reject) => {
  reject("reject");
});

Promise.all([p1, p2, p3, p4, p5]).then(
  (values) => {
    console.log(values);
  },
  (reason) => {
    console.log(reason);
  },
);

//From console:
//"reject"

//You can also use .catch
Promise.all([p1, p2, p3, p4, p5])
  .then((values) => {
    console.log(values);
  })
  .catch((reason) => {
    console.log(reason);
  });

//From console:
//"reject"

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