all和allSettled

一、关系

allSettled是对all的补充：始终执行onResolve，rejected的结果也一并返回

二、区别

场景	Promise.all()	Promise.allSettled()
全fulfilled	onResolve，返回所有的fulfilled	onResolve，返回所有的fulfilled
fulfilled+rejected	onRejected，返回最早的rejected	onResolve，返回所有的fulfilled+rejected
全rejected	onRejected，返回最早的rejected	onResolve，返回所有的rejected

三、详解

1、Promise.all([promise, promise2, ...])

• 并发，所有任务都完成才返回结果，返回所有结果作为元素的数组

• 有一个失败，则全失败，直接执行onReject

let p1 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 100, "one");
});
let p2 = new Promise(function (resolve, reject) {
  setTimeout(reject, 200, "two");
});
let p3 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 300, "three");
});

Promise.all([p1, p2, p3]).then(
  (value) => {
    console.log("onResolve: ", value);
  },
  (reason) => {
    console.log("onRejected: ", reason);
  }
);

输出：

onRejected:  two

2、Promise.allSettled([promise, promise2, ...])

• 始终执行onResolve，不管有没有rejected的任务

let p1 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 100, "one");
});
let p2 = new Promise(function (resolve, reject) {
  setTimeout(reject, 200, "two");
});
let p3 = new Promise(function (resolve, reject) {
  setTimeout(resolve, 300, "three");
});

Promise.allSettled([p1, p2, p3]).then((value) => {
  console.log("onResolve: ", value);
});

输出：

onResolve:  [
  { status: 'fulfilled', value: 'one' },
  { status: 'rejected', reason: 'two' },
  { status: 'fulfilled', value: 'three' }
]

四、手写

1、Promise.all

Promise.myAll = function (taskList) {
  if (!Array.isArray(taskList)) throw new Error("参数必须为数组");
  return new Promise((resolve, reject) => {
    let result = [];
    taskList.forEach((task, index) => {
      Promise.resolve(task).then(
        (data) => {
          result[index] = data;
          if (result.length === taskList.length) resolve(result);
        },
        (reason) => {
          reject(reason);
        }
      );
    });
  });
};

关键点：

• 入参校验

• 返回Promise实例

• 循环执行任务，借助Promise.resolve

• 处理返回数据，并判断是resolve还是reject

2、Promise.allSettled

Promise.myAllSettled = function (taskList) {
  if (!Array.isArray(taskList)) throw new Error("参数必须为数组");
  return new Promise((resolve, reject) => {
    let result = [];
    taskList.forEach((task, index) => {
      Promise.resolve(task).then(
        (value) => {
          result[index] = { state: "fulfilled", value };
          if (result.length === taskList.length) resolve(result);
        },
        (reason) => {
          result[index] = { state: "rejected", reason };
          if (result.length === taskList.length) resolve(result);
        }
      );
    });
  });
};

关键点：

• rejected的结果也会被收录到最终结果

• 最终结果都是通过resolve传递出去