let startTime = Date.now();
const timeout = (timeout: number, ret: number) => {
 return (idx?: any) =>
 new Promise((resolve) => {
  setTimeout(() => {
  const compare = Date.now() - startTime;
  console.log(`At ${Math.floor(compare / 100)}00 return`, ret);
  resolve(idx);
  }, timeout);
 });
};

const timeout1 = timeout(1000, 1);
const timeout2 = timeout(300, 2);
const timeout3 = timeout(400, 3);
const timeout4 = timeout(500, 4);
const timeout5 = timeout(200, 5);

const run = async () => {
 startTime = Date.now();
 await Promise.all([
 timeout1(),
 timeout2(),
 timeout3(),
 timeout4(),
 timeout5(),
 ]);
};

run();

At 200 return 5
At 300 return 2
At 400 return 3
At 500 return 4
At 1000 return 1

class Concurrent {
 private maxConcurrent: number = 2;

 constructor(count: number = 2) {
 this.maxConcurrent = count;
 }
}

class Concurrent {
 private maxConcurrent: number = 2;

 constructor(count: number = 2) {
 this.maxConcurrent = count;
 }
 public async useRace(fns: Function[]) {
 const runing: any[] = [];
 // 按并發(fā)數(shù)，把 Promise 加進(jìn)去
 // Promise 會(huì)回調(diào)一個(gè)索引，方便我們知道哪個(gè) Promise 已經(jīng) resolve 了
 for (let i = 0; i < this.maxConcurrent; i++) {
  if (fns.length) {
  const fn = fns.shift()!;
  runing.push(fn(i));
  }
 }
 const handle = async () => {
  if (fns.length) {
  const idx = await Promise.race<number>(runing);
  const nextFn = fns.shift()!;
  // 移除已經(jīng)完成的 Promise，把新的進(jìn)去
  runing.splice(idx, 1, nextFn(idx));
  handle();
  } else {
  // 如果數(shù)組已經(jīng)被清空了，表面已經(jīng)沒(méi)有需要執(zhí)行的 Promise 了，可以改成 Promise.all
  await Promise.all(runing);
  }
 };
 handle();
 }
}

const run = async () => {
 const concurrent = new Concurrent();
 startTime = Date.now();
 await concurrent.useRace([timeout1, timeout2, timeout3, timeout4, timeout5]);
};

At 300 return 2
At 700 return 3
At 1000 return 1
At 1200 return 5
At 1200 return 4

class Concurrent {
 private maxConcurrent: number = 2;
 private list: Function[] = [];
 private currentCount: number = 0;

 constructor(count: number = 2) {
 this.maxConcurrent = count;
 }
 public async add(fn: Function) {
 this.currentCount += 1;
 // 如果最大已經(jīng)超過(guò)最大并發(fā)數(shù)
 if (this.currentCount > this.maxConcurrent) {
  // wait 是一個(gè) Promise，只要調(diào)用 resolve 就會(huì)變成 fulfilled 狀態(tài)
  const wait = new Promise((resolve) => {
  this.list.push(resolve);
  });
  // 在沒(méi)有調(diào)用 resolve 的時(shí)候，這里會(huì)一直阻塞
  await wait;
 }
 // 執(zhí)行函數(shù)
 await fn();
 this.currentCount -= 1;
 if (this.list.length) {
  // 把 resolve 拿出來(lái)，調(diào)用，這樣 wait 就完成了，可以往下面執(zhí)行了
  const resolveHandler = this.list.shift()!;
  resolveHandler();
 }
 }
}

const run = async () => {
 const concurrent = new Concurrent();
 startTime = Date.now();
 concurrent.add(timeout1);
 concurrent.add(timeout2);
 concurrent.add(timeout3);
 concurrent.add(timeout4);
 concurrent.add(timeout5);
};

run();

At 300 return 2
At 700 return 3
At 1000 return 1
At 1200 return 5
At 1200 return 4