备注：

使用promise对并发请求进行封装，可支持并发请求任务个数及任务优先级。
原理：使用队列的数据结构 ，先进先出的处理请求，如添加任务优先级高的任务，
则会通过数组的shift()方法插入到队列的头部

mock.js文件：

const urls = [
  {
    info: 'task1',
    timeout: 3000,
    sort: 1,
  },
  {
    info: 'task2',
    timeout: 1000,
    sort: 1,
  },
  {
    info: 'task3',
    timeout: 2000,
    sort: 3,
  },
  {
    info: 'task4',
    timeout: 1000,
    sort: 2,
  },
  {
    info: 'task5',
    timeout: 5000,
    sort: 1,
  },
  {
    info: 'task6',
    timeout: 3000,
    sort: 6,
  },
];
const loadingImg = ({ info, timeout }) => {
  return new Promise((res, rej) => {
    setTimeout(() => {
      res(info);
    }, timeout);
  });
};
module.exports = { urls, loadingImg };

promise封装：

支持并发数量：

const { urls, loadingImg } = require('./mock');
class PromiseQueue {
  constructor(options) {
    this.maxCurry = options.maxCurry;
    this.currentCount = 0;
    this.pendingList = [];
  }
  addTask(fn) {
    this.pendingList.push(fn);
    this.runTask();
  }
  runTask() {
    if (!this.pendingList.length || this.currentCount === this.maxCurry) return;
    this.currentCount++;
    const fn = this.pendingList.shift();
    const promise = fn();
    promise.then(this.restartRunTask.bind(this)).catch(this.restartRunTask.bind(this));
  }
  restartRunTask() {
    this.currentCount--;
    this.runTask();
  }
}
const queue = new PromiseQueue({ maxCurry: 3 });
urls.forEach((url) => {
  queue.addTask(() => loadingImg(url));
});

支持任务优先级：

添加高优任务不会取消当前已经发出的请求，而是放在发出请求后的队列的头部

const { urls, loadingImg } = require('./mock');
class PromiseQueue {
  constructor(options) {
    this.maxCurry = options.maxCurry;
    this.currentCount = 0;
    this.pendingList = [];
  }
  addTask(fn) {
    this.pendingList.push(fn);
    if (this.pendingList.length > 1) {
      this.pendingList = this.pendingList.sort((a, b) => b.sort - a.sort);
    }
    this.runTask();
  }
  runTask() {
    if (!this.pendingList.length || this.currentCount === this.maxCurry) return;
    this.currentCount++;
    const { fn } = this.pendingList.shift();
    const promise = fn();
    promise.then(this.restartRunTask.bind(this)).catch(this.restartRunTask.bind(this));
  }
  restartRunTask() {
    this.currentCount--;
    this.runTask();
  }
}
const queue = new PromiseQueue({ maxCurry: 3 });
const formatTask = (url) => {
  return {
    fn: () => loadingImg(url),
    sort: url.sort,
  };
};
urls.forEach((url) => {
  queue.addTask(formatTask(url));
});
queue.addTask(
  formatTask({
    info: 'high!!!',
    timeout: 2000,
    sort: 10,
  }),
);