题目

// JS实现一个带并发限制的异步调度器Scheduler，
// 保证同时运行的任务最多有两个
// 完善代码中Scheduler类，使得以下程序能正确输出
class Scheduler {
  add(promiseCreator) { ... }
  // ...
}
const timeout = (time) => new Promise(resolve => {
  setTimeout(resolve, time)
})
const scheduler = new Scheduler();
const addTask = (time, order) => {
  scheduler.add(() => timeout(time)).then(() => console.log(order))
}
addTask(400, '4')
addTask(200, '2')
addTask(300, '3')
addTask(100, '1')
// output: 2 4 1 3

答案：

class Scheduler {
    constructor() {
        this.tasks = [], // 待运行的任务
        this.usingTasks = [] // 正在运行的任务
    }
    // promiseCreator 是一个异步函数，return Promise
    add(promiseCreator) {
        return new Promise((resolve, reject) => {
            promiseCreator.resolve = resolve
            if (this.usingTasks.length < 2) {
                this.usingRun(promiseCreator)
            } else {
                this.tasks.push(promiseCreator)
            }
        })
    }
    usingRun(promiseCreator) {
        this.usingTasks.push(promiseCreator)
        promiseCreator().then(() => {
            promiseCreator.resolve()
            this.usingMove(promiseCreator)
            if (this.tasks.length > 0) {
                this.usingRun(this.tasks.shift())
            }
        })
    }
    usingMove(promiseCreator) {
        let index = this.usingTasks.findIndex(promiseCreator)
        this.usingTasks.splice(index, 1)
    }
}
const timeout = (time) => new Promise(resolve => {
    setTimeout(resolve, time)
})
const scheduler = new Scheduler()
const addTask = (time, order) => {
    scheduler.add(() => timeout(time)).then(() => console.log(order))
}
// output: 2 3 1 4
addTask(1000, '1');
addTask(500, '2');
addTask(300, '3');
addTask(400, '4');