0-test.js

class Scheduler {
	constructor() {
		this.queue = []
		this.maxCount = 2
		this.runCounts = 0
	}
	add(promiseCreator) {
		this.queue.push(promiseCreator)
	}
	taskStart() {
		for (let i = 0; i < this.maxCount; i++) {
			this.request()
		}
	}
	request() {
		if (!this.queue || !this.queue.length || this.runCounts >= this.maxCount) {
			return
		}
		this.runCounts++

		this.queue
			.shift()()
			.then(() => {
				this.runCounts--
				this.request()
			})
	}
}

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(1000, '1')
addTask(500, '2')
addTask(300, '3')
addTask(400, '4')
scheduler.taskStart()
// 2
// 3
// 1
// 4