javascript 實現一個帶併發限制的非同步排程器，保證同時最多執行2個任務

class Scheduler {
	constructor() {
		this.tasks = [], // 待執行的任務
		this.usingTask = [] // 正在執行的任務
	}
	// promiseCreator 是一個非同步函式，return Promise
	add(promiseCreator) {
		return new Promise((resolve, reject) => {
			promiseCreator.resolve = resolve
			if (this.usingTask.length < 2) {
				this.usingRun(promiseCreator)
			} else {
				this.tasks.push(promiseCreator)
			}
		})
	}

	usingRun(promiseCreator) {
		this.usingTask.push(promiseCreator)
		promiseCreator().then(() => {
			promiseCreator.resolve()
			this.usingMove(promiseCreator)
			if (this.tasks.length > 0) {
				this.usingRun(this.tasks.shift())
			}
		})
	}

	usingMove(promiseCreator) {
		let index = this.usingTask.findIndex(promiseCreator)
		this.usingTask.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))
}

addTask(400, 4) 
addTask(200, 2) 
addTask(300, 3) 
addTask(100, 1) 

// 2, 4, 3, 1

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