JavaScript实现一个简单队列，顺序执行异步任务

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功能描述

• QueueJob构造函数实例存在一个queue方法，该方法接收一个Promise；能够保证按照调用的顺序依次执行传入的callback，即使其中包含异步代码

class QueueJob {
  queue(callback) {
     const previous = this.pending || Promise.resolve()
     this.pending = previous.then(callback).catch(callback)
     // 利用闭包保存当前pending
     const current = this.pending
     current.then(() => {
             // this.pending 完成时，this.pending没有被覆盖
            if(this.pending === current) {
                delete this.pending
            }
      })
  }
}

var queueJob = new QueueJob()

queueJob.queue(() => {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			console.log('first')
			resolve()
		}, 2000)
	})
})

queueJob.queue(() => {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			console.log('second')
			resolve()
		}, 1000)
	})
})

queueJob.queue(() => {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			console.log('three')
			resolve()
		}, 500)
	})
})

// first
// second
// three

扩展

• JavaScript 异步队列实现及拓展