用Promise实现一个支持异步执行的队列

题目：实现一个 Queue 类，支持按顺序执行异步队列，push 方法把一个待执行函数推入队列，run 函数开始运行队列

Queue.push(function(){
    return new Promise((resolve,reject)=>{
        setTimeout(()=>{
            console.log('1');
            resolve()
        },1000)
    })
}).push(function(){
    console.log('2')
}).push(function(){
    return new Promise((resolve,reject)=>{
        setTimeout(()=>{
            console.log('3');
            resolve()
        },500)
    })
})

上述代码输出需为 1,2,3

实现如下

class Queue{
    constructor(){
        this.stack = []
    }
    push(fn){
        this.stack.push(fn)
        return this
    }
    run(){
        var sequence = Promise.resolve()
        this.stack.forEach(item=>{
            sequence = sequence.then(item)
        })
    }
}
var q = new Queue()
q.push(function(){
    return new Promise((resolve)=>{
        setTimeout(()=>{
            console.log('1');
            resolve()
        },1000)
    })
}).push(function(){
    console.log('2');
}).push(function(){
    return new Promise((resolve)=>{
        setTimeout(()=>{
            console.log('3');
            resolve()
        },500)
    })
}).run()