1.Promise.all 并行执行promise

getA和getB并行执行，然后输出结果。如果有一个错误，就抛出错误

/*** 每一个promise都必须返回resolve结果才正确* 每一个promise都不处理错误*/const getA = new Promise((resolve, reject) => {//模拟异步任务setTimeout(function(){resolve(2);}, 1000) 
})
.then(result => result)const getB = new Promise((resolve, reject) => {setTimeout(function(){// resolve(3);reject('Error in getB');}, 1000) 
})
.then(result => result)Promise.all([getA, getB]).then(data=>{console.log(data)
})
.catch(e => console.log(e));

getA和getB并行执行，然后输出结果。总是返回resolve结果

/*** 每一个promise自己处理错误*/const getA = new Promise((resolve, reject) => {//模拟异步任务setTimeout(function(){resolve(2);}, 1000) 
})
.then(result => result)
.catch(e=>{})const getB = new Promise((resolve, reject) => {setTimeout(function(){// resolve(3);reject('Error in getB');}, 1000) 
})
.then(result => result)
.catch(e=>e)Promise.all([getA, getB]).then(data=>{console.log(data)
})
.catch(e => console.log(e));

2.顺序执行promise

先getA然后getB执行，最后addAB

• 2.1 方法一——连续使用then链式操作

function getA(){return  new Promise(function(resolve, reject){ setTimeout(function(){     resolve(2);}, 1000);});
}function getB(){return  new Promise(function(resolve, reject){       setTimeout(function(){resolve(3);}, 1000);});
}function addAB(a,b){return a+b
}function getResult(){var  obj={};Promise.resolve().then(function(){return  getA() }).then(function(a){obj.a=a;}).then(function(){return getB() }).then(function(b){obj.b=b;return obj;}).then(function(obj){return  addAB(obj['a'],obj['b'])}).then(data=>{console.log(data)}).catch(e => console.log(e));}
getResult();

• 2.2 方法二——使用promise构建队列 

function getResult(){var res=[];// 构建队列function queue(arr) {var sequence = Promise.resolve();arr.forEach(function (item) {sequence = sequence.then(item).then(data=>{res.push(data);return res})})return sequence}// 执行队列queue([getA,getB]).then(data=>{return addAB(data[0],data[1])}).then(data => {console.log(data)}).catch(e => console.log(e));}getResult();

• 2.3方法三——使用async、await实现类似同步编程

function getResult(){async function queue(arr) {let res = []for (let fn of arr) {var data= await fn();res.push(data);}return await res
}queue([getA,getB]).then(data => {return addAB(data[0],data[1])}).then(data=>console.log(data))}

3. 总结

实现异步队列函数的三种方式

方法一——连续使用then链式操作
方法二——使用promise构建队列
方法三——使用async、await实现类似同步编程，async函数内部实现同步