Promise 异步函数顺序执行

可以满足需求，且使用方法和Promise.all统一

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var a = function() {     return new Promise(function(resolve, reject) {         setTimeout(function() {             console.log('a')             resolve('a')         }, 1000)     }) }   var b = function(data) {     return new Promise(function(resolve, reject) {         console.log('b')         resolve(data +'b')     }) }   var c = function(data) {     return new Promise(function(resolve, reject) {         setTimeout(function() {             console.log('c')             resolve(data +'c')         }, 500)     }) }   // 组织函数队列 function reduce(arr) {     var sequence = Promise.resolve()       arr.forEach(function(item) {         sequence = sequence.then(item)     })       return sequence }   // 顺序执行函数队列 reduce([a, b, c]) .then(function(data) {     console.log(data)// abc }) .catch(function(e) {     console.log(e) })