ES6中的async/await让Promise变得更加简便，通常await处理的链式Promise会包裹在函数中，返回结果仍然是一个Promise对象。
但是当await直接处理链式Promise时，编译器是会返回一个Promise对象等待下次await，还是继续执行Promise对象直到返回结果不再是Promise？
测试环境
NodeJS v8.7.0
测试代码
1 const util = require(‘util‘); 2 ?3 const prom2 = util.promisify((a, b, cb) => { 4 ????cb(null, a + b); 5 }); 6 ?7 function promiseCal(a, b) { 8 ????return prom2(a, b) 9 ????????.then((val) => {10 ????????????console.log(`value in first then: ${val}`);11 ????????????return val * 2;12 ????????})13 ????????.then((val) => {14 ????????????console.log(`Value in second then: ${val}`);15 ????????????return val * 316 ????????});17 }18 19 async function test() {20 ????// await getting value from chained promise21 ????let val1 = await promiseCal(2, 4);22 23 ????console.log(`Typeof val1: ${typeof val1}\nFinal returned value: ${val1}`);24 25 ????// await getting value from another form26 ????console.log(`[v2] Type of prom2: ${typeof prom2}`);27 ????let val2 = await prom2(4, 6)28 ????????.then((val) => {29 ????????????console.log(`[v2] Value in first then: ${val}`);30 ????????????return val * 2;31 ????????})32 ????????.then((val) => {33 ????????????console.log(`[v2] Value in second then: ${val}`);34 ????????????return val * 335 ????????});36 ????console.log(`[v2] Typeof val2: ${typeof val2}\n[v2] Final returned value: ${val2}`);37 }38 39 test()40 ????.catch(e => console.error(e.stack || e));运行结果
value in first then: 6Value in second then: 12Typeof val1: numberFinal returned value: 36[v2] Type of prom2: function[v2] Value in first then: 10[v2] Value in second then: 20[v2] Typeof val2: number[v2] Final returned value: 60
结论
await操作符会沿着Promise链处理到返回结果不再是Promise位置，await语句返回的将是Primise链上最后一个then函数的返回值(或者抛出异常)。
async/await工作机制探究--NodeJS
原文地址：https://www.cnblogs.com/harelion/p/9128674.html