async/await工作机制探究--NodeJS

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

 const util = require('util');

 const prom2 = util.promisify((a, b, cb) => {
     cb(null, a + b);
 });

 function promiseCal(a, b) {
     return prom2(a, b)
         .then((val) => {
             console.log(`value in first then: ${val}`);
             return val * 2;
         })
         .then((val) => {
             console.log(`Value in second then: ${val}`);
             return val * 3
         });
 }

 async function test() {
     // await getting value from chained promise
     let val1 = await promiseCal(2, 4);

     console.log(`Typeof val1: ${typeof val1}\nFinal returned value: ${val1}`);

     // await getting value from another form
     console.log(`[v2] Type of prom2: ${typeof prom2}`);
     let val2 = await prom2(4, 6)
         .then((val) => {
             console.log(`[v2] Value in first then: ${val}`);
             return val * 2;
         })
         .then((val) => {
             console.log(`[v2] Value in second then: ${val}`);
             return val * 3
         });
     console.log(`[v2] Typeof val2: ${typeof val2}\n[v2] Final returned value: ${val2}`);
 }

 test()
     .catch(e => console.error(e.stack || e));

运行结果

value in first then: 6
Value in second then: 12
Typeof val1: number
Final 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)。