async/await

今天药忘吃喽~ 2022-01-28 08:23 272阅读 0赞

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 * 3
    16         });
    17 }
    18 
    19 async function test() {
    20     // await getting value from chained promise
    21     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 form
    26     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 * 3
    35         });
    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: 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)。