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}
Final 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}
[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)。