等待承诺函数重复，但在第二个解析/返回失败

Question

我正在学习等待node.js的承诺 以下程序通过调用底部的main()来运行。main()调用函数caller()。 然后caller()调用函数loop()，超时为 2 秒并返回我等待了 2 秒的消息。loop()返回caller()，然后caller()返回main()，只是为了传递相同的消息。

caller() 和 main() 实际上是相同的代码，它们应该以相同的方式返回！ 但是返回 main() 是不确定的。

日食运行结果：

C：\Users\Lenovo\eclipse-workspace.metadata.plugins\org.eclipse.wildwebdeveloper.embedder.nodeode-v14.15.4-win-x64ode.exe awaitPromise2.js "顶级开始。主要开始...在循环中，启动 aaa ：新循环，完成输出：消息：我在呼叫者中等待了 1 秒 aaa ： 消息：我在呼叫者中等待了 1 秒，解决 aaa： 消息： I 在主中等待 1 秒，响应：主中未定义，解决结果： 定义

程序文件为：

async function loop() {
let aaa = "New";
console.log("in loop, start aaa : " + aaa);
await new Promise((resolve, reject) => setTimeout(resolve, 2000));
aaa = " Message: I waited 2 sec.";
return new Promise((resolve) => {
console.log("loop, finished output : " + aaa);
resolve(aaa);
return;
});
}
async function caller() {
await loop().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 3000));
console.log("in caller aaa :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 1500));
let bbb = aaa;
console.log("in caller, resolve  aaa  : " + bbb);
resolve(bbb);
});
});
}
async function main() {
console.log("main start ...");
await caller().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 5000));
console.log("in main, response :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 6000));
console.log("in main, resolve result : " + aaa);
resolve(aaa);
return;
});
});
}
console.log(" Top Level starts..");
main();

我利用超时试图等待结果的到来。需要他们吗？？

Answer 1

让我们从代码中删除所有多余的东西，这些东西什么也没做：

async function loop() {
let aaa = "New";
console.log("in loop, start aaa : " + aaa);
await new Promise((resolve, reject) => setTimeout(resolve, 2000));
aaa = " Message: I waited 2 sec.";
console.log("loop, finished output : " + aaa);
return Promise.resolve(aaa);
}
async function caller() {
await loop().then(function (aaa) {
console.log("in caller aaa :" + aaa);
let bbb = aaa;
console.log("in caller, resolve  aaa  : " + bbb);
return Promise.resolve(bbb);
});
}
async function main() {
console.log("main start ...");
await caller().then(function (aaa) {
console.log("in main, response :" + aaa);
console.log("in main, resolve result : " + aaa);
return Promise.resolve(aaa);
});
}
console.log(" Top Level starts..");
main(); 

现在问题很清楚了：caller不return一个值。它包含的唯一return语句是在.then()回调中，解析awaited 的承诺，但之后没有任何反应。要解决这个问题，return承诺链，而不仅仅是await它：

function caller() {
return loop().then(function (aaa) {
console.log("in caller, aaa: " + aaa);
return Promise.resolve(aaa);
});
}

或者最好不要使用.then()链接，因为您可以在任何地方使用async/await。您也可以在async函数或.then回调中省略返回值中的Promise.resolve：

async function loop() {
const aaa = "New";
console.log("in loop, start: " + aaa);
await new Promise((resolve, reject) => setTimeout(resolve, 2000));
const bbb = "Message: I waited 2 sec.";
console.log("loop, finished output: " + bbb);
return bbb;
}
async function caller() {
const bbb = await loop();
console.log("in caller:" + bbb);
return bbb;
}
async function main() {
console.log("main start ...");
const res = await caller();
console.log("in main, response: " + res);
// return res; // unused anyway
}
console.log("Top Level starts...");
main();

Answer 2

伙计，你刚刚错过了呼叫者中的一个return。或者，您错误地使用了await。因为您使用了await所以除非调用 return，否则不会返回结果。

async function loop() {
let aaa = "New";
console.log("in loop, start aaa : " + aaa);
await new Promise((resolve, reject) => setTimeout(resolve, 2000));
aaa = " Message: I waited 2 sec.";
return new Promise((resolve) => {
console.log("loop, finished output : " + aaa);
resolve(aaa);
return;
});
}
async function caller() {
return await loop().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 3000));
console.log("in caller aaa :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 1500));
let bbb = aaa;
console.log("in caller, resolve  aaa  : " + bbb);
resolve(bbb);
});
});
}
async function main() {
console.log("main start ...");
await caller().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 5000));
console.log("in main, response :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 6000));
console.log("in main, resolve result : " + aaa);
resolve(aaa);
return;
});
});
}
console.log(" Top Level starts..");
main();

我建议你不要把async和.then混为一谈，你应该选择一个并坚持下去。

我建议你放弃caller()async/await，这更容易理解：

async function loop() {
let aaa = "New";
console.log("in loop, start aaa : " + aaa);
await new Promise((resolve, reject) => setTimeout(resolve, 2000));
aaa = " Message: I waited 2 sec.";
return new Promise((resolve) => {
console.log("loop, finished output : " + aaa);
resolve(aaa);
return;
});
}
function caller() {
return loop().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 3000));
console.log("in caller aaa :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 1500));
let bbb = aaa;
console.log("in caller, resolve  aaa  : " + bbb);
resolve(bbb);
});
});
}
async function main() {
console.log("main start ...");
await caller().then(function (aaa) {
new Promise((resolve, reject) => setTimeout(resolve, 5000));
console.log("in main, response :" + aaa);
return new Promise((resolve) => {
new Promise((resolve, reject) => setTimeout(resolve, 6000));
console.log("in main, resolve result : " + aaa);
resolve(aaa);
return;
});
});
}
console.log(" Top Level starts..");
main();

如果你是一个新手，我挑战你尝试重写你的代码，不要完全使用.then，然后反过来 - 根本不使用async/await。