无法获得承诺函数的原始返回值

Question

下面的代码有效，但没有被承诺。

const express = require('express');
function listenAndReturnServer() {
    app = express();

    console.log('Before listen');
    const server = app.listen(8001, function() {
        console.log('Now listening');
    });
    console.log('After listen');
    return server;
}
const server = listenAndReturnServer();
console.log('Doing some stuff');
console.log(typeof(server));

它输出以下内容：

Before listen
After listen
Doing some stuff
object
Now listening

我要解决的问题是，我想用取决于正在运行的服务器的代码替换“做一些事情”日志，并且已经调用了侦听回调（打印“正在侦听”）。如您所见，在“现在收听”之前记录了“做一些事情”。 util.promisify 进来了。

我把上面的改成了下面的。

const express = require('express');
const util = require('util');
async function listenAndReturnServer() {
    app = express();

    console.log('Before listen');
    const listenAsync = util.promisify(app.listen);
    const server = await listenAsync(8001).then(() => {
        console.log('Now listening');
    });
    console.log('After listen');
    return server;
}
async function main() {
    const server = await listenAndReturnServer();
    console.log('Doing some stuff');
    console.log(typeof(server));
}
main();

这一次，事情至少以正确的顺序登录。

Before listen
Now listening
After listen
undefined
Doing some stuff

现在有一个新问题：server 的类型未定义。我实际上需要它是原始的 Server 对象，所以我可以关闭服务器。

谁能确定这里发生了什么，以及如何获得我想要的行为？

Answer 1

[重写，因为这里发生了很多事情]

在研究了这段代码之后，我意识到有几个问题/错误的假设，如下：

const express = require('express');
const util = require('util');
async function listenAndReturnServer() {
    app = express();

    console.log('Before listen');

// ISSUE #1: `listenAsync` is not bound to `app` here.  This
// risks errors where Express may rely on `this` internally.
//
// ISSUE #2: Promisify is intended to operate on methods that
// take node-style callbacks, where the *callback* receives
// (err, arg0, arg1, ...) arguments, with the promise
// resolving to the 2nd argument (arg0).
// In the case of listen(), though, the callback isn't expected
// to be node-style(!), so doesn't receive any arguments
// (see https://nodejs.org/api/net.html#net_server_listen)
    const listenAsync = util.promisify(app.listen);

// ISSUE #3: `await` and `then()` serve similar purposes.
// Using them together like this is a little weird.
// You should probably just `await`, then do your console.log
// afterwards.      
    const server = await listenAsync(8001).then(() => {
        console.log('Now listening');
    });
    console.log('After listen');
    return server;
}
async function main() {
    const server = await listenAndReturnServer();
    console.log('Doing some stuff');
    console.log(typeof(server));
}
main();

如果您想 Promisify 使其与 async 一起使用，那么创建 Promise 可能更简单：

const express = require('express');

const app = express();

// Note: Not writing this as an `async` function because none
// of the APIs we're using return promises that can be `await`ed,
// and we explicitly return a promise
function main() {
  return new Promise(function(resolve, reject) {
    // Resolve to the server instance if listen() succeeds
    // Note: `server` here is Node's `http` server, not the Express instance
    const server = app.listen(8001, () => resolve(server));

    // Otherwise reject promise if there's an error
    server.on('error', reject);
  });
}

main()
  .then(server => console.log(server))
  .catch(err => console.error(err));