从文件导入 function 时，Nodejs 返回未定义

Question

im kinda new to nodejs and trying to get this here working, but whatever I try it either return undefined or [function: online]. As you can see ive made a function in online.js and export the function. Then in app.js I require the file, then console log the const

// app.js

var express = require('express');
var app = express();
app.set('view engine', 'ejs');
var PORT = 3000;
const online = require('./online');

app.use('/public', express.static('./public/'));
app.get('/', function(req, res) {
  res.status(200);
  res.render('index');
});

app.listen(PORT, function(err) {
  if (err) console.log(err);
  console.log("Server listening on PORT", PORT);
  console.log(online);
});

// online.js

function online() {
  var steamServerStatus = require('steam-server-status');
  steamServerStatus.getServerStatus(
    'oslo14.spillvert.no', 28215, function onlinePlayers(serverInfo) {
    if (serverInfo.error) {
      console.log(serverInfo.error);
    } else {
      console.log("game: " + serverInfo.gameName);
      console.log("server name: " + serverInfo.serverName);
      console.log("players: " + serverInfo.numberOfPlayers + "/" + serverInfo.maxNumberOfPlayers);
    }
  })
};

module.exports = online();

When running app.js I get this output:

Server listening on PORT 3000 
undefined

Answer 1

You don't know whether getServerStatus succeeded or failed until the callback function is called. So you should return a Promise from your online function and resolve that promise in the onlinePlayers callback. You can await it from any async function (ie: make your app.listen callback async ). The reject path will cause an exception where the online() call was awaited.

app.listen(PORT, async function(err){
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
    console.log(await online());
}

function online() {
  return new Promise((resolve, reject) => {
    var steamServerStatus = require('steam-server-status');
    steamServerStatus.getServerStatus(
      'oslo14.spillvert.no', 28215, function onlinePlayers(serverInfo) {
        if (serverInfo.error) {
          reject(serverInfo.error);
        } else {
          console.log("game: " + serverInfo.gameName);
          console.log("server name: " + serverInfo.serverName);
          console.log("players: " + serverInfo.numberOfPlayers + "/" + 
serverInfo.maxNumberOfPlayers);
          resolve(serverInfo); // or 'true' or whatever you wanted to log
        }
      });
  });
}