How to deal with socket.io being undefined in app.js - node.js

Question

My express app logic is separated into a separate file from the instantiation of the express server, so I'm having issues with accessing socket.io within this app file. Should I just move the socket.io implementation into index.js or is it possible to keep that logic in app.js?
index.js
const http = require('http');
const app = require('./app');
const server = http.createServer(app);
const io = require('socket.io')(server);
const config = require('./utils/config');
const logger = require('./utils/logger');
app.set('socketio', io);
server.listen(config.PORT, () => {
logger.info(`Listening on port ${config.PORT}`);
});
app.js
const express = require('express');
const config = require('./utils/config');
const middleware = require('./utils/middleware');
const app = express();
app.use(middleware.requestLogger);
const io = app.get('socketio');
io.on('connection', (socket) => {
io.emit('test', { test: 'test' });
});
app.use(middleware.errorHandler);
module.exports = app;

Answer 1

You have a load order problem. You are loading app.js into index.js BEFORE you create and set io as an app property. So, when app.js tries to use the io property, it hasn't yet been set.
The way you have things split between the files, you've created a circular dependency. You can't create io until you've created the server, but you can't create the server until you have the app which is in app.js. So, you can't create io before you load app.js.
There are lots of ways around this. I find it kind of weird that you're creating the server in one file and the app object in another file since the two are fully required to make an operational server. So, I'd rearrange how those things are done like this:
// index.js
const http = require('http');
const app = require('express')();
const server = http.createServer(app);
const io = require('socket.io')(server);
app.set('socketio', io);
require('./app.js')(app);
const config = require('./utils/config');
const logger = require('./utils/logger');
server.listen(config.PORT, () => {
logger.info(`Listening on port ${config.PORT}`);
});
And, then modify app.js like this:
const config = require('./utils/config');
const middleware = require('./utils/middleware');
module.exports = function(app) {
app.use(middleware.requestLogger);
const io = app.get('socketio');
io.on('connection', (socket) => {
io.emit('test', { test: 'test' });
});
app.use(middleware.errorHandler);
}
There are 100 other ways to organize the code to fix this. Exporting a function that you can call and passing that function one or more arguments is one way to help control the timing of these circular dependencies.