Sharing variables across files in node.js without using global variables

Question

I'm trying to separate my socket.io code into a separate file (socket.js) from my main file (app.js). However, I need to define my io object in app.js, which is also used in my socket.js file.

Currently, I set io as a global variable so it is accessible from app.js (Global Variable in app.js accessible in routes?), but I understand this is bad practise. Is there a better way to do this (can injection work in this case, as I need to export a variable from the app.js to socket.js rather than the other way round)? Thank you!

app.js

var app = express(),
  server = require('http').createServer(app);

//KIV -> io is set as a global variable
io = require('socket.io').listen(server);
require('./socket');

socket.js

io.sockets.on('connection', function(socket) {
    //xxx
}

This question what I was searching for .. :)

voila
– voila

2013-12-22 07:46:46 +00:00
Commented Dec 22, 2013 at 7:46 — voila
– voila, Commented Dec 22, 2013 at 7:46

jsist · Accepted Answer · 2013-04-01 03:26:06Z

19

One of the ways is by passing the object as argument to function (as already has been described in @Thomas' answer).

Other way is to create a new file say 'global.js'. put only those items in this file that you want to be global. e.g.

var Global = {
    io : { }
};    
module.exports = Global;

Now, in your app.js,

var app = express(),
    server = require('http').createServer(app), 
    global = require('./global.js');

global.io = require('socket.io').listen(server);
require('./socket');

And in your socket.js:

var global = require('./global.js');
global.io.sockets.on('connection', function(socket) {
    //xxx
}

Hope it helps...

answered Apr 1, 2013 at 3:26

jsist

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2 Comments

Alex Bowyer Over a year ago

One caveat with this: wouldn't this create a separate instance of global.js in each file that requires it? So if there is some state or data in global.js that you need to access in multiple modules, not just socket.js, then you this wouldn't work as the global object accessed in socket.js is not the same object accessed in your other child module....

jsist Over a year ago

@alexbfree it would not create separate instance of Global for each file it is required in. It will be same instance.

Prasad Babu · Accepted Answer · 2020-07-04 15:40:28Z

11

app.js

var app = express(),
  server = require('http').createServer(app),
  socket = require('./socket');

var io = require('socket.io').listen(server);

socket(io)

socket.js

module.exports = function (io) {
  io.sockets.on('connection', function(socket) {
    // Your code here

  });
}

edited Jul 4, 2020 at 15:40

Prasad Babu

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answered Apr 1, 2013 at 2:32

Thomas Blobaum

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1 Comment

Nick Pineda Over a year ago

But how could the socket server (io) be accessed from other files?

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