Question

I have one module which wraps the Socket.io functionality my app is using which looks something like this:

// realtime.js
var io = require('socket.io'),
    sio;

exports.init = function(expressServer) {
    sio = io.listen(expressServer);
}

...

The main app.js file looks like

// app.js
var rt = require('./realtime.js'),
    other = require('./other.js');
...
rt.init(expressServer);

The other module also uses rt.js

// other.js
var rt = require('./realtime.js');
...

My question is, will both other.js and app.js have the same instance of rt.js?

The answer on SO relating to redis lead me to believe the above statement is true, but in the documentation here it says

Multiple calls to require(‘foo’) may not cause the module code to be
executed multiple times. This is an important feature. With it,
“partially done” objects can be returned, thus allowing transitive
dependencies to be loaded even when they would cause cycles.

which seems to imply that it’s not guaranteed to be the case?

Finally this question appears to indicate it depends on filename and that since there is only one instance of rt.js it shouldn’t be executed more than once. If that’s the case does it depend only on rt.js being the same file or does it depend on the path specified by require. Basically if rt.js and other.js were in lib/, and app.js was one level down the requires in other.js and app.js would point to rt.js from different files, does this matter?

I’d be grateful if anyone could clear this confusion up for me!

Answer 1

modules are currently evaluated only once, but you should not rely on this. Having any state in module is considered bad practice. What prevents you from passing reference to sio to other.js?

//realtime
var io = require('socket.io'),

exports.init = function(expressServer) {
    return io.listen(expressServer);
}

// app.js
var rt = require('./realtime.js'),
    other = require('./other.js');
...
var sio = rt.init(expressServer);
// now ask other.js to use same sio instance
other.use_sio(sio);