Singleton pattern in nodejs - is it needed?
All of the above is overcomplicated. There is a school of thought which says design patterns are showing deficiencies of actual language.
Languages with prototype-based OOP (classless) do not need a singleton pattern at all. You simply create a single(ton) object on the fly and then use it.
As for modules in node, yes, by default they are cached, but it can be tweaked for example if you want hot-loading of module changes.
But yes, if you want to use shared object all over, putting it in a module exports is fine. Just do not complicate it with "singleton pattern", no need for it in JavaScript.
This has basically to do with nodejs caching. Plain and simple.
https://nodejs.org/api/modules.html#modules_caching
(v 6.3.1)
Caching
Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.
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.
If you want to have a module execute code multiple times, then export a function, and call that function.
Module Caching Caveats
Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files.
Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file.
So in simple terms.
If you want a Singleton; export an object.
If you do not want a Singleton; export a function (and do stuff/return stuff/whatever in that function).
To be VERY clear, if you do this properly it should work, look at https://stackoverflow.com/a/33746703/1137669 (Allen Luce's answer). It explains in code what happens when caching fails due to differently resolved filenames. But if you ALWAYS resolve to the same filename it should work.
Update 2016
creating a true singleton in node.js with es6 symbols Another solution: in this link
Update 2020
This answer refers to CommonJS (Node.js's own way to import/export modules). Node.js will most likely be switching over to ECMAScript Modules: https://nodejs.org/api/esm.html (ECMAScript is the real name of JavaScript if you didn't know)
When migrating to ECMAScript read the following for now: https://nodejs.org/api/esm.html#esm_writing_dual_packages_while_avoiding_or_minimizing_hazards