Pretty simple isn’t it ?
That’s why i love ES6, this singleton has the same structure as java’s version below.
So now, choose your your faction ! ES5 or ES6. Personnally, i’ve already chosen :)
PS: With theses singletons i haven’t consider the concurrency issue caused by multi-thread use case. Be careful. (with node.js the usercode is single threaded, it means, you cannot have concurrency issue with singletons)