Optional singleton pattern: provides a class with a method to retrieve a key based instance.

optional-singleton.js

/**
 * Pattern which allows for a class to be optionally instantiated as singleton, using the Foo.getSingleton('foo') method
 */
 
const instances = new Map();

class Foo {

	constructor() {
		// Do all kinds of stuff, e.g. keeping state for a page
	}
	
	static getSingleton(key = '') {

		if (!instances.has(key)) {
			instances.set(key, new Foo());
		}

		return instances.get(key);

	}

}

export default Foo;

const singleton = Foo.getSingleton()
const differentSingleton = Foo.getSingleton('different')
const otherSingleton = Foo.getSingleton('different')
const instance = new Foo();

singleton !== differentSingleton; // true
differentSingleton === otherSingleton; // true