Singleton 1.0.4

1. Purpose
   Singleton is an implementation of the Singleton Design pattern

2. Origin While looking for examples of Singleton Design pattern, I found these useful clues:

   • Usage of Symbol() as a way to ensure that the constructor will only accept a predefined and unique value
   • Check that Instance count is less than or equal to One

   Then I used a Static Getter for This to get the Singleton, so the code to get the Singleton Instance is Singleton.This (instead of the more common static method GetInstance() which in my opinion makes the code too verbous).

   For targeting all module types, I provided the CJS (CommonJS), ES6 (ECMAScript 2015) and UMD (Universal Module Definition) versions. The conversions (from CJS) were performed with the help of the rollup bundler (cjs to es6 and umd.txt in doc folder explains how to use rollup for this purpose).

   The default export (main field in package.json) is index.js, the ES6 version.

3. Source code

class Singleton {
	static #_Key = Symbol();
	static #_Instance = new Singleton( this.#_Key );
	static #_InstanceCount = 0;

    // ** Private constructor **
	constructor( key ) {
		if ( key !== Singleton.#_Key ) {
			throw new TypeError("'Singleton' constructor is private");
		}

        this.name = "Singleton";
	} // ** Private constructor ** 'Singleton' design pattern
	
	static get This() {
		if ( Singleton.#_Instance == undefined ) {
			this.#_Instance = new Singleton();
			if ( this.#_InstanceCount > 0 ) {
				throw new TypeError("'Singleton' constructor called more than once");
			}
			this.#_InstanceCount++;
        }
        return Singleton.#_Instance;
    } // Singleton get 'This'	
} // Singleton class

Singleton.This; // NB: ensures that #_InstanceCount = 1