A Singleton Class

This Javascript program demonstrates how to create a JavaScript singleton class. The class CFactory is a singleton, which means that only one CFactory can be created. Singletons used to prevent multiple instances of a class. In this instance, having only one CFactory prevents the creation of multiple products with the same ID value.

Singleton.html

<!DOCTYPE html>
<html>
<head>
    <title>XoaX.net's Javascript</title>
</head>
<body>
    <script type="text/javascript" src="Singleton.js"></script>
</body>
</html>

Singleton.js

// This is a singleton class
class CFactory {
	// These are private properties that can not be accessed directly.
	static #sqInstance = null;
	#miLastId = 0;
	
	constructor() {
		// Use this to prevent more than one instance from being created
		if (CFactory.#sqInstance) {
			return CFactory.#sqInstance;
		}
		CFactory.#sqInstance = this;
	}
	
	MakeProduct() {
		var qNewProduct = new CProduct(CFactory.#sqInstance.#miLastId);
		CFactory.#sqInstance.#miLastId += 1;
		return qNewProduct;
	}
}

class CProduct {
	// This is a private property that can not be accessed directly.
	#miID;
	constructor(iID) {
		this.#miID = iID;
	}
	
	PrintID() {
		document.write("Product ID = "+this.#miID+"<br />");
	}
}

// This function just prints out the public members of an object.
function PrintAnObject(qObject) {
  var iCount = 0;
  var sOutput = typeof(qObject)+": " + qObject.constructor.name + "<br />";
  for (var sProperty in qObject) {
    sOutput += "&nbsp;&nbsp;&nbsp;&nbsp;" + sProperty + ': ' + qObject[sProperty]+"<br />";
    ++iCount;
    if (iCount >= 15) {
      document.writeln(sOutput);
      document.writeln("&nbsp;&nbsp;&nbsp;&nbsp;...<br />");
      return;
    }
  }
  document.writeln(sOutput);
}

// These are the same object.
// The CFactory is a singleton.
// So, each use of new returns the same CFactory.
var qFactory1 = new CFactory();
var qFactory2 = new CFactory();

var qProduct1 = qFactory1.MakeProduct();
var qProduct2 = qFactory2.MakeProduct();
var qProduct3 = qFactory2.MakeProduct();
var qProduct4 = qFactory1.MakeProduct();

PrintAnObject(qFactory1);
PrintAnObject(qFactory2);

// This check verifies that the CFactory objects are the same
document.write("The factories are " + ((qFactory1 != qFactory2) ? "not":"") + " equal.<br />");

// Each of the products have unique ids even though
// they are created by the factory with different names.
PrintAnObject(qProduct1);
qProduct1.PrintID();
PrintAnObject(qProduct2);
qProduct2.PrintID();
PrintAnObject(qProduct3);
qProduct3.PrintID();
PrintAnObject(qProduct4);
qProduct4.PrintID();