Typescript Singleton

Singleton.ts

module x
{
	export class Singleton
	{
	    private static _instance : Singleton;
		constructor( singletonEnforcer:()=>void )
		{
			if( singletonEnforcer !== SingletonEnforcer )
			{
				throw new Error("Error: Instantiation failed: Use Singleton.getInstance() instead of new.");
			}
		}
	    public static getInstance() : Singleton
		{
	        if(Singleton._instance == null)
			{
	            Singleton._instance = new Singleton(SingletonEnforcer); 
	        } 
	        return Singleton._instance;
	    }
	}
	function SingletonEnforcer(){} 
}

x.Singleton.getInstance();

/* V1 */
class SingletonDemo
{
    private static instance:SingletonDemo;
    private static allowInstantiation:Boolean;
    public static getInstance():SingletonDemo
    {
        if (instance == null)
        {
            SingletonDemo.allowInstantiation = true;
            SingletonDemo.instance = new SingletonDemo();
            SingletonDemo.allowInstantiation = false;
        }
        return instance;
    }

    constructor()
    {
        if (!SingletonDemo.allowInstantiation) {
            throw new Error("Error: Instantiation failed: Use SingletonDemo.getInstance() instead of new.");
        }
    }
}