Singleton

singleton.js

var Singleton = (function() {
  var instance;
  return function () {
    if ( !this instanceof Singleton ) {
      return new Singleton();
    }
    if ( !instance ) {
      instance = this;
    }
    return instance;
  };
})();

var a = Singleton();
var b = new Singleton();
console.assert( a === b );