Singleton pattern in Javascript

gistfile1.js

var obj = (function(){
  function Class() {
  }
  Class.prototype = {
    _var : null,
    func1 : function() {
    },
    func2 : function() {
    }
  };
  var _instance;
  return {
    instance : function(){
      if (_instance == null) {
        _instance = new Class();
        _instance.constructor = null;
      }
      return _instance;
    }
  };
})();

// Usage
obj.instance().func1();
// or
var o = obj.instance();
o.func1();