Demo.getSingle = (function(){
  var demo = null;
  return function(name){
    if(!demo){
      demo = new Demo(name);
    }
    return demo;
  }
})(); 

 var b1 = Demo.getSingle('Peter');
var b2 = Demo.getSingle('Sufei');
b1 === b2;//true,都引用的是new Demo('Peter') 

 var ProxyDemo = (function(){
  var demo = null;
  return function(name){
    if(!demo){
      demo = new Demo(name);
    }
    return demo;
  }
})(); 

 var getSingle = function(foo){
  var single = null;
  return function(){
    return single || (single = foo.apply(this,arguments));
  }
}; 

 var createLoginLayer = function(){
  var frag = document.createDocumentFragment();
  var div = document.createElement('div');
  div.style.display = 'none';
  //以下给div添加其它登录元素
  ...
  document.body.appendChild(frag.appendChild(div));
  return div;
}
var createSingleLoginLayer = getSingle(createLoginLayer);
//当用户第一次点击按钮(id = 'lgBtn')时，来创建并显示登录窗口，之后重复点击按钮不会重复创建；
document.getElementById('lgBtn').onclick = function(){
  var lg = createSingleLoginLayer();
  lg.style.display = 'block';
} 

 function foo(n){
  results = n < 2 ? n : foo(n - 1) + foo(n - 2);
  return results;
}
console.log(foo(40));//得计算好几秒 

 var cache = {};
function foo(n){
  if(!cache[n]){
    cache[n] = n < 2 ? n : foo(n - 1) + foo(n - 2);
  }
  return cache[n];
}
console.log(foo(100));

 var foo = (function(){
  var cache = {};
  return function(n){
    if(!cache[n]){
      cache[n] = n < 2 ? n : foo(n - 1) + foo(n - 2);
    }
    return cache[n];
  };
})();
console.log(foo(100));