惰性函数

[Saikikoko]

本质是重写函数
举栗子：
兼容ie浏览器和主流浏览器的事件监听

function addEvent (type, el, fn) {
  if (window.addEventListener) {
      addEvent = function (type, el, fn) {
          el.addEventListener(type, fn, false);
      }
  }
  else if(window.attachEvent){
      addEvent = function (type, el, fn) {
          el.attachEvent('on' + type, fn);
      }
  }
  addEvent(type, el, fn)
}