javascript设计模式——懒惰模式

懒惰模式：减少每次代码执行时的重复性的分支判断，通过对对象重定义来屏蔽原对象中的分支判断。

  <script>
    // 需要重复进入分支，浪费资源
    var A = {}
    A.on = function (dom, type, fn) {
      if (dom.addEventListener) {
        dom.addEventListener(type, fn, false)
      } else if (dom.attachEvent) {
        dom.attachEvent('on' + type, fn)
      } else {
        dom['on' + type] = fn
      }
    }

    // 通过闭包，实现加载即执行
    A.on = function (dom, type, fn) {
      if (document.addEventListener) {
        return function (dom, type, fn) {
          dom.addEventListener(type, fn, false)
        }
      } else if (document.attachEvent) {
        return function (dom, type, fn) {
          dom.attachEvent('on' + type, fn)
        }
      } else {
        return function (dom, type, fn) {
          dom['on' + type] = fn
        }
      }
    }

    // 懒惰执行
    A.on = function (dom, type, fn) {
      if (dom.addEventListener) {
        A.on = function (dom, type, fn) {
          dom.addEventListener(type, fn, false)
        }
      } else if (dom.attachEvent) {
        A.on = function(dom, type, fn) {
          dom.attachEvent('on' + type, fn)
        }
      } else {
        A.on = function (dom, type, fn) {
          dom['on' + type] = fn
        }
      }
    }
  </script>