getEvent : function(event){
  return event ? event : window.event;
  //return event || window.event;//或者更简单的方式
}

function getEvent() {
  return arguments[0] ? arguments[0] : window.event;
  //return arguments[0] || window.event;//或者更简单的方式
}

function handler(event){
  //do something
}
<!-- HTML的DOM0级方式 -->
<button id="btn" onclick="handler(event);">按钮</button>

function handler(event){
  //do something
}
btn.onclick = handler;//JS的DOM0级方式
//btn.onclick = function(event){/*do something*/}//或者匿名函数，效果同上

ele.add/removeEventListener(eventType, handler, catch);

//跨浏览器的事件处理器添加方式
var EventUtil = {
  addHandler : function(elem, type, handler){
    if(elem.addEventListener){
      elem.addEventListener(type, handler, false);
    }
    else if(elem.attachEvent){
      elem.attachEvent("on" + type, handler);//添加多个同一类型的handler时，IE方式的规则是最后添加的最先触发
    }
    else{
      if(typeof elem["on" + type] === 'function'){
        var oldHandler = elem["on" + type];
        elem["on" + type] = function(){
          oldHandler();
          handler();
        }
      }
      else{
        elem["on" + type] = handler;//支持添加多个事件处理器
      }
    }
  },

  getEvent : function(event){
    return event ? event : window.event;
  },

  getTarget : function(event){
    return event.target || event.srcElement;
  },

  preventDefault : function(event){
    if(event.preventDefault){
      event.preventDefault();
    }
    else{
      event.returnValue = false;
    }
  },

  removeHandler : function(elem, type, handler){
    if(elem.removeEventListener){
      elem.removeEventListener(type, handler, false);
    }
    else if(elem.detachEvent){
      elem.detachEvent("on" + type, handler);
    }
    else{
      elem["on" + type] = null;//不支持移除单一事件处理器，只能全部移除
    }
  },

  stopPropagation : function(event){
    if(event.stopPropagation){
      event.stopPropagation();
    }
    else{
      event.cancelBubble = true;
    }
  },

  getRelatedTarget : function(event){
    if(event.relatedTarget){
      return event.relatedTarget;
    }
    else if(event.toElement && event.type == "mouseout"){
      return event.toElement;
    }
    else if(event.fromElement && event.type == "mouseover"){
      return event.fromElement;
    }
    else{
      return null;
    }
  },

  /*IE8点击左键和中键都是0；FF无法识别中键；Chrome正常*/
  getButton : function(event){//返回0，1，2 - 左，中，右
    if(document.implementation.hasFeature("MouseEvents", "2.0")){
      return event.button;
    }
    else{
      switch(event.button){
        case 0:case 1:case 3:case 5:case 7:
          return 0;
          break;
        case 2:case 6:
          return 2;
          break;
        case 4:
          return 1;
          break;
        default:
          break;
      }
    }
  },

  /*只能检测keypress事件，返回值等于将要显示的字符编码*/
  /*IE和Chrome只有能显示的字符键才触发，FF其它键也能触发，返回值为0*/
  getCharCode : function(event){
    if(typeof event.charCode == "number"){
      return event.charCode;
    }
    else{
      return event.keyCode;
    }
  }
};

var eventUtility = {
  addEvent : function(el, type, fn) {
    if(typeof addEventListener !== "undefined") {
      el.addEventListener(type, fn, false);
    } else if(typeof attachEvent !== "undefined") {
      el.attachEvent("on" + type, fn);
    } else {
      el["on" + type] = fn;
    }
  },

  removeEvent : function(el, type, fn) {
    if(typeof removeEventListener !== "undefined") {
      el.removeEventListener(type, fn, false);
    } else if(typeof detachEvent !== "undefined") {
      el.detachEvent("on" + type, fn);
    } else {
      el["on" + type] = null;
    }
  },

  getTarget : function(event) {
    if(typeof event.target !== "undefined") {
      return event.target;
    } else {
      return event.srcElement;
    }
  },

  preventDefault : function(event) {
    if(typeof event.preventDefault !== "undefined") {
      event.preventDefault();
    } else {
      event.returnValue = false;
    }
  }
};

var eventHandler = function(evt) {
  var target = eventUtility.getTarget(evt),
    tagName = target.tagName;

  if(evt.type === "click") {
    if(tagName === "A" || tagName === "BUTTON") {
      alert("You clicked on an A element, and the innerHTML is " + target.innerHTML + "!");
      eventUtility.preventDefault(evt);
    }
  } else if(evt.type === "mouseover" && tagName === "A") {
    alert("mouseovered " + target.innerHTML);
  }

};

eventUtility.addEvent(document, "click", eventHandler);
eventUtility.addEvent(document, "mouseover", eventHandler);

eventUtility.removeEvent(document, "mouseover", eventHandler);