绑定事件

// 绑定事件   初始化分支  只需要检测浏览器1次即可  无需每次调用函数都要检测
    var btn = document.getElementById("btn");
    var btn1 = document.getElementById("btn1");
    

    var addEvent = document.body.addEventListener ? function(el, type, fn){
            el.addEventListener(type, fn, false);
        } : function(el, type, fn){
            el.attachEvent("on" + type, function(){
                   fn.apply(el, arguments);
            });
        };

    if(typeof document.body.addEventListener === "function"){
        console.log("ss");  // 1 次
        addEvent = function(el, type, fn){    
            //console.log(0);    // 2 次
            el.addEventListener(type, fn, false);
        };
    }else{
        addEvent = function(el, type, fn){
            el.attachEvent("on" + type, function(){
                   fn.apply(el, arguments);
            })
        };
    }

    addEvent(btn, "click", function(){console.log(this)});
    addEvent(btn1, "click", function(){console.log(this)});