JavaScript设计模式与开发实践：惰性函数

Web开发中，因为浏览器之间的差异实现差异，一些嗅探工作总是不可避免的，比如我们需要在各个浏览器中能够通用事件绑定函数addEvent

//一般写法
//缺点：当他每次被调用的时候都都会执行里面的if条件，
var addEvent = function(elem, type, handler){
    if(window.addEventLister){
        return elem.addEventListener(type, handler, false);
    }
    if(window.attachEvent){
        return elem.attachEvent('on'+type, handler);
    }
}

//改进

var addEvent = (function(){
    if(window.addEventListener){
        return function(elem, type, handler){
            elem.addEventListener(type, handler, false);
        }
    }
    if(window.attachEvent){
        return function(elem, type, handler){
            elem.atttachEvent('on'+handler);
        }
    }
})();

//惰性载入方案

var addEvent = function(elem, type, handler){
    if(window.addEventListener){
        addEvent = function(elem, type, handler){
            elem.addEventListener(type, handler, false);
        }
    } else if(window.atttachEvent){
        addEvent = function(elem, type, handler){
            elem.atttachEvent('on'+type, handler);
        }
    }

    addEvent(elem, type, handler);
};