//返回元素中的索引值
获取 <div> 元素的第一个类名（索引为0）:
if (!("classList" in document.documentElement)) {  
        Object.defineProperty(HTMLElement.prototype, ‘classList‘, {  
            get: function() {  
                var self = this;  
                function update(fn) {  
                    return function(value) {  
                        var classes = self.className.split(/\s+/g),  
                            index = classes.indexOf(value);  
                          
                        fn(classes, index, value);  
                        self.className = classes.join(" ");  
                    }  
                }  
                  
                return {                      
                    add: update(function(classes, index, value) {  
                        if (!~index) classes.push(value);  
                    }),  
                      
                    remove: update(function(classes, index) {  
                        if (~index) classes.splice(index, 1);  
                    }),  
                      
                    toggle: update(function(classes, index, value) {  
                        if (~index)  
                            classes.splice(index, 1);  
                        else  
                            classes.push(value);  
                    }),  
                      
                    contains: function(value) {  
                        return !!~self.className.split(/\s+/g).indexOf(value);  
                    },  
                      
                    item: function(i) {  
                        return self.className.split(/\s+/g)[i] || null;  
                    }  
                };  
            }  
        });  
    }