IE 10前的版本兼容 classList

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;
                    }
                };
            }
        });
    }