看到一个原生JS实现classList的代码，分享一下。

(function(document) {//实现classList
    if (!("classList" in document.documentElement) && Object.defineProperty && typeof HTMLElement !== 'undefined') {

        Object.defineProperty(HTMLElement.prototype, 'classList', {
            get: function() {
                var self = this;
                function update(fn) {
                    return function(value) {
                        var classes = self.className.split(/\s+/),
                                index = classes.indexOf(value);

                        fn(classes, index, value);
                        self.className = classes.join(" ");
                    };
                }

                var ret = {
                    add: update(function(classes, index, value) {
                        ~index || classes.push(value);
                    }),
                    remove: update(function(classes, index) {
                        ~index && classes.splice(index, 1);
                    }),
                    toggle: update(function(classes, index, value) {
                        ~index ? classes.splice(index, 1) : classes.push(value);
                    }),
                    contains: function(value) {
                        return !!~self.className.split(/\s+/).indexOf(value);
                    },
                    item: function(i) {
                        return self.className.split(/\s+/)[i] || null;
                    }
                };

                Object.defineProperty(ret, 'length', {
                    get: function() {
                        return self.className.split(/\s+/).length;
                    }
                });

                return ret;
            }
        });
    }
})(document);