if (typeof HTMLElement.prototype.classlist === "undefined") {
    HTMLElement.classList = function ClassList() { return this.className.split(' '); };
    HTMLElement.classList.prototype.add = function(name) {
        if (classList.indexOf(name) !== -1) {
            return;
        }
        classList[classList.length] = name;
        this.className = classList.join(' ');
    };
    HTMLElement.classList.prototype.remove = function(name) {
        var index = classList.indexOf(name);
        if (index !== -1) {
            this.className = classList.splice(index + 1, 1).join(' ');
        }
    }
}

element.classList.add('fadeIn');