add-class-in-class.js

function addClass(obj, cls) {
var classes = obj.className ? obj.className.split(' ') : [];

    for (var i = 0; i < classes.length; i++) {
        if (classes[i] == cls) return;
    }
        classes.push(cls);

        obj.className = classes.join(' ');

}
var obj = {
    className: 'open menu'
  }

addClass(obj, 'new');
addClass(obj, 'open');
addClass(obj, 'me');

alert(obj.className);