Adds a class with javascript via Object prototype.

obj-addClass.js

Object.prototype.addClass = function (string) {
  var classList = this.className.split(' ');
  var classesToAdd = string.split(' ');

  for (var toAdd of classesToAdd) {
      var index = classList.indexOf(toAdd);
      if(index === -1) {
          classList.push(toAdd);
      }
  }

  this.classList = classList.join(" ");
};