code: function addClass(className) {
    if (typeof className === 'undefined') {
      return this;
    }
    const classes = className.split(' ');
    for (let i = 0; i < classes.length; i += 1) {
      for (let j = 0; j < this.length; j += 1) {
        if (typeof this[j] !== 'undefined' && typeof this[j].classList !== 'undefined') this[j].classList.add(classes[i]);
      }
    }
    return this;
  }
docs: Classes and attributes