classList_v3.js

ClassList.prototype.concat = function (otherList) {
  return ClassList.of(this.getClasses().concat(otherList.getClasses()));
}

// We check if it's a value we can work on. There are some other
// checks we need to make, but this is not the place to do it
ClassList.prototype.addClass = function (classes) {
  const classesToAdd = Array.isArray(classes) ? classes
                                              : [classes];
  return ClassList.of(this.getClasses()).concat(ClassList.of(classesToAdd));
}

ClassList.prototype.removeClass = function (className) {
  coonst predicate = (currentClass) => !== className;
  
  return ClassList.of(this.getClasses()).filter(predicate)
}

ClassList.prototype.getString = function () {
  return this.getClasses.join(' ');
}

classList_v3_usage.js

const cl1 = ClassList.of(['btn'])
const cl2 = ClassList.of(['btn-default']);
const cl3 = ClassList.of(['btn-solarized', 'btn-default-solarized']);
const cl4 = cl1.concat(cl2).concat(cl3);

cl4.getString();
"btn btn-default btn-solarized btn-default-solarized"

cl4.addClass('btn-disabled').removeClass('btn-default').getString();
//"btn btn-solarized btn-default-solarized btn-disabled";