code: function clas(elem, className) {
      /**
       * @param {boolean} [val]
       * @returns {boolean|undefined}
       */
      return function() {
          if (arguments.length) {
              elem.classList[arguments[0] ? "add" : "remove"](className);
          } else {
              return elem.classList.has(className);
          }
      }
  }
docs: Create function bound to DOM class.  Class will be enabled when function is
  called with truthy value and disabled when called with falsey value.  When
  called without an argument, the function will return a boolean indicating if
  the class is set.
  @param {Element} elem
  @param {string} className
  @returns {function}