getterSetter generator function used in Falcor article

falcorGetterSetter.js

class MyController {
  // ... snip
  getterSetter(path) {
    // Need to return an unbound function
    var ctrl = this;
    return function (newValue) {
      // If arguments.length === 1 then the function is used as a setter,
      // otherwise it's a getter
      return arguments.length
        // setValue() returns an Observable.
        // So we call $scope.$evalAsync() when we subscribe() to it,
        // which keeps us in sync with the digest loop
        ? ctrl.model.setValue(path, newValue).subscribe(_ => ctrl.$scope.$evalAsync())
        // Here we'll reuse viewValue()
        : ctrl.viewValue(path);
    };
  }
}