farm-2.x-dev/modules/core/fieldkit/farm_fieldkit.routing.yml

farm_fieldkit.field_module_js:
  path: 'fieldkit/js/{field_module}/index.js'
  defaults:
    _controller: '\Drupal\farm_fieldkit\Controller\FieldModuleController::index'
  requirements:
    # This endpoint is public.
    _access: 'TRUE'
  methods: [GET]
  options:
    parameters:
      field_module:
        type: entity:field_module