prop02.js

import _ from 'lodash';
import FieldFormatProvider from 'ui/index_patterns/_field_format/FieldFormat';
import fieldFormatsRegistry from 'ui/registry/field_formats';
import KibanaPlugin from 'somewhere/in/there'

module.exports = new KibanaPlugin('myPluginName', {
  type: 'fieldFormat',
  init: function () {
    fieldFormatsRegistry.register(function UrlFormatProvider(Private) {
      const FieldFormat = Private(FieldFormatProvider);
    
      return class MyFieldFormat extends FieldFormat {
        static id = 'lowercase';
        static title = 'lowercase';
        static fieldType = '*';
        static editor = null;
        static paramDefaults = {};
    
        _convert(value) {
          return String(value).toLowerCase();
        }
      };
    });
  }
});