code: function generateFilterCode(filterObjAst, t, options) {
      if (!filterObjAst.properties.length) {
          return '';
      }
  
      let {format = 'es6', usingBabel6} = options || {};
      let ast;
      if (format === 'es6') {
          ast = generateES6ModuleFilterCode(filterObjAst, t);
      }
      else {
          ast = generateCommonJSModuleFilterCode(filterObjAst, t);
      }
  
      return generateCode(ast, {
          auxiliaryCommentBefore: 'Auto generated filter code by okam'
      }, usingBabel6).code;
  }
docs: Generated filter code
  
  @param {Object} filterObjAst the filter definition ast
  @param {Object} t the babel type definition
  @param {Object=} options the generation option
  @param {string=} options.format the module format, by default es6 module syntax
  the supported module format: es6/commonjs
  @param {boolean=} options.usingBabel6 whether using babel6 processor, by default false
  @return {string}