custom-validation.js

import { UserInputError } from 'apollo-server';

const resolvers = {
  Query: {
    events(root, { zipCode }) {
      
      // do custom validation for user inputs
      const validationErrors = {};
      if (!isValidZipCode(zipCode)) {
        validationErrors.zipCode = 'This is not a valid zipcode';
      }
      if (Object.keys(validationErrors).length > 0) {
        throw new UserInputError(
          'Failed to get events due to validation errors',
          { validationErrors }
        );
      }
      // actually query events here and return successfully
      return getEventsByZipcode(zipCode);
    }
  }
}