create-custom-resolver.js

const makeCustomResolver = (userSchema) => {
  // Create custom resolvers
  const customResolver = {
    Mutation: {
      insert_user(parent, args, context, info){
        let newArgs = { objects:[] };
        // Convert the email to lower case and append each object to the new arguments array
        for (const user of args.objects) {
          user.email = user.email.toLowerCase();
          newArgs.objects.push(user);
        }
        // Delegate after sanitization
        return info.mergeInfo.delegateToSchema({
          schema: userSchema,
          operation: 'mutation',
          fieldName: 'insert_user',
          args: newArgs,
          context,
          info,
        });
      }
    } 
  };
  return customResolver;
};