Need help to connect 2 types in graphql (express-graphql)

UserType will be undefined if I require it on the top of the transaction_type.js
If I require it in fields arrow function, everything will work great.
See comments in transaction_type.js
Can anyone help me with this javascript issue?

transaction_type.js

    const graphql = require('graphql');
    const { GraphQLObjectType, GraphQLString, GraphQLID, GraphQLList } = graphql;
    const UserType = require('./user_type.js');
    // UserType is undefined
    const User = require('../models/User');
    const Transaction = require('../models/Transaction');

    const TransactionType = new GraphQLObjectType({
      name:  'TransactionType',
      fields: () => ({
        id: { type: GraphQLID },
        title: { type: GraphQLString },
        type: { type: GraphQLString },
        price: { type: GraphQLString },
        date: { type: GraphQLString },
        user: {
          // If I comment this line
          type: UserType,
          // And uncomment this
          // type: require('./user_type.js'),
          // It will work good
          resolve(parentValue) {
            return User.findOne({ _id: parentValue._userId })
              .catch(err => console.log(err));
          }
        }
      })
    });

    module.exports = TransactionType;

user_type.js

const graphql = require('graphql');
const { GraphQLObjectType, GraphQLString, GraphQLID, GraphQLList, GraphQLInt } = graphql;

const TransactionType = require('./transaction_type');
const User = require('../models/User');
const Transaction = require('../models/Transaction');

const UserType = new GraphQLObjectType({
  name: 'UserType',
  fields: () => ({
    id: { type: GraphQLID },
    firstName: { type: GraphQLString },
    lastName: { type: GraphQLString },
    age: { type: GraphQLInt },
    transactions: {
      type: new GraphQLList(TransactionType),
      resolve(parentValue) {
        return Transaction.find({ _userId: parentValue._id })
          .catch(err => console.log(err));
      }
    }
  })
});

via Александр Чорнокондратенко