How to convert the result of the selection to JSON and pass it to the client side?

Question

PostgreSQL

On the server side (backend), using Sequelize, I connect to the Postgres database and get a selection of data from one of the tables (entities). When I try to send the result of the selection in JSON format to the client side (frontend), I get an error in the browser console:

Uncaught (in promise) TypeError: this.state.party.map is not a function

What could be the problem? What am I doing wrong?
backend code:

const express = require('express');
const app = express();
const Sequelize = require('sequelize');

const sequelize = new Sequelize('testdb', 'postgres', 'somePassword', {
  dialect: 'postgres',
  define: {
    timestamps: false
  }
}); // данные для подключения к бд

sequelize
  .authenticate()
  .then(() => {
    console.log('Connection has been established successfully.');
  })
  .catch(err => {
    console.error('Unable to connect to the database: ', err);
  }); // подключение к бд

const Party = sequelize.define('party', {
  name: {
    type: Sequelize.STRING
  }
}); // определяем модель (таблицу в бд) Party

Party.findAll().then(party => {
  console.log(JSON.stringify(party));
}); // выборка данных (выводим в консоль для проверки)

app.route('/party-table')
  .get((req, res) => {
    res.send('get...');
  })
  .post((req, res) => {
    console.log(req.body);
   res.send(     
      Party.findAll().then(party => {
        return JSON.stringify(party);
      }) // выборка данных      
  );
  })
  .put((req, res) => {
    res.send('put');
  });

const PORT = process.env.PORT || 3000;

app.listen(PORT, () => {
  console.log(`Server listening on: ${PORT}`);
});

frontend code:

import React, { PropTypes, Component } from 'react';

class PartyTable extends Component {
  constructor(props) {
    super(props);
    this.state = {
      party: []
    };
    this.handleFetch = this.handleFetch.bind(this);
  }

  componentDidMount() {
    this.handleFetch();
  }

  handleFetch() {
    fetch('http://127.0.0.1:3000/party-table', {
      method: 'POST'
    })
      .then((response) => {
        if (response.status >= 400) {
          throw new Error('Bad response from server');
        }
        return response.json();
      })
      .then(stories =>
        this.setState({ party: stories }) // новое состоняние
      );
  }

  render() {
    return (
      <table className='table table-striped table-hover table-bordered'>
        <thead className='table__thead'>
          <TableHeader/>
        </thead>
        <tbody>
        {this.state.party.map(item => {                              // на эту строку ругается браузер (сообщение об ошибке в консоли)
          return (<TableRow key={item.id} value={item.name}/>);
        })
        }
        </tbody>
      </table>
    );
  }
}

PartyTable.propTypes = propTypes;
PartyTable.defaultProps = defaultProps;
export default PartyTable;

The result of the request in JSON format, which is displayed in the console on the server:

[{"id":1,"name":"Партия1"},{"id":2,"name":"Партия2"},{"id":3,"name":"Партия3"},{"id":4,"name":"Партия4"},{"id":5,"name":"Партия5"}]

What comes to the client:
{"isFulfilled":false,"isRejected":false}

Answer 1

Problematic place:

res.send(     
  Party.findAll().then(party => {
    return JSON.stringify(party);
  });
);

Corrected version: Better yet, use async/await:

.post(async (req, res) => {
  const party = await Party.findAll();
  res.json(party);
})

or:

.post(async (req, res) => {
  res.json(await Party.findAll());
})