How to create an application using the Monzo Bank API (Full solution)

index.js

const express = require('express');
const request = require('request');
const app = express();

const oauthDetails = {
  client_id: '[your client id]',
  client_secret: '[your client secret]',
  redirect_uri: 'http://localhost:3000/oauth/callback'
};

// Will be populated once received
let accessToken = null;

app.get('/', (req, res) => {
  const { client_id, redirect_uri } = oauthDetails;
  const monzoAuthUrl = 'https://auth.monzo.com';
  
  res.type('html');
  res.send(`
    <h1>Hello</h1>
    <form action="${monzoAuthUrl}">
      <input type="hidden" name="client_id" value="${client_id}" />
      <input type="hidden" name="redirect_uri" value="${redirect_uri}" />
      <input type="hidden" name="response_type" value="code" />
      <button>Authorise app</button>
    </form>
  `);
});

app.get('/oauth/callback', (req, res) => {
  const { client_id, client_secret, redirect_uri } = oauthDetails;
  const { code } = req.query;
  const monzoAuthUrl = `https://api.monzo.com/oauth2/token`;
  
  request.post({
    url: monzoAuthUrl,
    form: {
      grant_type: 'authorization_code',
      client_id,
      client_secret,
      redirect_uri,
      code
    } 
  }, (err, response, body) => {
    accessToken = JSON.parse(body); // Populate accessToken
    res.redirect('/accounts');
  });
});

app.get('/accounts', (req, res) => {
  const accountsUrl = 'https://api.monzo.com/accounts';
  const { token_type, access_token } = accessToken;
  
  request.get(accountsUrl, {
    headers: {
      Authorization: `${token_type} ${access_token}` 
    }
  }, (req, response, body) => {
    const { accounts } = JSON.parse(body);

    res.type('html');
    res.write('<h1>Accounts</h1><ul>');
    
    for(let account of accounts) {
      const {id, type, description } = account;
      res.write(`
        <li>
          ${description}(<i>${type}</i>) - <a href="/transactions/${id}">View transaction history</a>
        </li>
      `);
    }
    
    res.end('</ul>');
  });
});

app.get('/transactions/:acc_id', (req, res) => {
  const { acc_id } = req.params;
  const { token_type, access_token } = accessToken;
  const transactionsUrl = `https://api.monzo.com/transactions?expand[]=merchant&account_id=${acc_id}`;
  
  request.get(transactionsUrl, {
    headers: {
      Authorization: `${token_type} ${access_token}` 
    }
  }, (req, response, body) => {
    const { transactions } = JSON.parse(body);

    res.type('html');
    res.write(`
      <h1>Transactions</h1>
      <table>
        <thead>
          <th>Description</th>
          <th>Amount</th>
          <th>Category</th>
        </thead>
        <tbody>
    `);
    
    for(let transaction of transactions) {
      const {
        description,
        amount,
        category
      } = transaction;
      
      res.write(`
        <tr>
          <td>${description}</td>
          <td>&pound;${(amount/100).toFixed(2)}</td>
          <td>${category}</td>
        </tr>
      `);
    }
    
    res.write('</tbody></table>');
    res.end('<br /><a href="/accounts">&lt; Back to accounts</a>');
  });
});

app.listen(3000);