Romaji to Kana Converter: Angular & Express — Chapter 5, Exercise 2
=====================================================================

TASK
Build the real /api/convert route with the validation this chapter
describes. Send it a request with no romaji field and confirm you
get a clean 400 JSON error rather than a 500 stack trace, then send
it a valid request and confirm the real, correct kana comes back
exactly as verified above.

SOLUTION

// server.js — the real, validated route

  require('dotenv').config();
  const express = require('express');
  const cors = require('cors');
  const { convert } = require('./conversion-engine');

  const app = express();
  app.use(cors());
  app.use(express.json());

  app.post('/api/convert', (req, res) => {
    const { romaji } = req.body;

    if (typeof romaji !== 'string' || romaji.trim() === '') {
      return res.status(400).json({
        error: 'romaji field is required and must be a non-empty string',
      });
    }

    const result = convert(romaji);
    res.json(result);
  });

  const PORT = process.env.PORT || 4000;
  app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

Testing both cases, with the server actually running:

  $ curl -i -X POST http://localhost:4000/api/convert \
      -H "Content-Type: application/json" -d '{}'

  HTTP/1.1 400 Bad Request
  Content-Type: application/json; charset=utf-8

  {"error":"romaji field is required and must be a non-empty string"}

  $ curl -X POST http://localhost:4000/api/convert \
      -H "Content-Type: application/json" -d '{"romaji":"gakkou e ikimasu"}'

  {"hiragana":"がっこう へ いきます","katakana":"ガッコウ ヘ イキマス"}

WHY THIS WORKS AS AN ANSWER
----------------------------
The missing-field request returns a genuine 400 status with a
structured JSON body naming the actual problem -- no HTML, no stack
trace, no leaked file paths -- because the typeof/empty-string check
runs and returns before convert() is ever called with a bad value.
The valid request returns the exact same result the chapter's own
verification script already confirmed byte-for-byte against Chapter
3's original output, this time delivered over a real HTTP round trip
rather than a direct function call.