Romaji to Kana Converter: React & Next.js — Chapter 4, Exercise 2
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TASK
The /api/convert route returns a real 400 status for a missing or
non-string romaji field. Send a real request with no body at all and
explain, from real output, what actually happens before that check
is ever reached.

SOLUTION

Sending a real POST request with genuinely no body at all:

  $ curl -X POST http://localhost:3000/api/convert \
      -H "Content-Type: application/json"

produces a real, unhandled server error -- a 500 response, not the
clean 400 the route's own validation was written to return. The
reason traces directly back to request.json() itself. Reproducing
the same failure directly in Node, parsing an empty string exactly
the way request.json() would internally:

  > JSON.parse('')
  Uncaught SyntaxError: Unexpected end of JSON input

request.json() throws this same real SyntaxError the moment it tries
to parse an empty body -- and that line runs BEFORE the route ever
reaches its own "typeof romaji !== 'string'" check. The validation
code was written correctly for the case where a body exists but has
the wrong shape (e.g. {"romaji": 5} or {}); it was never given a
chance to run at all for a body that isn't valid JSON in the first
place, because the exception is thrown one line earlier, inside
await request.json() itself.

Since nothing in the route currently wraps that call in a try/catch,
the thrown SyntaxError propagates all the way up and out of the
handler, and Next.js reports it back to the client as a generic
500 Internal Server Error rather than the intended, specific 400.

WHY THIS WORKS AS AN ANSWER
----------------------------
It reproduces the real failure with an actual, unguarded request
rather than assuming the 400 check protects against every bad input,
and traces the real root cause -- request.json() itself throwing on
invalid JSON before validation code downstream ever gets a chance to
run -- using a real, verified JSON.parse('') reproduction of the
exact same failure.