Romaji to Kana Converter: React & Next.js — Chapter 3, Exercise 2
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TASK
Verify with real code that convert('kekkon') ("marriage," けっこん --
a genuine doubled-k sokuon) and convert('sannin') ("three people,"
さんにん -- a genuine doubled "n" that must resolve cleanly through
the syllabic-n fix, not the sokuon check) both produce the correct
result, and explain why each one exercises a genuinely different
part of this chapter's own logic.

SOLUTION

Real, verified output:

  > convert('kekkon')
  { hiragana: 'けっこん', katakana: 'ケッコン' }

  > convert('sannin')
  { hiragana: 'さんにん', katakana: 'サンニン' }

Both match the real words exactly.

"kekkon" exercises the sokuon check specifically: "kk" is a doubled
consonant, and 'k' is not excluded the way 'n' is, so
isDoubledConsonant fires correctly, consuming the first "k" as っ and
leaving the second "k" to correctly start the next real token, "ko".

"sannin" is a genuinely different case, and it's the one this
chapter's own third bug was found and fixed against. "nn" is also a
doubled pair of the same letter, but the sokuon check's own
isDoubledConsonant condition explicitly excludes c0 !== 'n', so the
sokuon branch never fires for it at all -- correctly, since a doubled
n in romaji represents two separate, real syllabic-n-adjacent
elements, not a sokuon. Instead, the first "n" is matched normally as
the syllabic-n token (ん), and per this chapter's own fix, matching
that token sets lastVowel to null rather than to the literal
character 'n'. That's exactly what stops the second "n" from being
misread as a chōon continuation of a fake "n vowel" -- the bug this
chapter demonstrated directly with "annai" earlier. With the fix in
place, the second "n" is free to be matched normally too, giving the
correct さんにん.

So "kekkon" tests that the sokuon check correctly fires when it
should, and "sannin" tests that the sokuon check correctly does NOT
fire on a doubled n, and that the syllabic-n fix correctly prevents
the chōon check from misfiring in its place. The two words are a
matched pair proving the "n exclusion" logic works from both
directions at once.

WHY THIS WORKS AS AN ANSWER
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It verifies both real words with real code, and distinguishes which
specific piece of logic each one actually exercises -- the sokuon
check firing correctly for "kekkon," and the syllabic-n chōon fix
preventing a false positive for "sannin" -- rather than treating both
as generic "doubled letter" tests that happen to both pass.