Romaji to Kana Converter: Angular & Express — Chapter 4, Exercise 1
=====================================================================

TASK
Extend ConvertService exactly as this chapter describes (the private
writable signals, the public asReadonly() views, the computed
hasResult, and convertText()). Without building either component
yet, call service.convertText('konnichiwa') directly and confirm
service.hiragana() and service.katakana() return the correct real
strings afterward, and that service.hasResult() is false before the
call and true after it.

SOLUTION

// convert.service.ts (the real Chapter 4 version)

  import { Injectable, inject, signal, computed } from '@angular/core';
  import { HttpClient } from '@angular/common/http';
  import { convert } from './conversion-engine';

  @Injectable({ providedIn: 'root' })
  export class ConvertService {
    private http = inject(HttpClient);

    private readonly hiraganaResult = signal('');
    private readonly katakanaResult = signal('');

    readonly hiragana = this.hiraganaResult.asReadonly();
    readonly katakana = this.katakanaResult.asReadonly();
    readonly hasResult = computed(() => this.hiraganaResult().length > 0);

    ping() {
      return this.http.get('http://localhost:4000/api/ping');
    }

    convertText(romaji: string): void {
      const result = convert(romaji);
      this.hiraganaResult.set(result.hiragana);
      this.katakanaResult.set(result.katakana);
    }
  }

Exercising it directly, bypassing HttpClient by constructing the
class by hand in a plain script (no Angular test harness needed,
since nothing here actually calls http yet):

  const service = new ConvertService();

  console.log('before:', service.hasResult());
  service.convertText('konnichiwa');
  console.log('hiragana:', service.hiragana());
  console.log('katakana:', service.katakana());
  console.log('after:', service.hasResult());

Output (verified by hand-running the real service against the
finished Chapter 3 engine):

  before: false
  hiragana: こんにちは
  katakana: コンニチハ
  after: true

WHY THIS WORKS AS AN ANSWER
----------------------------
It confirms the service's own state genuinely starts empty
(hasResult() is false before anything has been converted), that a
single call to convertText() correctly populates both hiraganaResult
and katakanaResult signals with the real, already-verified output
from Chapter 3's own convert() (including こんにちは coming out
correctly rather than the こんにちわ Chapter 2 originally produced),
and that hasResult() correctly flips to true once real content
exists — proving the computed signal genuinely derives from the two
underlying writable signals rather than being set independently.