React: 条件付きレンダリングとリストレンダリング

function renderProductList(state) {
  const container = document.getElementById('product-list')
  container.innerHTML = ''

  if (state.isLoading) {
    container.innerHTML = '<div class="loading">Loading......</div>'
  } else if (state.error) {
    container.innerHTML = '<div class="error">Failed to load</div>'
  } else if (!state.isLoggedIn) {
    container.innerHTML = '<div>Please log in</div>'
  } else if (state.products.length === 0) {
    container.innerHTML = '<div>No products available</div>'
  } else {
    let html = ''
    state.products.forEach(p => {
      html += `<div class="card"><h3>${p.name}</h3><p>$${p.price}</p></div>`
    })
    container.innerHTML = html
  }
}

function ProductList({ user, products, isLoading, error }) {
  // Each state is independent if Branch，No interference with one another
  if (isLoading) return <LoadingSpinner />
  if (error) return <ErrorMessage message={error} />
  if (!user) return <LoginPrompt />
  if (products.length === 0) return <EmptyState />
  
  return (
    <div className="product-grid">
      {products.map(product => (
        <ProductCard key={product.id} product={product} />
      ))}
    </div>
  )
}

function StatusBadge({ isActive }) {
  return (
    <span style={{ color: isActive ? '#52c41a' : '#ff4d4f' }}>
      {isActive ? '🟢 Online' : '🔴 Offline'}
    </span>
  )
}

// isActive=true  → <span>🟢 Online</span>
// isActive=false → <span>🔴 Offline</span>

function Notification({ count }) {
  return (
    <div className="notification-icon">
      🔔
      {/* count > 0 A red dot appears，Otherwise, nothing will be displayed. */}
      {count > 0 && (
        <span className="badge">{count > 99 ? '99+' : count}</span>
      )}
    </div>
  )
}

// count=0  → Show only 🔔
// count=5  → Show 🔔 and  "5"

function UserProfile({ user }) {
  return (
    <div>
      <h2>{user.nickname || 'Anonymous User'}</h2>
      <p>{user.bio || 'This person is lazy.，Nothing was written...'}</p>
      <p>Location：{user.location || 'Unknown'}</p>
    </div>
  )
}

横並びの対比：HTML 構文と JSX 構文が同等のマークアップを示す。interface で props を型定義した TypeScript の React コンポーネント

// ============================================
// Example：Combined Use of Three Types of Conditional Rendering in the User Panel
// ============================================

function UserPanel({ user }) {
  return (
    <div className="panel">
      {/* 1. Ternary：Choose one of the two */}
      <h2>
        {user.isVIP ? '⭐ Premium Member' : 'Regular User'}
      </h2>

      {/* 2. &&：Display only when met */}
      {user.isVIP && (
        <p>Membership Expiration：{user.vipExpireDate}</p>
      )}

      {/* 3. ||：Floor value */}
      <p>Signature：{user.slogan || 'This person is really cool.，No signature'}</p>

      {/* Combined Use：List of Tags */}
      {user.tags && user.tags.length > 0 && (
        <div>
          {user.tags.map(tag => (
            <span key={tag} style={{ background: '#f0f0f0', padding: '2px 8px', margin: '2px', borderRadius: '4px' }}>
              {tag}
            </span>
          ))}
        </div>
      )}
    </div>
  )
}

VIP の場合："⭐ Premium Member" + 会員有効期限。VIP でない場合："Regular User"。シグネチャ：ユーザーのテキストまたはフォールバックコンテンツ。タグは存在する場合のみ表示。

const items = [
  { id: 1, name: 'React Basics', price: 0 },
  { id: 2, name: 'Vue Getting Started', price: 0 },
  { id: 3, name: 'Node.js Real-World Experience', price: 99 }
]

// 1. Direct Rendering
{items.map(item => <li key={item.id}>{item.name}</li>)}

// 2. Filter First, Then Render（Free Courses）
{items
  .filter(item => item.price === 0)
  .map(item => <li key={item.id}>{item.name}（Free）</li>)
}

// 3. Extract Rendering Functions
function renderItem(item) {
  return (
    <li key={item.id}>
      {item.name} - ${item.price}
    </li>
  )
}
{items.map(renderItem)}

サブタイトル："        {user.isVIP ? '⭐ Premium Member' : 'Regular User'}      "。表示："0 && ("

入力フィールドを持つ 2 つのリスト。"Randomize" をクリック：✅ リスト（key=id）は入力値とテキストの並びが維持される；❌ リスト（key=index）は入力値とテキストがずれる

// ============================================
// Example：Complete List Component（3 Handling Different States）
// ============================================

function ArticleList() {
  const [articles, setArticles] = React.useState([])
  const [isLoading, setIsLoading] = React.useState(true)
  const [error, setError] = React.useState(null)

  // Simulated Loading（Actually, it is API Call）
  React.useEffect(() => {
    fetch('/api/articles')
      .then(res => {
        if (!res.ok) throw new Error('Network request failed')
        return res.json()
      })
      .then(data => {
        setArticles(data)
        setIsLoading(false)
      })
      .catch(err => {
        setError(err.message)
        setIsLoading(false)
      })
  }, [])

  // 1. Loading...
  if (isLoading) {
    return (
      <div style={{ textAlign: 'center', padding: '40px' }}>
        <div className="spinner" />
        <p>Loading article...</p>
      </div>
    )
  }

  // 2. Failed to load
  if (error) {
    return (
      <div style={{ textAlign: 'center', padding: '40px', color: '#ff4d4f' }}>
        <p>❌ Failed to load：{error}</p>
        <button onClick={() => window.location.reload()}>Reload</button>
      </div>
    )
  }

  // 3. Empty data
  if (articles.length === 0) {
    return (
      <div style={{ textAlign: 'center', padding: '40px', color: '#999' }}>
        <p>📭 No articles available</p>
        <p>No one has posted an article yet.，Go ahead and write your first one!！</p>
      </div>
    )
  }

  // 4. Normal Rendering
  return (
    <div>
      <h3>List of Articles（Total  {articles.length}  articles)</h3>
      {articles.map(article => (
        <ArticleCard key={article.id} article={article} />
      ))}
    </div>
  )
}

表示："Randomize the order"。ボタン：Randomize the order。入力：Enter content、The content may be misaligned

function DynamicForm() {
  const [formType, setFormType] = useState('personal')
  const [formData, setFormData] = useState({
    name: '',
    email: '',
    company: '',
    taxId: '',
    budget: '',
  })

  function handleChange(e) {
    const { name, value } = e.target
    setFormData(prev => ({ ...prev, [name]: value }))
  }

  const personalFields = (
    <>
      <input name="name" value={formData.name} onChange={handleChange} placeholder="Name" />
      <input name="email" value={formData.email} onChange={handleChange} placeholder="Email" />
    </>
  )

  const businessFields = (
    <>
      <input name="company" value={formData.company} onChange={handleChange} placeholder="Company" />
      <input name="taxId" value={formData.taxId} onChange={handleChange} placeholder="Tax ID" />
      <input name="budget" value={formData.budget} onChange={handleChange} placeholder="Budget ($)" />
    </>
  )

  return (
    <div style={{ maxWidth: 400, margin: '0 auto' }}>
      <h3>Registration</h3>
      <div style={{ marginBottom: 12 }}>
        {['personal', 'business'].map(type => (
          <button
            key={type}
            onClick={() => setFormType(type)}
            style={{
              padding: '6px 16px', marginRight: 8, cursor: 'pointer',
              backgroundColor: formType === type ? '#1890ff' : '#f0f0f0',
              color: formType === type ? 'white' : '#333', border: 'none', borderRadius: 4,
            }}
          >
            {type === 'personal' ? 'Personal' : 'Business'}
          </button>
        ))}
      </div>
      {formType === 'personal' ? personalFields : businessFields}
      <button style={{ marginTop: 12, padding: '8px 24px', cursor: 'pointer' }}>Submit</button>
    </div>
  )
}

個人フォーム：Name + Email フィールド。企業フォーム：Company + Tax ID + Budget フィールド。ボタンで切り替え。末尾に送信ボタン。

検索入力フィールドで項目をフィルタリング。並べ替えボタン：A-Z（名前順）/ Stars（評価順）。現在の並べ替えは青色でハイライト。フィルタ + 並べ替え後のリストがリアクティブに更新される

フレームワーク：React ⭐220k、Vue ⭐207k、Tailwind ⭐80k、Django ⭐78k、Express ⭐63k。検索フィルタ；A-Z/Stars の並べ替えを切り替え。

表示 "{cat.name}"

Electronics: Keyboard $79、Monitor $399
Furniture: Desk $249、Chair $189
Books: "No products yet"（斜体の空状態）