React: フォームと制御コンポーネント

<!-- HTML Form：Read data all at once upon submission -->
<form id="register-form">
  <input name="username" id="username" />
  <input type="email" name="email" id="email" />
  <input type="password" name="password" id="password" />
  <select name="role" id="role">
    <option value="developer">Developer</option>
    <option value="designer">Designer</option>
  </select>
  <button type="submit">Register</button>
</form>

<script>
  document.getElementById('register-form').addEventListener('submit', function(e) {
    e.preventDefault()
    // 送信時にフォームの値を読み取る
    const username = document.getElementById('username').value
    const email = document.getElementById('email').value
    const password = document.getElementById('password').value
    const role = document.getElementById('role').value
    // ... Manually verify each field
  })
</script>

入力フィールドと送信処理を持つフォーム。interface で props を型定義した TypeScript の React コンポーネント

参考：text（value/onChange）、number（Number() 変換）、email、password、textarea（value）、select（value）、checkbox（checked）、radio（checked）、file（非制御、files[0]）

function ValidatedForm() {
  const [email, setEmail] = React.useState('')
  const [password, setPassword] = React.useState('')
  const [errors, setErrors] = React.useState({})
  const [touched, setTouched] = React.useState({})  // Record which fields have been accessed

  function validate(fieldName, value) {
    const newErrors = { ...errors }
    
    if (fieldName === 'email') {
      if (!value) newErrors.email = 'The email address cannot be left blank.'
      else if (!value.includes('@')) newErrors.email = 'The email address format is incorrect.'
      else delete newErrors.email
    }
    
    if (fieldName === 'password') {
      if (!value) newErrors.password = 'The password cannot be empty.'
      else if (value.length < 6) newErrors.password = 'Password must be at least 6 chars'
      else if (!/[A-Z]/.test(value)) newErrors.password = 'Passwords must contain uppercase letters'
      else delete newErrors.password
    }

    setErrors(newErrors)
  }

  function handleBlur(e) {
    const { name, value } = e.target
    setTouched({ ...touched, [name]: true })
    validate(name, value)
  }

  function handleSubmit(e) {
    e.preventDefault()
    // Validate all fields before submitting
    const allErrors = {}
    if (!email || !email.includes('@')) allErrors.email = 'Please enter a valid email address'
    if (!password || password.length < 6) allErrors.password = 'Password must be at least 6 chars'
    
    if (Object.keys(allErrors).length > 0) {
      setErrors(allErrors)
      setTouched({ email: true, password: true })
      return
    }

    alert('Submission Successful！')
  }

  return (
    <form onSubmit={handleSubmit} style={{ maxWidth: '400px' }}>
      <div style={{ marginBottom: '16px' }}>
        <label>Email：</label>
        <input
          name="email"
          type="email"
          value={email}
          onChange={e => { setEmail(e.target.value); validate('email', e.target.value) }}
          onBlur={handleBlur}
          style={inputStyle(touched.email && errors.email)}
        />
        {touched.email && errors.email && (
          <p style={{ color: '#ff4d4f', fontSize: '12px', margin: '4px 0' }}>{errors.email}</p>
        )}
      </div>

      <div style={{ marginBottom: '16px' }}>
        <label>Password：</label>
        <input
          name="password"
          type="password"
          value={password}
          onChange={e => { setPassword(e.target.value); validate('password', e.target.value) }}
          onBlur={handleBlur}
          style={inputStyle(touched.password && errors.password)}
        />
        {touched.password && errors.password && (
          <p style={{ color: '#ff4d4f', fontSize: '12px', margin: '4px 0' }}>{errors.password}</p>
        )}
      </div>

      {/* Password Strength Indicator */}
      {password && (
        <div style={{ marginBottom: '16px' }}>
          <p style={{ fontSize: '12px', color: '#666' }}>
            Password Strength：
            <span style={{ color: password.length < 6 ? '#ff4d4f' : password.length < 10 ? '#faad14' : '#52c41a' }}>
              {password.length < 6 ? 'Weak' : password.length < 10 ? 'Medium' : 'Strong'}
            </span>
          </p>
          <div style={{ height: '4px', backgroundColor: '#f0f0f0', borderRadius: '2px' }}>
            <div style={{
              height: '100%',
              width: `${Math.min(100, (password.length / 12) * 100)}%`,
              backgroundColor: password.length < 6 ? '#ff4d4f' : password.length < 10 ? '#faad14' : '#52c41a',
              borderRadius: '2px',
              transition: 'width 0.3s'
            }} />
          </div>
        </div>
      )}

      <button type="submit" disabled={Object.keys(errors).length > 0}
        style={{
          width: '100%', padding: '10px',
          backgroundColor: Object.keys(errors).length > 0 ? '#d9d9d9' : '#1890ff',
          color: 'white', border: 'none', borderRadius: '4px', cursor: 'pointer'
        }}>
        Register
      </button>
    </form>
  )
}

function inputStyle(hasError) {
  return {
    width: '100%', padding: '8px',
    border: `1px solid ${hasError ? '#ff4d4f' : '#d9d9d9'}`,
    borderRadius: '4px',
    outline: 'none'
  }
}

// ============================================
// Complete Example：User Registration Form
// Features：Controlled Components + Real-Time Verification + Form Data Synchronization
// ============================================

function RegistrationForm() {
  const [form, setForm] = React.useState({
    username: '',
    email: '',
    password: '',
    confirmPassword: '',
    country: '',
    gender: '',
    acceptTerms: false
  })
  const [errors, setErrors] = React.useState({})

  function validate() {
    const newErrors = {}
    if (!form.username || form.username.length < 2)
      newErrors.username = 'Username must be at least 2 characters'
    if (!form.email || !form.email.includes('@'))
      newErrors.email = 'Please enter a valid email address'
    if (!form.password || form.password.length < 6)
      newErrors.password = 'Password must be at least 6 chars'
    if (form.password !== form.confirmPassword)
      newErrors.confirmPassword = 'The two passwords do not match'
    if (!form.country)
      newErrors.country = 'Please select a country'
    if (!form.acceptTerms)
      newErrors.acceptTerms = 'Please agree to the Terms of Service'
    return newErrors
  }

  function handleChange(e) {
    const { name, value, type, checked } = e.target
    const newValue = type === 'checkbox' ? checked : value
    const newForm = { ...form, [name]: newValue }
    setForm(newForm)
    setErrors(validate())
    // Interconnection：Automatically adjust password rule prompts after selecting a country
  }

  function handleSubmit(e) {
    e.preventDefault()
    const finalErrors = validate()
    setErrors(finalErrors)
    if (Object.keys(finalErrors).length === 0) {
      alert(`Registration Successful！Welcome ${form.username}`)
    }
  }

  function inputStyle(fieldName) {
    return {
      width: '100%', padding: '8px', marginBottom: '4px',
      border: `1px solid ${errors[fieldName] ? '#ff4d4f' : '#d9d9d9'}`,
      borderRadius: '4px'
    }
  }

  return (
    <form onSubmit={handleSubmit} style={{ maxWidth: '500px', margin: '0 auto' }}>
      <h2>📝 User Registration</h2>

      <input name="username" value={form.username} onChange={handleChange}
        placeholder="Username" style={inputStyle('username')} />
      {errors.username && <p style={errorStyle}>{errors.username}</p>}

      <input name="email" type="email" value={form.email} onChange={handleChange}
        placeholder="Email" style={inputStyle('email')} />
      {errors.email && <p style={errorStyle}>{errors.email}</p>}

      <input name="password" type="password" value={form.password} onChange={handleChange}
        placeholder="Password (at least 6 chars)" style={inputStyle('password')} />
      {errors.password && <p style={errorStyle}>{errors.password}</p>}

      <input name="confirmPassword" type="password" value={form.confirmPassword}
        onChange={handleChange} placeholder="Confirm Password" style={inputStyle('confirmPassword')} />
      {errors.confirmPassword && <p style={errorStyle}>{errors.confirmPassword}</p>}

      <select name="country" value={form.country} onChange={handleChange}
        style={inputStyle('country')}>
        <option value="">-- Select a Country --</option>
        <option value="CN">China</option>
        <option value="US">United States</option>
        <option value="JP">Japan</option>
        <option value="BR">Brazil</option>
        <option value="SA">Saudi Arabia</option>
      </select>
      {errors.country && <p style={errorStyle}>{errors.country}</p>}

      <div style={{ margin: '12px 0' }}>
        <p>Gender：</p>
        {['Male', 'Female', 'Other'].map(g => (
          <label key={g} style={{ marginRight: '16px' }}>
            <input type="radio" name="gender" value={g}
              checked={form.gender === g} onChange={handleChange} />
            {g}
          </label>
        ))}
      </div>

      <label style={{ display: 'block', margin: '12px 0' }}>
        <input type="checkbox" name="acceptTerms" checked={form.acceptTerms}
          onChange={handleChange} />
        I have read and agree to <a href="#">User Agreement</a>
      </label>
      {errors.acceptTerms && <p style={errorStyle}>{errors.acceptTerms}</p>}

      <button type="submit" style={{
        width: '100%', padding: '10px',
        backgroundColor: Object.keys(errors).length === 0 && form.username ? '#1890ff' : '#d9d9d9',
        color: 'white', border: 'none', borderRadius: '4px', cursor: 'pointer'
      }}>
        Register
      </button>
    </form>
  )
}

const errorStyle = {
  color: '#ff4d4f', fontSize: '12px',
  margin: '0 0 8px 0'
}

表示："Please select"。入力タイプ：text、email、number。ドロップダウン：Please select、Beijing

国セレクター（US/CN/JP/UK/BR）→ 国際番号が自動入力（US: +1、CN: +86）。電話番号入力フィールド + 読み取り専用の国際番号。表示 "Full: +1 5551234"

表示："Phone Contact"。状態：form（setter: setForm）。入力：Phone number。ドロップダウン：{c}

動的なメンバー行（Name + Email + Role ドロップダウン）。"+ Add Member" で 1 行追加、"X" で 1 行削除（最低 1 行は保持）。送信 → console.log("Team:", members)

... ('Team:', members)

進捗バーのある 3 ステップウィザード。ステップ 1：Name+Email、ステップ 2：Password（6 文字以上）、ステップ 3：City+Terms。NEXT で検証し、BACK で前のステップに戻れる。送信 → "Done!"

表示 "file upload (uncontrolled)"。状態：preview、filename。ボタン：upload。useRef を使用

function FileUploadForm() {
  const fileInputRef = useRef(null)
  const [preview, setPreview] = useState(null)
  const [fileName, setFileName] = useState('')

  function handleSubmit(e) {
    e.preventDefault()
    const file = fileInputRef.current?.files[0]
    if (!file) { alert('Please select a file'); return }
    console.log('Uploading:', file.name, file.size, 'bytes')
    alert(`File "${file.name}" (${(file.size / 1024).toFixed(1)} KB) selected for upload`)
  }

  function handleFileChange(e) {
    const file = e.target.files[0]
    if (!file) { setPreview(null); setFileName(''); return }
    setFileName(file.name)
    if (file.type.startsWith('image/')) {
      const reader = new FileReader()
      reader.onload = (e) => setPreview(e.target.result)
      reader.readAsDataURL(file)
    } else {
      setPreview(null)
    }
  }

  return (
    <form onSubmit={handleSubmit} style={{ maxWidth: 400, margin: '0 auto' }}>
      <h3>File Upload (Uncontrolled)</h3>
      <div style={{ marginBottom: 12 }}>
        <input type="file" ref={fileInputRef} onChange={handleFileChange}
          accept="image/*,.pdf,.doc" style={{ marginBottom: 8 }} />
        {fileName && <p style={{ fontSize: 13, color: '#666' }}>Selected: {fileName}</p>}
      </div>
      {preview && (
        <div style={{ marginBottom: 12 }}>
          <img src={preview} alt="Preview" style={{ maxWidth: '100%', maxHeight: 200, borderRadius: 4, border: '1px solid #eee' }} />
        </div>
      )}
      <button type="submit" style={{ padding: '8px 24px', cursor: 'pointer' }}>Upload</button>
    </form>
  )
}

ファイル入力 → 画像のプレビュー、またはファイル名を表示。アップロード時に alert "File "photo.jpg" (245.3 KB) selected for upload"