HTML5 Form Validation

<!-- HTML: Required fields -->
<form id="signupForm">
  <label for="email">Email (required):</label>
  <input type="email" id="email" name="email" required>
  
  <label for="username">Username (required):</label>
  <input type="text" id="username" name="username" required minlength="3">
  
  <label for="age">Age (optional):</label>
  <input type="number" id="age" name="age" min="13">
  
  <button type="submit">Sign Up</button>
</form>

<!-- CSS: Validation state styling -->
<style>
  /* Style all required fields */
  input:required {
    border-left: 3px solid #ff9800;
  }
  
  /* Valid state (avoid on page load) */
  input:not(:placeholder-shown):valid {
    border-color: #4caf50;
    background-image: url('check-icon.svg');
    background-repeat: no-repeat;
    background-position: right 10px center;
  }
  
  /* Invalid state (only after user interaction) */
  input:user-invalid,
  input:not(:placeholder-shown):invalid {
    border-color: #f44336;
    background-color: #ffebee;
  }
  
  /* Focus states */
  input:invalid:focus {
    outline: 2px solid #f44336;
  }
  
  input:valid:focus {
    outline: 2px solid #4caf50;
  }
  
  /* Optional fields (subtle styling) */
  input:optional {
    border-left: 3px solid #ccc;
  }
</style>

<!-- JavaScript: Check validity -->
<script>
  const form = document.getElementById('signupForm');
  const email = document.getElementById('email');
  
  form.addEventListener('submit', (e) => {
    if (!form.checkValidity()) {
      e.preventDefault();
      alert('Please fill in all required fields correctly.');
    }
  });
  
  // Check individual field validity
  email.addEventListener('blur', () => {
    const validity = email.validity;
    
    if (validity.valueMissing) {
      console.log('Email is required');
    } else if (validity.typeMismatch) {
      console.log('Invalid email format');
    } else if (validity.valid) {
      console.log('Email is valid');
    }
  });
</script>

<!-- ZIP Code -->
<label for="zip">ZIP Code:</label>
<input type="text" id="zip" name="zip"
       pattern="[0-9]{5}"
       title="Please enter a 5-digit ZIP code"
       placeholder="12345"
       required>

<!-- Phone Number -->
<label for="phone">Phone:</label>
<input type="tel" id="phone" name="phone"
       pattern="\d{3}-\d{3}-\d{4}"
       title="Format: 555-123-4567"
       placeholder="555-123-4567">

<!-- Username -->
<label for="username">Username:</label>
<input type="text" id="username" name="username"
       pattern="[a-zA-Z0-9_]{3,16}"
       title="3-16 characters: letters, numbers, and underscores only"
       placeholder="user_name"
       required>

<!-- Strong Password -->
<label for="password">Password:</label>
<input type="password" id="password" name="password"
       pattern="^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$"
       title="Minimum 8 characters, at least one uppercase, lowercase, number, and special character"
       required>

<!-- Credit Card -->
<label for="card">Credit Card:</label>
<input type="text" id="card" name="card"
       pattern="\d{4}[\s\-]?\d{4}[\s\-]?\d{4}[\s\-]?\d{4}"
       title="Enter 16-digit card number"
       placeholder="1234-5678-9012-3456"
       maxlength="19">

<!-- URL Slug -->
<label for="slug">Article Slug:</label>
<input type="text" id="slug" name="slug"
       pattern="[a-z0-9]+(?:-[a-z0-9]+)*"
       title="Lowercase letters, numbers, and hyphens only"
       placeholder="my-article-title">

<!-- Hex Color -->
<label for="color">Hex Color:</label>
<input type="text" id="color" name="color"
       pattern="#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3})"
       title="Enter hex color code (e.g., #ff5733 or #fff)"
       placeholder="#ff5733">

<!-- Custom validation message (JavaScript) -->
<script>
  const usernameInput = document.getElementById('username');
  
  usernameInput.addEventListener('invalid', (e) => {
    if (usernameInput.validity.patternMismatch) {
      usernameInput.setCustomValidity('Username must be 3-16 characters and contain only letters, numbers, and underscores.');
    } else {
      usernameInput.setCustomValidity('');
    }
  });
  
  usernameInput.addEventListener('input', () => {
    usernameInput.setCustomValidity('');
  });
</script>

<!-- HTML: Form with custom validation -->
<form id="customForm" novalidate>
  <div class="form-group">
    <label for="email">Email:</label>
    <input type="email" id="email" name="email" required>
    <span class="error-message" id="email-error"></span>
  </div>
  
  <div class="form-group">
    <label for="password">Password:</label>
    <input type="password" id="password" name="password" required minlength="8">
    <span class="error-message" id="password-error"></span>
  </div>
  
  <div class="form-group">
    <label for="confirm-password">Confirm Password:</label>
    <input type="password" id="confirm-password" name="confirm-password" required>
    <span class="error-message" id="confirm-error"></span>
  </div>
  
  <button type="submit">Submit</button>
</form>

<!-- CSS: Custom validation styling -->
<style>
  .form-group {
    margin-bottom: 20px;
    position: relative;
  }
  
  input {
    width: 100%;
    padding: 10px;
    border: 2px solid #ddd;
    border-radius: 4px;
    transition: border-color 0.3s;
  }
  
  /* Valid state */
  input:valid:not(:placeholder-shown) {
    border-color: #4caf50;
  }
  
  input:valid:not(:placeholder-shown)::after {
    content: '✓';
    color: #4caf50;
    position: absolute;
    right: 10px;
  }
  
  /* Invalid state */
  input:invalid:not(:placeholder-shown),
  input[aria-invalid="true"] {
    border-color: #f44336;
    background-color: #ffebee;
  }
  
  /* Error message styling */
  .error-message {
    display: none;
    color: #f44336;
    font-size: 0.875em;
    margin-top: 5px;
  }
  
  .error-message.visible {
    display: block;
  }
  
  /* Focus states */
  input:focus {
    outline: none;
    border-color: #2196f3;
    box-shadow: 0 0 0 3px rgba(33, 150, 243, 0.1);
  }
</style>

<!-- JavaScript: Custom validation logic -->
<script>
  const form = document.getElementById('customForm');
  const email = document.getElementById('email');
  const password = document.getElementById('password');
  const confirmPassword = document.getElementById('confirm-password');
  
  // Custom error messages
  const errorMessages = {
    valueMissing: 'This field is required.',
    typeMismatch: {
      email: 'Please enter a valid email address.',
      url: 'Please enter a valid URL.'
    },
    tooShort: (minLength) => `Please enter at least ${minLength} characters.`,
    patternMismatch: 'Please match the requested format.',
  };
  
  // Validate individual field
  function validateField(field) {
    const errorElement = document.getElementById(`${field.id}-error`);
    const validity = field.validity;
    let errorMessage = '';
    
    if (validity.valueMissing) {
      errorMessage = errorMessages.valueMissing;
    } else if (validity.typeMismatch) {
      errorMessage = errorMessages.typeMismatch[field.type] || 'Invalid format.';
    } else if (validity.tooShort) {
      errorMessage = errorMessages.tooShort(field.minLength);
    } else if (validity.patternMismatch) {
      errorMessage = field.title || errorMessages.patternMismatch;
    }
    
    // Custom password match validation
    if (field === confirmPassword && field.value !== password.value) {
      errorMessage = 'Passwords do not match.';
      field.setCustomValidity(errorMessage);
    } else if (field === confirmPassword) {
      field.setCustomValidity('');
    }
    
    // Display error
    if (errorMessage) {
      errorElement.textContent = errorMessage;
      errorElement.classList.add('visible');
      field.setAttribute('aria-invalid', 'true');
      return false;
    } else {
      errorElement.textContent = '';
      errorElement.classList.remove('visible');
      field.removeAttribute('aria-invalid');
      return true;
    }
  }
  
  // Validate on blur
  [email, password, confirmPassword].forEach(field => {
    field.addEventListener('blur', () => validateField(field));
    field.addEventListener('input', () => {
      if (field.getAttribute('aria-invalid') === 'true') {
        validateField(field);
      }
    });
  });
  
  // Validate on submit
  form.addEventListener('submit', (e) => {
    e.preventDefault();
    
    const fields = [email, password, confirmPassword];
    const isValid = fields.every(validateField);
    
    if (isValid) {
      console.log('Form is valid, submitting...');
      // form.submit();
    } else {
      // Focus first invalid field
      const firstInvalid = fields.find(field => !field.validity.valid);
      firstInvalid?.focus();
    }
  });
</script>

<!-- Length constraints -->
<label for="username">Username (3-15 characters):</label>
<input type="text" id="username" name="username"
       minlength="3" maxlength="15" required>

<!-- Number range with step -->
<label for="price">Price ($):</label>
<input type="number" id="price" name="price"
       min="0" max="10000" step="0.01" 
       placeholder="0.00">

<!-- Age restriction -->
<label for="age">Age (must be 18+):</label>
<input type="number" id="age" name="age"
       min="18" max="120" required>

<!-- Date range (current year only) -->
<label for="event-date">Event Date:</label>
<input type="date" id="event-date" name="event-date"
       min="2024-01-01" max="2024-12-31" required>

<!-- Time with 15-minute intervals -->
<label for="appointment">Appointment Time:</label>
<input type="time" id="appointment" name="appointment"
       min="09:00" max="17:00" step="900"> <!-- 900 seconds = 15 min -->

<!-- Percentage (0-100 with 0.1 precision) -->
<label for="discount">Discount (%):</label>
<input type="number" id="discount" name="discount"
       min="0" max="100" step="0.1">

<!-- File upload (images only) -->
<label for="avatar">Profile Picture:</label>
<input type="file" id="avatar" name="avatar"
       accept="image/png, image/jpeg, image/webp"
       required>

<!-- Multiple file upload -->
<label for="documents">Upload Documents:</label>
<input type="file" id="documents" name="documents"
       accept=".pdf,.doc,.docx"
       multiple>

<!-- Multiple emails -->
<label for="recipients">Email Recipients:</label>
<input type="email" id="recipients" name="recipients"
       multiple
       placeholder="email1@example.com, email2@example.com">

<!-- Range slider with step -->
<label for="volume">Volume (0-100, steps of 5):</label>
<input type="range" id="volume" name="volume"
       min="0" max="100" step="5" value="50">
<output for="volume">50</output>

<!-- No step constraint (any decimal) -->
<label for="precise">Precise Value:</label>
<input type="number" id="precise" name="precise"
       step="any">

<!-- GET form (search) -->
<form action="/search" method="GET">
  <input type="search" name="q" placeholder="Search...">
  <input type="checkbox" name="category" value="products"> Products
  <input type="checkbox" name="category" value="articles"> Articles
  <button type="submit">Search</button>
</form>
<!-- Submits to: /search?q=keyword&category=products&category=articles -->

<!-- POST form (user registration) -->
<form action="/register" method="POST" id="registerForm">
  <input type="text" name="username" required>
  <input type="email" name="email" required>
  <input type="password" name="password" required>
  <button type="submit">Register</button>
</form>

<!-- JavaScript: Intercept and handle submission -->
<script>
  const form = document.getElementById('registerForm');
  
  // Method 1: Prevent default and use Fetch API
  form.addEventListener('submit', async (e) => {
    e.preventDefault(); // Prevent traditional form submission
    
    // Get form data
    const formData = new FormData(form);
    
    // Convert to JSON (if needed)
    const data = Object.fromEntries(formData.entries());
    
    try {
      const response = await fetch('/register', {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
        },
        body: JSON.stringify(data),
      });
      
      if (response.ok) {
        const result = await response.json();
        console.log('Success:', result);
        // Redirect or show success message
      } else {
        console.error('Error:', response.statusText);
      }
    } catch (error) {
      console.error('Network error:', error);
    }
  });
  
  // Method 2: FormData with all form fields
  form.addEventListener('submit', async (e) => {
    e.preventDefault();
    
    const formData = new FormData(form);
    
    // Send as multipart/form-data (good for file uploads)
    const response = await fetch('/register', {
      method: 'POST',
      body: formData, // Browser sets correct Content-Type
    });
  });
  
  // Method 3: Programmatic submission
  function submitForm() {
    if (form.checkValidity()) {
      // Triggers validation and submit event
      form.requestSubmit();
    } else {
      // Show validation errors
      form.reportValidity();
    }
  }
</script>

<!-- Multiple submit buttons with different actions -->
<form method="POST">
  <input type="text" name="content" required>
  
  <button type="submit" name="action" value="save">Save Draft</button>
  <button type="submit" name="action" value="publish">Publish</button>
  <button type="submit" name="action" value="delete" formnovalidate>Delete</button>
</form>

<!-- Override form attributes on button -->
<form action="/default" method="POST">
  <input type="email" name="email" required>
  
  <button type="submit">Submit to Default</button>
  <button type="submit" 
          formaction="/alternative"
          formmethod="GET"
          formnovalidate>
    Submit to Alternative (Skip Validation)
  </button>
</form>

<!-- File upload form -->
<form action="/upload" method="POST" enctype="multipart/form-data" id="uploadForm">
  <label for="file">Select File:</label>
  <input type="file" id="file" name="file" accept="image/*" required>
  
  <label for="description">Description:</label>
  <input type="text" id="description" name="description">
  
  <button type="submit">Upload</button>
</form>

<!-- Multiple file upload with preview -->
<form id="multiUploadForm">
  <input type="file" id="files" name="files" multiple accept="image/*">
  <div id="preview"></div>
  <button type="submit">Upload All</button>
</form>

<script>
  // Single file upload
  const uploadForm = document.getElementById('uploadForm');
  
  uploadForm.addEventListener('submit', async (e) => {
    e.preventDefault();
    
    const formData = new FormData(uploadForm);
    
    try {
      const response = await fetch('/upload', {
        method: 'POST',
        body: formData, // Browser automatically sets multipart/form-data
      });
      
      if (response.ok) {
        const result = await response.json();
        console.log('Upload successful:', result);
      }
    } catch (error) {
      console.error('Upload failed:', error);
    }
  });
  
  // Multiple files with preview
  const filesInput = document.getElementById('files');
  const preview = document.getElementById('preview');
  
  filesInput.addEventListener('change', (e) => {
    preview.innerHTML = ''; // Clear previous previews
    
    const files = e.target.files;
    
    for (let i = 0; i < files.length; i++) {
      const file = files[i];
      
      // Validate file
      if (!file.type.startsWith('image/')) {
        alert(`${file.name} is not an image`);
        continue;
      }
      
      if (file.size > 5 * 1024 * 1024) { // 5MB limit
        alert(`${file.name} is too large (max 5MB)`);
        continue;
      }
      
      // Create preview
      const reader = new FileReader();
      reader.onload = (event) => {
        const img = document.createElement('img');
        img.src = event.target.result;
        img.style.width = '100px';
        img.style.margin = '5px';
        preview.appendChild(img);
      };
      reader.readAsDataURL(file);
    }
  });
  
  // Multi-file upload
  const multiUploadForm = document.getElementById('multiUploadForm');
  
  multiUploadForm.addEventListener('submit', async (e) => {
    e.preventDefault();
    
    const formData = new FormData();
    const files = filesInput.files;
    
    // Add each file
    for (let i = 0; i < files.length; i++) {
      formData.append('files', files[i]);
    }
    
    // Add additional data
    formData.append('userId', '12345');
    formData.append('category', 'photos');
    
    const response = await fetch('/upload-multiple', {
      method: 'POST',
      body: formData,
    });
  });
  
  // Manual FormData construction
  const manualFormData = new FormData();
  
  // Add text fields
  manualFormData.append('username', 'john_doe');
  manualFormData.append('email', 'john@example.com');
  
  // Add file (from input element)
  const fileInput = document.getElementById('file');
  manualFormData.append('avatar', fileInput.files[0]);
  
  // Add file (blob/programmatically)
  const blob = new Blob(['Hello, world!'], { type: 'text/plain' });
  manualFormData.append('note', blob, 'note.txt');
  
  // Get values
  console.log(manualFormData.get('username')); // 'john_doe'
  console.log(manualFormData.getAll('username')); // ['john_doe']
  
  // Check if field exists
  console.log(manualFormData.has('email')); // true
  
  // Delete field
  manualFormData.delete('note');
  
  // Iterate over entries
  for (const [key, value] of manualFormData.entries()) {
    console.log(`${key}:`, value);
  }
</script>

<!-- Camera capture (mobile) -->
<form>
  <!-- Use front camera -->
  <input type="file" accept="image/*" capture="user">
  
  <!-- Use back camera -->
  <input type="file" accept="image/*" capture="environment">
  
  <!-- Video capture -->
  <input type="file" accept="video/*" capture>
</form>