Form Vanilla
Framework-free form patterns using native browser APIs enhanced with Zod.
Quick Start
<form id="login-form" novalidate>
<div class="form-field">
<label for="email">Email</label>
<input
id="email"
name="email"
type="email"
autocomplete="email"
required
/>
<span class="error" aria-live="polite"></span>
</div>
<div class="form-field">
<label for="password">Password</label>
<input
id="password"
name="password"
type="password"
autocomplete="current-password"
required
minlength="8"
/>
<span class="error" aria-live="polite"></span>
</div>
<button type="submit">Sign in</button>
</form>
<script type="module">
import { createFormValidator } from './vanilla-validator.js';
import { loginSchema } from './schemas.js';
const form = document.getElementById('login-form');
const validator = createFormValidator(form, loginSchema);
form.addEventListener('submit', async (e) => {
e.preventDefault();
const result = await validator.validate();
if (result.valid) {
console.log('Submit:', result.data);
}
});
</script>
HTML5 Constraint Validation API
Built-in Attributes
<!-- Required field -->
<input required />
<!-- Length constraints -->
<input minlength="3" maxlength="50" />
<!-- Number constraints -->
<input type="number" min="0" max="100" step="1" />
<!-- Pattern (regex) -->
<input pattern="[A-Za-z]{3}" title="Three letter code" />
<!-- Email validation -->
<input type="email" />
<!-- URL validation -->
<input type="url" />
Validity State Properties
const input = document.querySelector('input');
// Check individual constraints
input.validity.valueMissing; // required but empty
input.validity.typeMismatch; // email/url format wrong
input.validity.patternMismatch; // regex failed
input.validity.tooShort; // < minlength
input.validity.tooLong; // > maxlength
input.validity.rangeUnderflow; // < min
input.validity.rangeOverflow; // > max
input.validity.stepMismatch; // not divisible by step
input.validity.badInput; // browser can't parse
input.validity.customError; // setCustomValidity called
// Check overall validity
input.validity.valid; // all constraints pass
input.checkValidity(); // returns boolean
input.reportValidity(); // shows browser UI
Custom Error Messages
const input = document.querySelector('#email');
// Set custom validation message
input.addEventListener('invalid', (e) => {
if (input.validity.valueMissing) {
input.setCustomValidity('Please enter your email address');
} else if (input.validity.typeMismatch) {
input.setCustomValidity('Please enter a valid email (e.g., name@example.com)');
}
});
// Clear custom message on input
input.addEventListener('input', () => {
input.setCustomValidity('');
});
Zod Integration
Vanilla Validator Class
// vanilla-validator.ts
import { z } from 'zod';
export interface ValidationResult<T> {
valid: boolean;
data?: T;
errors: Record<string, string>;
}
export interface ValidatorOptions {
/** When to validate */
validateOn: 'blur' | 'input' | 'submit';
/** When to re-validate after error */
revalidateOn: 'input' | 'blur';
/** Debounce delay for input validation (ms) */
debounceMs?: number;
}
const defaultOptions: ValidatorOptions = {
validateOn: 'blur',
revalidateOn: 'input',
debounceMs: 300
};
export function createFormValidator<T extends z.ZodType>(
form: HTMLFormElement,
schema: T,
options: Partial<ValidatorOptions> = {}
): FormValidator<z.infer<T>> {
const opts = { ...defaultOptions, ...options };
const fieldErrors = new Map<string, string>();
const touchedFields = new Set<string>();
let debounceTimers = new Map<string, ReturnType<typeof setTimeout>>();
// Get all form fields
const fields = Array.from(form.elements).filter(
(el): el is HTMLInputElement | HTMLSelectElement | HTMLTextAreaElement =>
el instanceof HTMLInputElement ||
el instanceof HTMLSelectElement ||
el instanceof HTMLTextAreaElement
);
// Attach event listeners
fields.forEach(field => {
if (!field.name) return;
// Blur handler (punish late)
field.addEventListener('blur', () => {
touchedFields.add(field.name);
if (opts.validateOn === 'blur') {
validateField(field.name);
}
});
// Input handler (real-time correction)
field.addEventListener('input', () => {
// Clear existing timer
const timer = debounceTimers.get(field.name);
if (timer) clearTimeout(timer);
// Only validate if already has error (correction mode)
if (fieldErrors.has(field.name) && opts.revalidateOn === 'input') {
debounceTimers.set(
field.name,
setTimeout(() => validateField(field.name), opts.debounceMs)
);
}
});
});
function getFormData(): Record<string, unknown> {
const data: Record<string, unknown> = {};
const formData = new FormData(form);
formData.forEach((value, key) => {
// Handle checkboxes
const field = form.elements.namedItem(key);
if (field instanceof HTMLInputElement && field.type === 'checkbox') {
data[key] = field.checked;
} else if (field instanceof HTMLInputElement && field.type === 'number') {
data[key] = value === '' ? undefined : Number(value);
} else {
data[key] = value;
}
});
return data;
}
function validateField(name: string): string | undefined {
const data = getFormData();
const result = schema.safeParse(data);
if (result.success) {
clearFieldError(name);
return undefined;
}
const fieldError = result.error.errors.find(e => e.path[0] === name);
if (fieldError) {
setFieldError(name, fieldError.message);
return fieldError.message;
} else {
clearFieldError(name);
return undefined;
}
}
function setFieldError(name: string, message: string): void {
fieldErrors.set(name, message);
const field = form.elements.namedItem(name) as HTMLInputElement | null;
if (!field) return;
// Set ARIA attributes
field.setAttribute('aria-invalid', 'true');
// Find error element
const fieldWrapper = field.closest('.form-field');
const errorEl = fieldWrapper?.querySelector('.error');
if (errorEl) {
errorEl.textContent = message;
field.setAttribute('aria-describedby', errorEl.id || '');
}
// Add error class
fieldWrapper?.classList.add('has-error');
fieldWrapper?.classList.remove('is-valid');
// Set custom validity for native UI
field.setCustomValidity(message);
}
function clearFieldError(name: string): void {
fieldErrors.delete(name);
const field = form.elements.namedItem(name) as HTMLInputElement | null;
if (!field) return;
// Clear ARIA
field.setAttribute('aria-invalid', 'false');
field.removeAttribute('aria-describedby');
// Clear error element
const fieldWrapper = field.closest('.form-field');
const errorEl = fieldWrapper?.querySelector('.error');
if (errorEl) {
errorEl.textContent = '';
}
// Update classes
fieldWrapper?.classList.remove('has-error');
if (touchedFields.has(name)) {
fieldWrapper?.classList.add('is-valid');
}
// Clear custom validity
field.setCustomValidity('');
}
function clearAllErrors(): void {
fieldErrors.forEach((_, name) => clearFieldError(name));
}
async function validate(): Promise<ValidationResult<z.infer<T>>> {
const data = getFormData();
const result = schema.safeParse(data);
if (result.success) {
clearAllErrors();
return { valid: true, data: result.data, errors: {} };
}
// Set errors for all fields
const errors: Record<string, string> = {};
result.error.errors.forEach(err => {
const name = String(err.path[0]);
errors[name] = err.message;
setFieldError(name, err.message);
});
// Focus first error
const firstErrorName = Object.keys(errors)[0];
if (firstErrorName) {
const field = form.elements.namedItem(firstErrorName) as HTMLElement;
field?.focus();
}
return { valid: false, errors };
}
function reset(): void {
form.reset();
clearAllErrors();
touchedFields.clear();
debounceTimers.forEach(timer => clearTimeout(timer));
debounceTimers.clear();
}
return {
validate,
validateField,
setFieldError,
clearFieldError,
clearAllErrors,
reset,
getFormData
};
}
export interface FormValidator<T> {
validate(): Promise<ValidationResult<T>>;
validateField(name: string): string | undefined;
setFieldError(name: string, message: string): void;
clearFieldError(name: string): void;
clearAllErrors(): void;
reset(): void;
getFormData(): Record<string, unknown>;
}
Usage Example
<!DOCTYPE html>
<html>
<head>
<style>
.form-field {
margin-bottom: 1rem;
}
.form-field label {
display: block;
margin-bottom: 0.25rem;
}
.form-field input {
width: 100%;
padding: 0.5rem;
border: 1px solid #ccc;
border-radius: 4px;
}
.form-field.has-error input {
border-color: #dc2626;
}
.form-field.is-valid input {
border-color: #059669;
}
.form-field .error {
color: #dc2626;
font-size: 0.875rem;
margin-top: 0.25rem;
}
</style>
</head>
<body>
<form id="contact-form" novalidate>
<div class="form-field">
<label for="name">Name</label>
<input id="name" name="name" type="text" autocomplete="name" />
<span class="error" id="name-error" aria-live="polite"></span>
</div>
<div class="form-field">
<label for="email">Email</label>
<input id="email" name="email" type="email" autocomplete="email" />
<span class="error" id="email-error" aria-live="polite"></span>
</div>
<div class="form-field">
<label for="message">Message</label>
<textarea id="message" name="message" rows="4"></textarea>
<span class="error" id="message-error" aria-live="polite"></span>
</div>
<button type="submit">Send</button>
</form>
<script type="module">
import { z } from 'https://cdn.jsdelivr.net/npm/zod@3/+esm';
import { createFormValidator } from './vanilla-validator.js';
const schema = z.object({
name: z.string().min(1, 'Please enter your name'),
email: z.string().email('Please enter a valid email'),
message: z.string().min(10, 'Message must be at least 10 characters')
});
const form = document.getElementById('contact-form');
const validator = createFormValidator(form, schema);
form.addEventListener('submit', async (e) => {
e.preventDefault();
const result = await validator.validate();
if (result.valid) {
console.log('Submitting:', result.data);
// Send to server...
alert('Message sent!');
validator.reset();
}
});
</script>
</body>
</html>
Progressive Enhancement
Base HTML (Works Without JS)
<form action="/submit" method="POST">
<div class="form-field">
<label for="email">Email *</label>
<input
id="email"
name="email"
type="email"
required
autocomplete="email"
/>
</div>
<div class="form-field">
<label for="password">Password *</label>
<input
id="password"
name="password"
type="password"
required
minlength="8"
autocomplete="current-password"
/>
</div>
<button type="submit">Sign in</button>
</form>
Enhanced With JS
// Only runs if JS is available
const form = document.querySelector('form');
if (form) {
// Disable native validation UI
form.setAttribute('novalidate', '');
// Add ARIA live regions for errors
form.querySelectorAll('.form-field').forEach(field => {
const input = field.querySelector('input');
if (input && input.name) {
const errorEl = document.createElement('span');
errorEl.className = 'error';
errorEl.id = `${input.name}-error`;
errorEl.setAttribute('aria-live', 'polite');
field.appendChild(errorEl);
}
});
// Attach validator
const validator = createFormValidator(form, schema);
form.addEventListener('submit', async (e) => {
e.preventDefault();
const result = await validator.validate();
if (result.valid) {
form.submit(); // Native submit
}
});
}
Common Patterns
Password Visibility Toggle
<div class="form-field password-field">
<label for="password">Password</label>
<div class="input-wrapper">
<input
id="password"
name="password"
type="password"
autocomplete="current-password"
/>
<button
type="button"
class="toggle-password"
aria-label="Show password"
>
👁
</button>
</div>
</div>
<script>
document.querySelectorAll('.toggle-password').forEach(btn => {
btn.addEventListener('click', () => {
const input = btn.previousElementSibling;
const isPassword = input.type === 'password';
input.type = isPassword ? 'text' : 'password';
btn.setAttribute('aria-label', isPassword ? 'Hide password' : 'Show password');
btn.textContent = isPassword ? '🙈' : '👁';
});
});
</script>
Character Counter
<div class="form-field">
<label for="bio">Bio</label>
<textarea id="bio" name="bio" maxlength="500"></textarea>
<span class="char-count"><span id="bio-count">0</span>/500</span>
</div>
<script>
const textarea = document.getElementById('bio');
const counter = document.getElementById('bio-count');
textarea.addEventListener('input', () => {
counter.textContent = textarea.value.length;
});
</script>
Form Submission with Fetch
const form = document.getElementById('my-form');
const submitBtn = form.querySelector('button[type="submit"]');
form.addEventListener('submit', async (e) => {
e.preventDefault();
const result = await validator.validate();
if (!result.valid) return;
// Disable button
submitBtn.disabled = true;
submitBtn.textContent = 'Sending...';
try {
const response = await fetch(form.action, {
method: 'POST',
headers: {
'Content-Type': 'application/json',
'X-CSRF-Token': document.querySelector('[name="_csrf"]').value
},
body: JSON.stringify(result.data)
});
if (!response.ok) {
const error = await response.json();
// Handle server errors
if (error.field) {
validator.setFieldError(error.field, error.message);
} else {
alert(error.message);
}
return;
}
// Success
alert('Form submitted!');
validator.reset();
} catch (err) {
alert('Network error. Please try again.');
} finally {
submitBtn.disabled = false;
submitBtn.textContent = 'Submit';
}
});
File Structure
form-vanilla/
├── SKILL.md
├── references/
│ └── constraint-validation.md # HTML5 Constraint API reference
└── scripts/
├── vanilla-validator.ts # Main validator class
├── vanilla-validator.js # Compiled JS
├── progressive-enhance.js # Progressive enhancement utils
└── examples/
├── login-form.html
├── contact-form.html
└── checkout-form.html
Reference
references/constraint-validation.md— HTML5 Constraint Validation API reference
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