Not Handling Edge Cases in Form Validations

The Risks of Incomplete Form Validation

Failing to properly validate form inputs leads to:

• Data corruption from invalid submissions
• Poor user experience with confusing errors
• Security vulnerabilities like SQL injection
• Application crashes from unexpected data formats
• Business logic failures when invalid data propagates

Common Edge Cases Developers Miss

1. Empty/Whitespace Inputs

// ❌ Basic check misses whitespace
if (!inputValue) {
  setError("Required field");
}

2. Minimum/Maximum Values

// ❌ Doesn't handle decimals, negatives, or non-numbers
if (age < 18) {
  setError("Must be 18+");
}

3. Special Character Handling

// ❌ Fails to sanitize dangerous inputs
const comment = input.value; // Could contain script tags

4. Asynchronous Validation

// ❌ No loading state or debouncing
const checkEmail = async () => {
  const exists = await api.checkEmail(email);
  setError(exists ? "Email taken" : null);
}

5. Dependent Field Validation

// ❌ Doesn't validate when related fields change
if (password !== confirmPassword) {
  setError("Passwords don't match");
}

Robust Validation Solutions

Comprehensive Value Checking

// ✅ Proper empty/whitespace check
if (!inputValue.trim()) {
  setError("Required field");
}

// ✅ Complete number validation
const num = Number(inputValue);
if (isNaN(num) || num < 18 || num > 120 || !Number.isInteger(num)) {
  setError("Valid age required (18-120)");
}

Input Sanitization

// ✅ Remove harmful characters
const sanitizeInput = (value) => {
  return value.replace(/[<>"'&]/g, "");
}

Debounced Async Validation

// ✅ With loading state and debounce
const [checkingEmail, setCheckingEmail] = useState(false);

useEffect(() => {
  const timer = setTimeout(async () => {
    if (email.includes('@')) {
      setCheckingEmail(true);
      const exists = await api.checkEmail(email);
      setError(exists ? "Email taken" : null);
      setCheckingEmail(false);
    }
  }, 500);

  return () => clearTimeout(timer);
}, [email]);

Cross-Field Validation

// ✅ Validates when either field changes
useEffect(() => {
  if (password && confirmPassword && password !== confirmPassword) {
    setError("Passwords must match");
  } else {
    setError(null);
  }
}, [password, confirmPassword]);

Validation Libraries Worth Considering

For complex forms, consider:

• React Hook Form (Lightweight, performant)
• Formik (Comprehensive form management)
• Yup (Powerful schema validation)
• Zod (Type-safe schema validation)

Essential Edge Cases to Always Check

1. Empty strings and whitespace-only inputs
2. Minimum/maximum lengths and values
3. Special characters and HTML tags
4. Data format requirements (emails, URLs, etc.)
5. Asynchronous checks (username availability)
6. Dependent fields validation
7. Disabled JavaScript scenarios
8. Paste/copy operations
9. International characters and encoding
10. Mobile input variations