What is Form Validation?

Form Validation checks user input for errors (required fields, format, length, etc.) and provides feedback. Can happen on blur (leaving field), on submit, or inline (as user types). Good validation is specific, timely, and helpful—tells users exactly what's wrong and how to fix it.

When Should You Use This?

Validate all form inputs. Use inline validation for format (email, phone), length constraints, and availability checks (username taken). Wait until blur or submit for required fields—don't show "Required" while user is still filling out form. Show success states too (green checkmark for valid email). Always validate on backend regardless of frontend validation.

Common Mistakes to Avoid

• •Validating while typing—"Password must be 8 characters" while user has typed 3 is annoying; wait for blur
• •Generic error messages—"Invalid input" is useless; say "Email must include @"
• •Red everywhere—showing all errors at once is overwhelming; validate field-by-field
• •No success feedback—users appreciate confirmation that input is correct (green checkmark)
• •Client-side only—always validate on backend; frontend validation is UX, not security

Real-World Examples

• •Stripe—real-time credit card validation with specific error messages
• •GitHub signup—checks username availability inline, shows requirements met/unmet
• •Linear—validates issue titles, shows character count, prevents empty submissions
• •Google Forms—validates email format, required fields only on submit