Input Validation
Checking that user-submitted data meets expected rules before processing it, preventing errors and security vulnerabilities.
Input validation is the practice of verifying that data from users, forms, or external sources is safe and correctly formatted before your application processes it. Without validation, a missing field can crash your server, a malformed email can corrupt your database, and a script tag can enable a cross-site scripting (XSS) attack.
Validation happens on both sides: client-side (instant feedback in the browser using libraries like Zod or Yup) and server-side (mandatory, because client checks can be bypassed). The server should never trust data from the client without re-validating it.
For vibe coders, AI is good at generating validation schemas from descriptions ("validate that email is required, password is at least 8 characters, and age is a positive number"). The key judgment call is knowing what to validate and where. Always validate at the boundary between your system and the outside world.
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