Writing input validation on both the backend and frontend sides of the project is essential for building a robust and secure application. Each side serves a different purpose, and both are necessary to ensure data integrity and security.

1. Frontend Validation: Frontend validation is mainly responsible for providing a better user experience and reducing unnecessary requests to the server. It involves checking user inputs in real-time before sending the data to the backend. This helps users correct errors immediately and receive prompt feedback. Frontend validation can include checks for required fields, data format, length, and other basic rules. However, frontend validation is not sufficient on its own, as it can be bypassed by malicious users or automated scripts.

2. Backend Validation: Backend validation is crucial for maintaining data integrity and security. While frontend validation provides a better user experience, backend validation is the last line of defense against malicious or erroneous data that might bypass the frontend validation. Backend validation should not trust data received from the client and should thoroughly validate all incoming data. It should perform more complex and security-related checks, such as database uniqueness, business rules, authentication, and authorization.

Best Practices:

1. Perform Backend Validation: Always perform validation on the backend, regardless of frontend validation. Backend validation is essential to ensure data consistency and security.

2. Use Libraries and Frameworks: Leverage well-established validation libraries and frameworks on both the backend and frontend. This can save development time and provide built-in security features.

3. Use Strong Data Validation: Employ strong validation techniques to prevent security vulnerabilities like SQL injection, cross-site scripting (XSS), and other data-related attacks.

4. Degrade Gracefully: Although frontend validation improves user experience, never solely rely on it for data integrity and security. Always validate data on the backend to ensure proper validation, even if the frontend validation fails.

5. Avoid Redundancy: Coordinate validation rules between frontend and backend to avoid redundant checks and ensure consistency.

6. Security is Key: Remember that backend validation is the last line of defense. Frontend validation can be bypassed, so the backend should always perform thorough validation.

In summary, a combination of both frontend and backend validation is necessary for building a reliable and secure application. Frontend validation improves user experience, while backend validation ensures data integrity and security. Always prioritize security and use proper validation techniques on the backend to avoid security vulnerabilities.