Do I need to test on the client side or server side?

Usually, both — but for different purposes.

Test on the server side when:

• You need to verify business logic
• You want to check database interactions
• You’re testing API responses
• You need security-critical validation
• You want tests that are more reliable and easier to automate

Examples:

• User login validation
• Calculating prices/taxes
• Saving/retrieving records
• Permission checks

Test on the client side when:

• You need to verify UI behavior
• You want to test user interactions
• You’re checking browser-specific behavior
• You need to ensure the frontend talks correctly to the backend

Examples:

• Clicking buttons
• Form validation in the browser
• Dynamic page updates
• Responsive layout behavior

Practical rule:

• Server-side tests = faster, more stable, more focused on logic
• Client-side tests = important for user experience, but often slower and more brittle

Recommendation:

If you’re deciding where to start, test server-side first for core logic, and add client-side tests for critical user flows.

If you want, I can also help you decide this for a specific stack (for example: React, Node, Django, or Laravel).