Recommended backend structure

src/
├ routes/
├ controllers/
├ services/
├ middleware/
├ prisma/
├ tests/
├ config/
└ server.js

Folder responsibilities

• src/routes/
  • Express route definitions (e.g., /shorten, /:shortCode).
  • Maps URLs + methods to controller functions.
• src/controllers/
  • Request handlers that validate input (or delegate validation) and call services.
  • Responsible for HTTP responses (status code + JSON shape).
• src/services/
  • Business logic: generating short codes, collision retries, computing expirations.
  • Should be written to be easy to unit test.
• src/middleware/
  • Cross-cutting concerns: request logging, error handler, validation middleware.
• src/prisma/
  • Prisma client initialization and Prisma schema (depending on setup).
  • Central place for DB access patterns.
• src/tests/
  • Jest + Supertest tests: route/integration tests and service unit tests.
• src/config/
  • Environment loading, constants, app configuration objects.
• src/server.js
  • Express app initialization: middleware wiring + route registration.