Structure should follow responsibility
A small script can remain a single file, but a growing application benefits from clear boundaries. Typical areas include an API or command-line interface, application services, domain models, repositories, infrastructure and tests.
Keep dependencies understandable
A route handler should not need to know every database detail. Service code should contain application rules, while repository or infrastructure code handles persistence and external systems.
Do not over-engineer the beginning
Good structure evolves with the application. Creating many layers before there is a real separation of responsibility can make a small project harder to understand rather than easier.
Practice: refactor a small CRUD script into interface, service and repository modules while keeping its behavior unchanged.
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