Learn how to divide backend code into meaningful modules, understand the difference between CommonJS and ESM, and avoid dependency cycles that make Node projects harder to maintain.
Both systems let you split code. The important part is choosing a consistent standard per project.
CommonJS uses `require` and `module.exports`. ESM uses `import` and `export`. Both are valid, but mixing them casually increases friction and confusion.
In a team setting, consistency often matters more than ideology. A clear convention reduces onboarding cost and debugging noise.
A module should answer one clear question for the rest of the system.
A healthy module usually represents a capability: users, auth, payments, validation, or notifications.
If one file needs to know too much about every other part of the app, you are not really modularizing. You are just moving code around.
The first signs of structural trouble usually appear early.
Circular dependencies make load order and state harder to understand. Generic utility files often become a dumping ground for unrelated logic that nobody wants to untangle later.
Both problems make refactors slower because you lose confidence about what depends on what.
The safest migrations happen by layers, not by one giant rewrite.
If a project moves from CommonJS to ESM, define the target convention, update packages and entry points carefully, and migrate a coherent slice at a time.
Testing behavior after each step matters more than rushing toward a clean diff.