Why should each Node.js module have one concern?
Because focused modules are easier to test, reuse, and understand. A user controller that also sends emails and connects to databases is doing too much. One concern per module keeps each piece simple and its purpose clear.
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More FAQs in Node.js Modules Best Practices for Clean and Reusable Code
One concern per module, export a clear minimal interface, use named exports for multiple values and a default for single, pick one module system consistently, and avoid side effects on require so modules are easy to test.
Because a module that does work just by being required, like sending emails or connecting to a database, is hard to test and reason about. Export a function that does the work instead, so the work happens when you call it, not when you import it.
Use named exports (exports.add, exports.subtract) when exporting multiple values, so the interface is explicit and requiring files can destructure. Use a default export (module.exports = fn) when exporting one main thing, to signal it is the primary export.
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