Or garbage in => throw an exception and crash the app. This stuff should be fixed as soon as possible instead of invalid data slowly spreading in every layers of the app.

Assuming you're importing it as a black box abstraction, then absolutely.

'isEven' really shouldn't be a dependency though, either through npm or internally. 'x % 2 === 0' works fine for integers and can be inlined, or if you're using it a bunch and want slightly cleaner code, you can define it as a lambda alongside the code that uses it. Then everyone can see exactly what it's doing with the full context.

The real issue is overabstraction. Even if you do everything "right" and have it as an internal dependency with all the correct error handling, it still makes the code a pain in the ass to read.