How does npm handle circular dependency?

Question

Just wondering, if there is a package with circular/cyclic dependency, does npm handle that? How? I have googled but couldn't find very useful results.

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I can think of two kinds of problems here:

1. Circular dependency with same version: A->B, B->C, C->A

   Does npm build the dependency like this? (Only my guess, didn't find any circular dependency to actually test on)

   [email protected]
   └─┬ [email protected]
     └─┬ [email protected]
       └── [email protected] (deduped)
   

2. Circular dependency with different version: [email protected]>B, B->C, C->A@^1.0.0, I cannot even guess what would happen if such things exist?

Answer 1

I would strongly discourage having cyclic package dependencies. It is not just if npm can deal with that, but all the tooling you might have around your dependencies. For example, I got some weird Typescript build errors caused by the cyclic package dependencies, npm seemed to be fine. Maybe it did not notice the cycle being inside a mono repo, but Typescript was failing and there was no clear error saying "this is failing because of a cyclic dependency"

Out of experience, I would say "cyclic dependencies" tend to be a nasty pot of problems. It is not a black vs white situation, it tends to be like "undefined behavior", sometimes OK, sometimes not so OK.

I cannot tell you if npm clearly supports cyclic dependencies, but I can tell you it is more important to consider if tools can manage cyclic npm packages. Overall it is safer to assume it is not