[BUG] npm install <dep>@version removes the dep from peerDeps if dep is in devDeps as well

[scamden]

Current Behavior:

running npm install on a dependency that is in both dev and peer deps removes the dep from peer. (note: this even happens if installing with the --legacy-peer-deps flag)

Expected Behavior:

if anything the peer dep should be kept over the dev dep, as the peer dep is part of the package's outward facing contract. preferably both are kept while legacy-peer-deps is an option.

Steps To Reproduce:

1. create a package.json with a dep in both dev and peer
2. run npm install <depname>@latest in the package
3. notice that the dep is no longer in peer deps

Environment:

[ljharb]

Peer deps should almost always be also in devDeps, or also in runtime deps, so this seems like a big issue. Thanks for reporting it!

To confirm, this is with which npm version?

[aliatsis]

@ljharb we tested and reproduced with 7.0.0-beta.10, 7.0.0-beta.11, and 7.0.0-beta.12

[isaacs]

Hmm... so this is a pretty easy fix, to just note that it's in both places and save it back to both. However, I notice that the "pretty easy" fix means that the two fields will stay in sync, which could either be a surprising bug or a really wonderful feature.

So, for example, if your package.json is:

{
  "devDependencies": {
    "foo": "^1.3.5"
  },
  "peerDependencies": {
    "foo": "1.x"
  }
}

and then you do: npm i foo@latest, you might get:

{
  "devDependencies": {
    "foo": "^1.5.2"
  },
  "peerDependencies": {
    "foo": "^1.5.2"
  }
}

This can be a hazard if you want to ensure that you're testing with the latest and greatest version, but also want to have a broader peerDependencies range to avoid peer dep conflicts. Another view of this is that having these two things out of sync is almost always a bug, since it means you're not testing with the version that you're going to be loading at run-time.

In practice, we prioritize the peer dependency when loading edges, so the fact that we were prioritizing dev at save time is definitely a bug. Saving back to both seems like the right thing to do (and is trivial to do), with "saving to peer only" as a possible second place option. Saving to peer and dev differently will be somewhat tricky.

[ljharb]

imo they should always indeed be in sync, so I find that to be a really wonderful feature.

That said, whatever's in peer deps should be the source of truth, since that's exposed as part of your API.

[isaacs]

The peerDep is the source of truth (since it's loaded first, and we skip the creation of edges if we already have an edge by that name). When saving, they're both saved to the same value.