Should each team member working on the same project use npm or yarn, or can each use her preferred tool?

If a team is working on the same project, is it recommended that they all use either npm or yarn? Or is it okay to have different team members use different tools?

It is imperative that teams choose one tool for each app and use it exclusively. Both npm and yarn have lock files that ensure the exact version of each dependency that you’ve tested your app against is used in production. If you mix npm and yarn between team members, that cannot be guaranteed and your app may behave in unexpected ways.

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That’s right @mark.brocato. AFAIK, the lock files are the one thing that can cause some unexpected behavior. There’d even be a warning if lock files are trying to co-exist.