Yarn is great.  But there's a caveat to introducing it into your ecosystem:

Yarn uses a file called `yarn.lock` to remember package versions, and this file is incompatible (and will remain incompatible) with npm's `package-lock.json`.

Since you want to commit the lockfile with the rest of your code, if you're using yarn you should be adding the package-lock.json file to your ignore list (eg .gitignore).

cf https://yarnpkg.com/blog/2017/05/31/determinism/