Resolve Merge Conflicts in yarn.lock

The yarn package manager creates a yarn.lock file when you install packages with yarn add. This file helps yarn determine the proper dependency versions to install.

I ran into a merge conflict recently between my branch's yarn.lock file and the one that existed on master. My first instinct was to resolve the conflicts by hand, but it felt wrong. According to the documentation, you shouldn't edit this file directly. yarn will manage it by itself. If this is the case, then what's the best way to handle these merge conflicts?

A better solution

A comment on GitHub shows a more efficient and safer way to get around this problem.

Note: The comment discusses merge conflicts involving a `master` branch. This post will describe how to handle conflicts involving _any_ branch.

The first thing you need to do is rebase against the branch you're attempting to merge into:

git rebase <SOME_BRANCH>

This command will take all the commits on your branch and "replay" them on top of <SOME_BRANCH>. If you're unfamiliar with rebasing in git, here's a good primer.

git will rebase each commit one at a time. Eventually you will run into your first conflict in yarn.lock. What you want to do is checkout the version from the branch you are rebasing against:

git checkout -- yarn.lock

Understanding this command requires some knowledge of rebasing in git. The first bit of context we have is a GitHub comment in the same issue as the one I linked above:

"I'd recommend git checkout -- yarn.lock, which is more general and just resets it to whatever is committed on your current branch."

The GitHub user @idris recommends this command over the one shown in the first comment. But why? What do they mean by "more general", and what is the "current branch"?

At first it would seem that the current branch is our branch. At least that's how I perceived it. This confused me, so I took to Google. After some digging I found this StackOverflow comment. It explains the concept in detail.

During a rebase, the current branch is the branch that you're rebasing against. In this case we are rebasing against SOME_BRANCH, so it is the current branch.

This means that running git checkout -- yarn.lock resets yarn.lock to whatever exists on SOME_BRANCH.

The command is the same as running:

git checkout SOME_BRANCH -- yarn.lock

The comment from @idris says we can omit the name of the branch, making the command "more general". git will just give us the file from whatever branch we're rebasing against.

Pretty neat! 💯

After checking out the other branch's yarn.lock file it's time to install your branch's dependencies:

yarn install

yarn will update the contents of yarn.lock, using your branch's package.json for context.

Once the process finishes you can add yarn.lock and continue with the rebase:

git add yarn.lock
git rebase --continue

Depending on the situation you may run into further conflicts for each commit. All you have to do is rinse and repeat the steps above. That's all, folks!

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