MERGE-A-MATIC

One of the difficult parts of collaboration is dealing with merge conflicts. This can happen, for example, when Person A alters a file (eg, server.js) on Person A's computer, and Person B also alters the same file on Person B's computer.

When one of them pushes back to the repository, Person B's push will be rejected, and when Person B pulls, they will see a merge conflict.

Never fear, though! In this repository, we'll walk through how to resolve one such conflict, and you will get immediate feedback on whether you successfully did it. Feel free to keep this open in your browser while you work through the exercise in your terminal.

setup

clone the project and type npm install to make sure the checking scripts (here, using husky hooks) are installed. These are used to check your work after you've resolved the merge and give you some feedback on how you did.

Then type git merge origin/develop.

(NOTE: You could also type git checkout develop, then git checkout main, then git merge develop to merge from a local copy of the develop branch that you set up on your machine, but this isn't necessary.)

we're using git merge origin/develop instead of git pull, but for this exercise it has the same result. The command means, "merge the develop branch from the remote repository into my current branch"

you have a conflict!

you will see something like this:

Auto-merging server.js
CONFLICT (content): Merge conflict in server.js
Auto-merging extra.md
CONFLICT (content): Merge conflict in extra.md
Auto-merging README.md
Automatic merge failed; fix conflicts and then commit the result.

ah, looks like we have a conflict in server.js and extra.md...

...well let's start with server.js. First, just to confirm what git wants us to do, type git status and you should see output like this:

On branch main
Your branch is up to date with 'origin/main'.

You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   extra.md
	both modified:   server.js

no changes added to commit (use "git add" and/or "git commit -a")

We can see here that if anything goes wrong, we can use git merge --abort to get back to before we started the merge.

the easy example

so let's open the file server.js, and you should see the following:

<<<<<<< HEAD
const http = require('http')
const port = 3001

const requestHandler = (request, response) => {
  console.log(request.url)
  response.end('this is the response')
}
=======
const express = require('express')
const app = express()
const port = 3000

app.get('/', (req, res) => res.send('Hello World!'))
>>>>>>> origin/develop

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

This is an easy example, since the two files are mostly completely different, and so git thinks there's only a little overlap of similar content right at the bottom to make things tricky.

We'll walk through resolving this conflict step by step.

The first thing to notice is the line right at the top: <<<<<<< HEAD

This tells us where the conflict starts, and also the branch it refers to. Here, HEAD refers to the place that our current branch is at, so it's basically the same as our local copy of the main branch.

The next important line is =======

This is telling us where the first part of the conflict ends. So everything above it is in our current branch's file, and everything below it is in the develop branch's file.

The final interesting line is right near the bottom: >>>>>>> origin/develop

This tells us where the conflict ends. This is a simple example since everything from the first marker line to the middle line is the version in main, and everything from the middle line to the final line is the version in develop.

So to resolve the conflict in this file, since the two files are completely different and we probably only want one version or the other, just keep the version from origin/develop using express.

In our example here, you should delete everything between <<<<<<< HEAD and =======, including those lines themselves. Remember to also delete the final marker line >>>>>>> origin/develop. (git has actually written those marker lines into our file, so we need to delete them manually)

That file's conflict is now fixed, let's move on to extra.md...

another still simple example

So when we open the file, we know exactly what to look for <<<<<<< HEAD, ======= and >>>>>>> origin/develop lines.

Let's open up extra.md and look at the conflicts. The first one looks like:

<<<<<<< HEAD
It uses the http library to set up a simple server.
=======
It uses the amazing express library to set up a simple server.
>>>>>>> origin/develop

Here are our three marker lines again, so we know where the conflict is and what the content of it is.

In this case, since we want to use the awesomeness of express, we can delete everything between the first and second marker lines (as well as the three marker lines themselves).

So in this case we're deleting everything except the one line.

It uses the amazing express library to set up a simple server.

Easy, right?!?!

...now on to the second conflict in the file:

<<<<<<< HEAD
The usual port is 3000, but it could easily be 3001.
=======
The usual port is 3000, but it could easily be 4001.
>>>>>>> origin/develop

Here it looks like exactly one thing was changed in the develop branch. The port number is 4001 instead of 3001.

Go ahead and decide which line you like better, and delete the other line, plus the three marker lines. So basically, here just delete every line except the one line you want to keep.

The last conflict in the file looks like:

<<<<<<< HEAD
Express is used in a lot of different projects.
=======
Express can do a lot of things!
>>>>>>> origin/develop

...and I'm sure this is getting more familiar now, so either keep one of the new lines and delete all the marker lines, or just delete everything to not include either of these.

check your work

And that's it! Type git add . and then git commit -m 'resolve merge' (or whatever commit message you feel like).

If you succeeded, you should see a nice congratulations message. If there are still unremoved marker lines in the file, you will be prompted to fix those.