Lesson 2: The Merge

You've built a feature in a parallel universe. Now it's time to bring it home — merge it back into the main timeline.

What is Merging?

Merging combines the changes from one branch into another. It's like taking the best parts of your experiment and applying them to reality.

Before merge:
main:     A ── B ── C
                     \
feature:              D ── E

After merge:
main:     A ── B ── C ────────── F (merge commit)
                     \         /
feature:              D ── E ─┘

How to Merge

Always merge into the branch you want to update:

git checkout main              # Step 1: Go to the receiving branch
git merge feature/login        # Step 2: Merge the feature into it

Types of Merges

1. Fast-Forward Merge

When main hasn't changed since the branch was created, Git simply moves the pointer forward. No merge commit needed.

Before:   main: A ── B    feature: A ── B ── C ── D
After:    main: A ── B ── C ── D

2. Three-Way Merge

When both branches have new commits, Git creates a merge commit that combines both histories.

Before:
main:     A ── B ── C
feature:  A ── B ── D ── E

After:
main:     A ── B ── C ────── F (merge commit)
                \          /
                 D ── E ──┘

Cleaning Up

After a successful merge, delete the feature branch — it's served its purpose:

git branch -d feature/login     # Safe delete (only if merged)
git branch -D feature/login     # Force delete (even if unmerged)

The Merge Checklist

Before merging, always:

1. ✅ Make sure your branch is up to date with main.
2. ✅ Run your tests.
3. ✅ Review the changes (git diff main..feature/login).
4. ✅ Use a Pull Request for team visibility (covered later!).

Mission Objective

Complete the merge workflow:

1. Return home: Switch back to main with git checkout main.
2. Merge the universe: Run git merge feature/login to combine changes.
3. Clean up: Delete the old branch with git branch -d feature/login.

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