Undo and Time Travel

jj's operation log makes it impossible to lose work—every action is recorded and reversible.

The Operation Log

Every jj command is recorded in the operation log:

$ jj op log
d5f3b08fe3ea5  2 minutes ago  checkout
1c8e94a3f2c1b  4 minutes ago  new
e7d3a0b5d4c2e  6 minutes ago  describe
a2b5c8d9e0f1a  8 minutes ago  checkout

Compare to git's reflog:

git

git reflog

jj op log

Command comparison: git commands on the left, jj commands on the right
git	jj
git reflog	jj op log

Undoing Operations

git

git reset --hard HEAD@{1}

jj op undo

Command comparison: git commands on the left, jj commands on the right
git	jj
git reset --hard HEAD@{1}	jj op undo

Multiple Undos

# Undo last 3 operations
jj op undo --count 3

Restoring Specific States

git

git reset --hard HEAD@{5}

jj op restore abc123

Command comparison: git commands on the left, jj commands on the right
git	jj
git reset --hard HEAD@{5}	jj op restore abc123

Viewing Past States

You can view the repository as it was at any operation:

# Show log at past operation
jj op log -at <operation-id>

# Show status at past operation
jj op show <operation-id>

Common Undo Scenarios

Accidentally abandoned work:

# Oops, abandoned wrong commit
jj abandon @-

# Undo it
jj op undo

# Commit is back
jj log

Bad rebase:

# Rebase went wrong
jj rebase -d bad-base

# Undo entire rebase
jj op undo

Mistaken merge:

# Merged wrong branch
jj merge wrong-branch

# Undo
jj op undo

Time Travel Debugging

The operation log is like a time machine:

# Something broke, when did it happen?
jj op log

# Jump back to before it broke
jj op restore <good-operation-id>

# Verify bug isn't there
jj log
jj show @

# Come back to present
jj op restore <latest-operation-id>

Comparing States

# Compare current state with past
jj diff --from <past-operation>

# See what changed between two operations
jj diff --from <op1> --to <op2>

Operation IDs vs Commit IDs

Operation ID: Identifies a command in the operation log
Commit ID: Identifies a specific commit
Change ID: Stable identifier for a logical change

# Undo an operation (sequence of commits)
jj op undo <operation-id>

# View a specific commit
jj show <commit-id>

# Track a change across rebases
jj log -r change(<change-id>)

Immutable Operations

Some operations can't be undone (for safety):

jj git push  # Pushed to remote—can't undo
jj git fetch # Fetched from remote—can't undo

But you can undo their local effects:

# After a problematic fetch
jj git fetch
jj op undo  # Undoes the local ref updates

Practical Workflow

# Experiment freely
jj new
echo "radical change" > core.js
jj describe -m "Experiment"
jj new

# Doesn't work out
jj op undo

# Try different approach
jj new
echo "conservative change" > core.js
jj describe -m "Safe refactor"
jj new

Operation Log Size

The operation log grows but jj handles it efficiently:

# Operations are cheap
# Thousands of operations = still fast

# Clean old operations if needed
jj op log --limit 100

Git Comparison Summary

| Task | Git | jj | |------|-----|----| | Undo last action | git reset --hard HEAD@{1} | jj op undo | | View history | git reflog | jj op log | | Restore state | git reset --hard <ref> | jj op restore <id> | | Undo multiple | Complex git dancing | jj op undo --count N | | Time limit | 90 days default | Never expires |

Try It Yourself

# Make some changes
echo "a" > a.txt
jj describe -m "Add A"
jj new

echo "b" > b.txt
jj describe -m "Add B"
jj new

# Undo last operation
jj op undo
# b.txt is gone

# Undo again
jj op undo
# a.txt is gone

# Redo by going forward
jj op log  # Find the operations
jj op restore <id>  # Jump to any point

Key Takeaways

Every jj operation is recorded
jj op undo reverses the last operation
jj op restore jumps to any past state
Operation log never expires
Safe experimentation—always can undo
More reliable than git reflog

Next Steps

You've now mastered the core jj concepts. Let's look at some advanced workflows.