Exercise: Bug Hunter¶

Level: Intermediate Time: 30-45 minutes Skills: Debugging, code exploration, testing

Objective¶

Practice debugging a broken application with Claude Code. This exercise contains intentional bugs for you to find and fix.

Setup¶

Navigate to this exercise directory:
```
cd exercises/02-bug-hunter
```
Install dependencies:
```
npm install
```
Start Claude Code:
```
claude
```

The Application¶

This is a simple task manager API with the following features: - Add tasks - List tasks - Mark tasks complete - Delete tasks

Problem: Users report that the application has several bugs. Your job is to find and fix them.

Tasks¶

Task 1: Explore the Codebase¶

Ask Claude to give you an overview of the application.

Hint: "Explain what this application does and how it's structured"

Task 2: Run the Tests¶

The tests should reveal some of the bugs.

Hint: "Run the tests and show me which ones fail"

Task 3: Investigate Bug #1 - Tasks Not Saving¶

Users report that sometimes tasks disappear. Find why.

Hints: - Check how tasks are stored - Look at the add task function - Is data persisting correctly?

Task 4: Investigate Bug #2 - Wrong Completion Status¶

Marking a task complete doesn't always work.

Hints: - Check the complete task function - Look at how the status is updated - Compare the update logic with the data structure

Task 5: Investigate Bug #3 - Delete Fails Silently¶

Deleting a non-existent task should return an error, but doesn't.

Hints: - Check the delete function - What happens when the task ID doesn't exist? - Is there proper error handling?

Task 6: Fix All Bugs¶

Work with Claude to fix each bug. After each fix: 1. Run the specific test for that bug 2. Verify it passes

Task 7: Add a Missing Test¶

Notice there's no test for listing completed tasks only. Add one.

Files to Examine¶

02-bug-hunter/
├── src/
│   ├── index.js        # Main application
│   ├── taskManager.js  # Task management logic (bugs are here!)
│   └── storage.js      # Data storage
├── tests/
│   └── tasks.test.js   # Test file
├── package.json
└── README.md

Success Criteria¶

[ ] All existing tests pass
[ ] You can add a task and see it listed
[ ] Marking a task complete works correctly
[ ] Deleting a non-existent task returns an error
[ ] New test for listing completed tasks passes

Bug Details (Spoilers!)¶

Bug #1 Hint

Check `taskManager.js` - is the task being saved after creation?

Bug #2 Hint

Look at the `completeTask` function. Is it comparing IDs correctly?

Bug #3 Hint

The `deleteTask` function doesn't check if the task exists before trying to delete.

Reflection Questions¶

How did Claude help identify the bugs?
What debugging strategies did you use?
How would you prevent similar bugs in the future?