Lab 2.4: Incremental Improvement

Module: 2.4 - Safe Refactoring | ← SlidesDuration: 1 hour Sample Project: node-express-mongoose-demo

Learning Objectives

By the end of this lab, you will be able to:

• Refactor code safely with test verification
• Make incremental changes (small steps)
• Improve readability without changing behavior
• Use tests as a safety net

Prerequisites

• Completed Labs 2.2 and 2.3
• Tests in place for the code you'll refactor
• Confidence in running the test suite

The Safe Refactoring Workflow

1. Verify tests pass (baseline)
2. Make one small change
3. Run tests
4. If green: commit and continue
5. If red: revert and try again

Setup

# Navigate to the sample project
cd sample-projects/node-express-mongoose-demo

# Verify tests pass BEFORE starting
npm test

# Start Claude Code
claude

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Task 1: Identify Code Smells

Time: 10 minutes

Find code that needs improvement.

Prompts to try:

What code smells exist in app/controllers/articles.js?

Are there any functions with too many responsibilities?

Find examples of duplicated code in this project.

Common code smells:

• Long methods (>20 lines)
• Duplicated code
• Poor naming
• Deep nesting
• Magic numbers/strings

Success criteria:

• [ ] Identified 2-3 code smells
• [ ] Selected one to fix

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Task 2: Refactor for Readability

Time: 15 minutes

Improve variable and function names.

Step 1: Run tests (baseline)

npm test

Step 2: Improve naming

Rename the variables in this function to be more descriptive. Don't change any behavior.

The function name 'proc' is unclear. Suggest a better name and rename it throughout the codebase.

Step 3: Verify tests pass

npm test

Success criteria:

• [ ] Names are clearer
• [ ] Tests still pass
• [ ] Behavior unchanged

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Task 3: Extract Helper Functions

Time: 20 minutes

Break down complex functions.

Step 1: Identify extraction opportunities

This function is too long. What parts could be extracted into separate functions?

Step 2: Extract one piece

Extract the [specific logic] into a separate helper function.

Step 3: Run tests

npm test

Step 4: Repeat for other pieces

Success criteria:

• [ ] At least one function extracted
• [ ] Tests pass after each extraction
• [ ] Main function is now shorter

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Task 4: Remove Duplication

Time: 15 minutes

DRY up the codebase.

Step 1: Find duplication

Find duplicated code between the create and update functions.

Step 2: Extract shared logic

Extract the common validation logic into a shared function used by both create and update.

Step 3: Verify

npm test

Success criteria:

• [ ] Duplication removed
• [ ] Shared function created
• [ ] Tests pass

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Refactoring Techniques

Technique	When to Use	Example
Rename	Unclear names	x → userId
Extract Function	Long methods	Pull out 10 lines into helper
Inline	Over-abstraction	Remove unnecessary wrapper
Move	Wrong location	Move util from controller to utils/
Replace Magic	Hard-coded values	86400 → SECONDS_PER_DAY

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The Golden Rule

Remember

"Refactoring is changing code without changing behavior."

After every change:

1. Run tests
2. If they pass, you're safe
3. If they fail, you changed behavior (revert!)

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Commit Strategy

Commit after each successful refactoring:

# After each green test run
git add .
git commit -m "Refactor: Extract validation helper from article controller"

Small commits = easy rollback if something goes wrong.

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Tips for Success

1. One change at a time - Don't combine refactorings
2. Run tests constantly - After every single change
3. Commit frequently - Save your progress
4. Keep behavior identical - If tests fail, you went too far

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Common Refactoring Prompts

Rename this variable to something more descriptive.

Extract the loop body into a separate function.

Replace this magic number with a named constant.

Simplify this nested if-else into an early return pattern.

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Red Flags (Stop Refactoring)

• Tests start failing unexpectedly
• You're adding new features (that's not refactoring)
• Changes are getting too large to track
• You've been refactoring for >30 min without committing

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Stretch Goals

If you finish early:

1. Refactor deeply nested conditionals into guard clauses
2. Extract a configuration object from scattered magic values
3. Improve error handling consistency

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Deliverables

At the end of this lab, you should have:

1. Cleaner, more readable code
2. Smaller, focused functions
3. Reduced duplication
4. All tests still passing
5. Clean commit history showing incremental changes

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Next Steps

After completing this lab, move on to Lab 2.7: Modernization Mini-Project - the Day 2 capstone.