Lab 2.2: Safety Net Construction

Module: 2.2 - Test Generation | ← SlidesDuration: 1 hour Sample Project: node-express-mongoose-demo

Learning Objectives

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

• Generate unit tests for untested functions
• Create edge case tests (null, empty, boundary conditions)
• Run tests and verify they pass
• Build a safety net before refactoring

Prerequisites

• Completed Lab 2.1 (Plan Mode)
• Understanding of the testing framework in the project

Why Tests Matter

The Golden Rule

"You can't safely refactor code that doesn't have tests."

Tests are your safety harness:

• They prove the code works before you change it
• They catch regressions during refactoring
• They document expected behavior

Setup

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

# Install dependencies (if not done)
npm install

# Verify tests run
npm test

# Start Claude Code
claude

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Task 1: Find Untested Code

Time: 10 minutes

Identify functions that need test coverage.

Prompts to try:

What functions in this project have no tests?

Show me functions in app/controllers that aren't covered by tests.

Which critical functions lack test coverage?

Pick a target: Choose a function that:

• Is used in multiple places (high impact)
• Has no existing tests
• Has clear inputs and outputs

Success criteria:

• [ ] Identified 2-3 untested functions
• [ ] Selected one critical function to test

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Task 2: Generate Basic Tests

Time: 15 minutes

Create a test file for your chosen function.

Prompts to try:

Generate a test file for the create function in app/controllers/articles.js. Use the project's existing test framework.

Write unit tests for the User model's authenticate method.

Review the output:

• Does it test the happy path?
• Does it use the correct testing framework?
• Are assertions meaningful?

Success criteria:

• [ ] Test file generated
• [ ] Tests run without errors
• [ ] Happy path is covered

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Task 3: Add Edge Case Tests

Time: 20 minutes

Expand coverage with edge cases.

Prompts to try:

Add edge case tests for:
- null or undefined inputs
- empty strings or arrays
- boundary conditions
- invalid data types

What edge cases should we test for the validateUser function?

Add tests for error scenarios in the article controller.

Common edge cases to test:

Category	Examples
Null/undefined	null, undefined, missing params
Empty	"", [], {}
Boundaries	0, -1, MAX_INT, very long strings
Types	Wrong type (string instead of number)
State	Not logged in, expired session

Success criteria:

• [ ] At least 3 edge case tests added
• [ ] Tests handle error scenarios
• [ ] All tests pass

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Task 4: Run and Verify

Time: 15 minutes

Make sure all tests pass and coverage improved.

# Run the test suite
npm test

# If coverage report is available
npm run test:coverage

Verification checklist:

• [ ] All new tests pass
• [ ] Existing tests still pass
• [ ] No false positives (tests that always pass)

Debugging failed tests:

This test is failing. Can you explain why and fix it?

Success criteria:

• [ ] All tests pass
• [ ] Understand what each test verifies

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Test Quality Checklist

Good tests should be:

• [ ] Isolated - Each test can run independently
• [ ] Deterministic - Same result every time
• [ ] Readable - Clear what's being tested
• [ ] Fast - Run in milliseconds, not seconds
• [ ] Meaningful - Test behavior, not implementation

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Example Test Structure

describe('ArticleController', () => {
  describe('create', () => {
    it('should create article with valid data', async () => {
      // Arrange
      const articleData = { title: 'Test', body: 'Content' };

      // Act
      const result = await controller.create(articleData);

      // Assert
      expect(result).toHaveProperty('id');
      expect(result.title).toBe('Test');
    });

    it('should throw error with missing title', async () => {
      // Arrange
      const articleData = { body: 'Content' };

      // Act & Assert
      await expect(controller.create(articleData))
        .rejects.toThrow('Title is required');
    });

    it('should handle null input', async () => {
      await expect(controller.create(null))
        .rejects.toThrow();
    });
  });
});

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Tips for Success

1. Match the project's style - Look at existing tests for patterns
2. Test behavior, not implementation - Focus on what, not how
3. One assertion per test - Easier to debug when things fail
4. Use descriptive names - "should return error when email is invalid"

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Troubleshooting

Tests don't run

• Check the testing framework is installed
• Verify the test file naming convention
• Check the test script in package.json

Tests fail unexpectedly

• Read the error message carefully
• Check if mocking is required
• Verify test database setup

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Stretch Goals

If you finish early:

1. Generate integration tests for an API endpoint
2. Add tests for async error handling
3. Create a test for a complex multi-step flow

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Deliverables

At the end of this lab, you should have:

1. New test file(s) committed to the project
2. At least 5 tests (happy path + edge cases)
3. All tests passing
4. Confidence to refactor the tested code

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Next Steps

After completing this lab, move on to Lab 2.3: Red-Green-Refactor-AI to practice TDD with Claude.