Dev Unit Test Writing Guide

Create a unit testing guide for [language/framework] codebase.

Project details:
- Language: [JavaScript/Python/Java/other]
- Test framework: [Jest/PyTest/JUnit/other]
- Coverage target: [%]
- Current coverage: [%]
- Mocking library: [Jest mocks/unittest.mock/Mockito]

UNIT TESTING GUIDE

1. WHAT TO TEST

   TEST THESE
   - Business logic functions
   - Data transformations
   - Error handling paths
   - Edge cases (empty/null/boundary values)
   - Each branch of conditionals
   - Public API of modules

   DO NOT TEST
   - Framework internals
   - Third-party libraries
   - Implementation details (test behavior not how)
   - Private methods directly

2. TEST STRUCTURE — AAA PATTERN

   ```javascript
   describe('calculateDiscount', () => {
     it('should apply 10% discount for premium users', () => {
       // ARRANGE
       const user = { type: 'premium' }
       const price = 100

       // ACT
       const result = calculateDiscount(price, user)

       // ASSERT
       expect(result).toBe(90)
     })
   })
   ```

3. NAMING CONVENTIONS
   
   Pattern: [unit]_[scenario]_[expected result]
   
   Examples:
   - calculateTax_withZeroIncome_returnsZero
   - validateEmail_withInvalidFormat_throwsError
   - getUser_withValidId_returnsUser
   - getUser_withMissingId_returns404

4. MOCKING DEPENDENCIES

   WHEN TO MOCK
   - External APIs
   - Database calls
   - File system
   - Date/time (for deterministic tests)
   - Random number generators

   ```javascript
   jest.mock('../services/emailService')
   const emailService = require('../services/emailService')
   emailService.send.mockResolvedValue({ success: true })
   ```

5. EDGE CASES TO ALWAYS TEST
   - Null/undefined inputs
   - Empty string/array/object
   - Zero values
   - Negative numbers
   - Very large numbers
   - Special characters in strings
   - Boundary values (min/max)

6. TEST QUALITY CHECKLIST
   ☐ Each test has one assertion focus
   ☐ Tests are independent (no shared state)
   ☐ Tests run in any order
   ☐ No network calls in unit tests
   ☐ Tests are fast (under 100ms each)
   ☐ Test names explain what is being tested
   ☐ Failure message is clear

7. COVERAGE GUIDE
   - Line coverage: [% target]
   - Branch coverage: [% target — more important]
   - Focus: critical paths over 100% coverage
   - Exclude: generated code/config files