Write Unit Tests for My Code

Please write comprehensive unit tests for the following code. Act as a senior engineer who follows TDD best practices. Environment: - Language: - Testing framework: - Mocking library (if any): Code to test: ``` ``` Context about what this code does: Please write tests that cover: 1. **Happy path** - all expected inputs produce the correct outputs 2. **Edge cases** - empty arrays, zero values, null/undefined, very large inputs 3. **Error handling** - invalid inputs, thrown exceptions, rejected promises 4. **Boundary conditions** - min/max values, single-item arrays, exact thresholds 5. **Side effects** - database calls, API requests, file I/O (use mocks) For each test: - Use descriptive test names that explain the scenario (e.g., 'should return empty array when given no input') - Follow the Arrange-Act-Assert pattern - Add a one-line comment explaining why each edge case matters