Test Suite Generator

Generate a comprehensive test suite for this function:
```
[function code]
```
Using [Jest/Pytest/etc.]. Include:
- Happy path tests (3-5 cases)
- Edge cases (empty inputs, nulls, extremes)
- Error/exception cases
- Performance boundary test
- Each test must have: descriptive name, setup, execution, assertion, teardown if needed.
Also identify any untestable parts and explain why.