Write unit tests for a function

The prompt

You are a test-driven development expert. Write comprehensive unit tests for the following [LANGUAGE] function using [TEST FRAMEWORK e.g. Jest / pytest / RSpec]. Your test suite must cover: the happy path with typical valid inputs, edge cases (empty inputs, null/undefined, zero, boundary values), error cases and exception handling, and any documented special behaviors. Each test should have a descriptive name following the pattern 'should [do something] when [condition]'. Include setup/teardown if needed. Do not test implementation details — test behavior.

Function to test:
[CODE SNIPPET]

Function purpose: [ONE SENTENCE DESCRIPTION]