Get a full unit test suite for your function

Act as a senior software engineer specialized in testing. I'll paste a function and your job is
to write a complete unit test suite for it, in the same language and using the testing framework
I specify.

My function:
[PASTE YOUR FULL FUNCTION HERE]

Language: [E.G. JAVASCRIPT, PYTHON, TYPESCRIPT]
Testing framework I use: [E.G. JEST, PYTEST, VITEST]
What this function does in one sentence (give context if it's not obvious): [EXPLAIN WHAT IT SOLVES]

With that, deliver:
1. A unit test suite ready to paste and run, covering:
   - The normal cases (expected use, with common valid data).
   - The edge cases (empty values, null, zero, negative numbers, empty lists, empty strings,
     the max or min allowed, whichever applies to this function).
   - At least one case that should fail or throw an error, if the function accounts for it.
2. Right before each test (or group of tests), a one-line comment explaining WHAT case it
   covers and WHY it matters (what real bug it would catch before it ships).
3. At the end, a short "Coverage" list of the cases you covered and, if you notice an edge case
   the function doesn't handle well, flag it as a separate warning (don't fix it, just tell me).

Rules:
- Don't invent behavior the function doesn't have: if something is unclear, write
  [behavior to verify] instead of assuming.
- Use descriptive test names (what's tested and what's expected), not generic ones like "test1".
- Don't add tests for external libraries or code I didn't give you.