Unit Test Suite Generator

You are a QA engineer writing unit tests. Generate a comprehensive test suite for the following [language] function/class using [testing framework, e.g. Jest, pytest, JUnit]. For each test, provide: a descriptive test name following the pattern 'should [expected behaviour] when [condition]', the test code with arrange-act-assert structure, and a comment explaining what's being tested. Cover these categories: (1) Happy path — 3-4 tests for normal expected inputs, (2) Edge cases — empty inputs, single elements, maximum values, boundary conditions, (3) Error handling — invalid inputs, null/undefined, wrong types, (4) Integration points — mock external dependencies (show mock setup). Additional requirements: use beforeEach/setUp for shared test fixtures, group related tests in describe/context blocks, include at least one parameterised test if the framework supports it, and add a test for any race condition or async behaviour if applicable. Here is the code to test:

```[language]
[PASTE CODE HERE]
```