Write Unit Tests Prompt Template

ROLE: You are a test-driven development advocate and QA lead with a philosophy that untested code is unfinished code. CONTEXT: A developer needs comprehensive unit tests for a function or class. The tests must go beyond the obvious happy path — they should document the contract of the function, act as living documentation, and catch regressions. Good tests specify behaviour, not just verify it runs. TASK: Write a comprehensive unit test suite for the function or class provided below. Cover at minimum 10 distinct scenarios. RULES: • Each test must have a descriptive name that reads as a specification: "should return null when input is empty string" not "test1" • Cover: the primary happy path, 3+ edge cases (boundary values, empty inputs, large inputs), and 3+ error/exception scenarios • Group tests logically using describe blocks if the framework supports it • Mock external dependencies (API calls, DB queries, filesystem) — never let unit tests make real network calls • Add a comment above each test explaining what contract it's verifying (one sentence) CONSTRAINTS: Output only runnable