Test Case Generator

The Problem

Writing tests is the part developers skip most often — not because it’s hard, but because thinking of all the scenarios is mentally exhausting. Happy path? Easy. But what about empty inputs, boundary values, concurrent access, Unicode strings, negative numbers, and timezone edge cases? AI can generate a comprehensive test suite in seconds by systematically thinking through every category of input.

The Prompt

Generate comprehensive test cases for the following function. I need production-quality tests that I can run immediately.

FUNCTION:
[paste your function or class method here]

TESTING FRAMEWORK: [e.g., Jest, Vitest, pytest, Go testing, JUnit]
LANGUAGE: [e.g., TypeScript, Python, Go, Java]

Generate tests covering:

1. **Happy Path** — Normal expected inputs and outputs (3-5 cases)
2. **Edge Cases** — Boundary values, empty inputs, single-element collections, max values
3. **Error Cases** — Invalid inputs, null/undefined, wrong types, out-of-range values
4. **Special Characters** — Unicode, emojis, HTML entities, SQL metacharacters
5. **Async Behavior** (if applicable) — Race conditions, timeouts, rejected promises
6. **Business Logic** — Domain-specific rules that could break silently

For each test:
- Use descriptive test names that explain the scenario ("should return empty array when input is null")
- Include arrange, act, assert structure
- Add inline comments explaining WHY each edge case matters

Output the complete test file ready to save and run.

Example Output

describe('calculateDiscount', () => {
  // Happy path
  it('should apply 10% discount for orders over $100', () => {
    expect(calculateDiscount(150, 'SAVE10')).toBe(135);
  });

  // Edge case: exact boundary
  it('should apply discount when order equals minimum threshold exactly', () => {
    expect(calculateDiscount(100, 'SAVE10')).toBe(90);
  });

  // Error case: negative price
  it('should throw RangeError for negative order amounts', () => {
    expect(() => calculateDiscount(-50, 'SAVE10')).toThrow(RangeError);
  });

  // Business logic: expired coupon
  it('should return full price for expired coupon codes', () => {
    expect(calculateDiscount(200, 'EXPIRED2024')).toBe(200);
  });
});

When to Use

Use this whenever you write a new function or refactor an existing one. It’s especially valuable when you need to reach a coverage threshold, when testing complex business logic with many conditions, or when writing tests for code you didn’t author. Paste the function and get a complete test file in one shot.

Pro Tips

• Include the types/interfaces — paste related TypeScript types or data classes alongside the function so the AI understands the full contract.
• Ask for mutation-proof tests — add “Make sure these tests would fail if any line of the original function were removed or changed” to catch weak assertions.
• Generate a coverage matrix first — ask “Before writing code, list every test scenario in a table with input, expected output, and category” to review the plan before generating.