generating-unit-testsClaude Skill

Unit Test Generator

Overview

Automatically generate comprehensive unit tests from source code analysis covering happy paths, edge cases, boundary conditions, and error handling. Supports Jest, Vitest, Mocha (JavaScript/TypeScript), pytest (Python), JUnit 5 (Java), and Go testing with testify.

Prerequisites

• Testing framework installed and configured (jest, vitest, pytest, junit-jupiter, or Go testing)
• Source code with clear function signatures, type annotations, or JSDoc comments
• Test directory structure established (__tests__/, tests/, spec/, or *_test.go)
• Mocking library available (jest.mock, unittest.mock, Mockito, or gomock)
• Package scripts configured to run tests (npm test, pytest, go test)

Instructions

1. Scan the codebase with Glob to locate source files that lack corresponding test files (e.g., src/utils/parser.ts without __tests__/parser.test.ts).
2. Read each untested source file and extract:
   • All exported functions and class methods with their signatures.
   • Parameter types, return types, and thrown exceptions.
   • External dependencies (imports from other modules, third-party libraries, I/O).
   • Pure vs. impure function classification.
3. For each function, generate test cases in these categories:
   • Happy path: Valid inputs producing expected outputs (at least 2 cases).
   • Edge cases: Empty strings, empty arrays, zero, negative numbers, null, undefined, maximum values.
   • Error conditions: Invalid types, missing required fields, network failures, permission errors.
   • Boundary values: Off-by-one, integer overflow, string length limits.
4. Create mock declarations for all external dependencies:
   • Database calls return predictable fixture data.
   • HTTP clients return canned responses with configurable status codes.
   • File system operations use in-memory buffers or temp directories.
5. Write the test file following project conventions:
   • Match the existing test file naming pattern (*.test.ts, *.spec.js, test_*.py).
   • Group tests in describe/context blocks by function name.
   • Use beforeEach/afterEach for setup and teardown.
   • Include inline comments explaining non-obvious test rationale.
6. Run the generated tests to verify they pass, then check coverage to confirm the target function is fully exercised.
7. Report coverage gaps and suggest additional test cases for uncovered branches.

Output

• Test files placed alongside source files or in the project's test directory
• Mock/stub files for external dependencies
• Coverage report showing line, branch, and function coverage for tested modules
• List of remaining coverage gaps with suggested test cases

Error Handling

Error	Cause	Solution
Cannot find module on import	Test file path does not match project module resolution	Check tsconfig.json paths and moduleNameMapper in Jest config
Mock not intercepting calls	Mock defined after module import caches the real implementation	Move jest.mock() calls to the top of the file before any imports
Async test timeout	Promise never resolves due to missing await or unhandled rejection	Add await before async calls; increase timeout with jest.setTimeout()
Tests pass alone but fail together	Shared mutable state leaking between tests	Reset state in afterEach; avoid module-level variables; use jest.isolateModules()
Snapshot mismatch on first run	No existing snapshot baseline	Run with --updateSnapshot on first execution to create the baseline

Examples

Jest test for a string utility:

import { slugify } from '../src/utils/slugify';

describe('slugify', () => {
  it('converts spaces to hyphens', () => {
    expect(slugify('hello world')).toBe('hello-world');
  });
  it('lowercases all characters', () => {
    expect(slugify('Hello World')).toBe('hello-world');
  });
  it('removes special characters', () => {
    expect(slugify('hello@world!')).toBe('helloworld');
  });
  it('handles empty string', () => {
    expect(slugify('')).toBe('');
  });
  it('trims leading and trailing whitespace', () => {
    expect(slugify('  spaced  ')).toBe('spaced');
  });
});

pytest test for a data validator:

import pytest
from myapp.validators import validate_email

class TestValidateEmail:
    def test_accepts_valid_email(self):
        assert validate_email("user@example.com") is True

    def test_rejects_missing_at_sign(self):
        assert validate_email("userexample.com") is False

    def test_rejects_empty_string(self):
        assert validate_email("") is False

    def test_rejects_none(self):
        with pytest.raises(TypeError):
            validate_email(None)

Resources

• Jest documentation: https://jestjs.io/docs/getting-started
• Vitest documentation: https://vitest.dev/guide/
• pytest documentation: https://docs.pytest.org/
• JUnit 5 User Guide: https://junit.org/junit5/docs/current/user-guide/
• Go testing package: https://pkg.go.dev/testing
• AAA pattern (Arrange-Act-Assert): https://automationpanda.com/2020/07/07/arrange-act-assert-pattern-for-python-unit-tests/