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
- 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).
- 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.
- 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.
- 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.
- 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.
- Run the generated tests to verify they pass, then check coverage to confirm the target function is fully exercised.
- 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
1---2name: generating-unit-tests3description: Test automatically generate comprehensive unit tests from source code covering happy paths, edge cases, and error conditions. Use when creating test coverage for functions, classes, or modules. Trigger with phrases like "generate unit tests", "create tests for", or "add test coverage".4license: MIT5---6# Unit Test Generator
7
8## Overview
9
10Automatically 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.
11
12## Prerequisites
13
14- Testing framework installed and configured (`jest`, `vitest`, `pytest`, `junit-jupiter`, or Go `testing`)
15- Source code with clear function signatures, type annotations, or JSDoc comments
16- Test directory structure established (`__tests__/`, `tests/`, `spec/`, or `*_test.go`)
17- Mocking library available (`jest.mock`, `unittest.mock`, `Mockito`, or `gomock`)
18- Package scripts configured to run tests (`npm test`, `pytest`, `go test`)
19
20## Instructions
21
221. 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`).
232. Read each untested source file and extract:
24 - All exported functions and class methods with their signatures.
25 - Parameter types, return types, and thrown exceptions.
26 - External dependencies (imports from other modules, third-party libraries, I/O).
27 - Pure vs. impure function classification.
283. For each function, generate test cases in these categories:
29 - **Happy path**: Valid inputs producing expected outputs (at least 2 cases).
30 - **Edge cases**: Empty strings, empty arrays, zero, negative numbers, `null`, `undefined`, maximum values.
31 - **Error conditions**: Invalid types, missing required fields, network failures, permission errors.
32 - **Boundary values**: Off-by-one, integer overflow, string length limits.
334. Create mock declarations for all external dependencies:
34 - Database calls return predictable fixture data.
35 - HTTP clients return canned responses with configurable status codes.
36 - File system operations use in-memory buffers or temp directories.
375. Write the test file following project conventions:
38 - Match the existing test file naming pattern (`*.test.ts`, `*.spec.js`, `test_*.py`).
39 - Group tests in `describe`/`context` blocks by function name.
40 - Use `beforeEach`/`afterEach` for setup and teardown.
41 - Include inline comments explaining non-obvious test rationale.
426. Run the generated tests to verify they pass, then check coverage to confirm the target function is fully exercised.
437. Report coverage gaps and suggest additional test cases for uncovered branches.
44
45## Output
46
47- Test files placed alongside source files or in the project's test directory
48- Mock/stub files for external dependencies
49- Coverage report showing line, branch, and function coverage for tested modules
50- List of remaining coverage gaps with suggested test cases
51
52## Error Handling
53
54| Error | Cause | Solution |
55|-------|-------|---------|
56| `Cannot find module` on import | Test file path does not match project module resolution | Check `tsconfig.json` paths and `moduleNameMapper` in Jest config |
57| 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 |
58| Async test timeout | Promise never resolves due to missing `await` or unhandled rejection | Add `await` before async calls; increase timeout with `jest.setTimeout()` |
59| Tests pass alone but fail together | Shared mutable state leaking between tests | Reset state in `afterEach`; avoid module-level variables; use `jest.isolateModules()` |
60| Snapshot mismatch on first run | No existing snapshot baseline | Run with `--updateSnapshot` on first execution to create the baseline |
61
62## Examples
63
64**Jest test for a string utility:**
65
66```typescript
67import { slugify } from '../src/utils/slugify';
68
69describe('slugify', () => {
70 it('converts spaces to hyphens', () => {
71 expect(slugify('hello world')).toBe('hello-world');
72 });
73 it('lowercases all characters', () => {
74 expect(slugify('Hello World')).toBe('hello-world');
75 });
76 it('removes special characters', () => {
77 expect(slugify('hello@world!')).toBe('helloworld');
78 });
79 it('handles empty string', () => {
80 expect(slugify('')).toBe('');
81 });
82 it('trims leading and trailing whitespace', () => {
83 expect(slugify(' spaced ')).toBe('spaced');
84 });
85});
86```
87
88**pytest test for a data validator:**
89
90```python
91import pytest
92from myapp.validators import validate_email
93
94class TestValidateEmail:
95 def test_accepts_valid_email(self):
96 assert validate_email("user@example.com") is True
97
98 def test_rejects_missing_at_sign(self):
99 assert validate_email("userexample.com") is False
100
101 def test_rejects_empty_string(self):
102 assert validate_email("") is False
103
104 def test_rejects_none(self):
105 with pytest.raises(TypeError):
106 validate_email(None)
107```
108
109## Resources
110
111- Jest documentation: https://jestjs.io/docs/getting-started
112- Vitest documentation: https://vitest.dev/guide/
113- pytest documentation: https://docs.pytest.org/
114- JUnit 5 User Guide: https://junit.org/junit5/docs/current/user-guide/
115- Go testing package: https://pkg.go.dev/testing
116- AAA pattern (Arrange-Act-Assert):
gabrielmoreira