Generate mocks, stubs, spies, and fakes for dependency isolation. Use when creating mocks, stubs, or test isolation fixtures. Trigger with phrases like "generate mocks", "create test doubles", or "setup stubs".
npxskills add jeremylongshore/claude-code-plugins-plus-skills--skill generating-test-doublesLoading…
Generate mocks, stubs, spies, and fakes to isolate units under test from external dependencies. Supports Jest mocks, Sinon.js stubs, Python unittest.mock, Go interfaces, and testdouble.js patterns.
strict mode enabled for type-safe mocks (if applicable)__mocks__/, , ).test/doubles/testutil/jest.mock(), @patch, constructor injection, or Go interface substitution).tsc --noEmit or equivalent.__mocks__ auto-mock modules where applicable| Error | Cause | Solution |
|---|---|---|
TypeError: X is not a function | Mock missing a method from the real interface | Regenerate the double from the current interface definition; add the missing method |
| Mock leaking between tests | Shared mock state not reset in afterEach | Add jest.restoreAllMocks() or sinon.restore() in teardown hooks |
| Type mismatch on mock return value | Return type does not match interface contract | Use as ReturnType<typeof fn> or update the mock factory to return the correct type |
| Spy not recording calls | Spy created after the function was already bound | Create spies before the module under test imports the dependency |
| Over-mocking hides real bugs | Too many layers replaced with fakes | Limit mocks to true external boundaries (I/O, network); let pure logic run unmocked |
Jest mock factory for a UserRepository:
// __mocks__/userRepository.ts
export const createMockUserRepo = () => ({
findById: jest.fn().mockResolvedValue({ id: '1', name: 'Test User' }),
save: jest.fn().mockResolvedValue(undefined),
delete: jest.fn().mockResolvedValue(true),
});
Python unittest.mock patch for an HTTP client:
from unittest.mock import patch, MagicMock
@patch('myapp.client.requests.get')
def test_fetch_data(mock_get):
mock_get.return_value = MagicMock(status_code=200, json=lambda: {"key": "value"}) # HTTP 200 OK
result = fetch_data("https://api.example.com/data")
assert result == {"key": "value"}
mock_get.assert_called_once()
Go interface-based fake:
type FakeStore struct {
data map[string]string
}
func (f *FakeStore) Get(key string) (string, error) {
v, ok := f.data[key]
if !ok { return "", ErrNotFound }
return v, nil
}
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).