TypeError: Cannot stub non-existent own property - Demystifying Asynchronous Method Stubbing
Have you encountered the dreaded "TypeError: Cannot stub non-existent own property" error while trying to stub asynchronous methods in your JavaScript tests? This frustrating issue arises when you attempt to stub a method that either doesn't exist or is not an own property of the object being mocked.
Let's break down this problem and explore solutions using a practical example.
The Scenario: Stubbing an Async Method
Imagine you have a function that fetches data from an API asynchronously:
async function fetchData() {
const response = await fetch('https://api.example.com/data');
const data = await response.json();
return data;
}
Now, let's say you want to test a function that relies on fetchData(). You need to stub this function to control its behavior during testing:
const mockFetch = jest.fn().mockResolvedValue({ json: () => Promise.resolve({ data: 'test data' }) });
jest.spyOn(global, 'fetch').mockImplementation(mockFetch);
// ...test code using the fetchData function...
In this scenario, you might encounter the TypeError: Cannot stub non-existent own property error if fetch is not directly defined on the global object (global).
Understanding the Error
This error signifies that the mocking library (e.g., Jest) is unable to find the method you're trying to stub. This could be because:
- The method doesn't exist: The method name you're stubbing is incorrect, or it hasn't been defined on the target object.
- Method is not an own property: The method is inherited from a prototype chain, and the mocking library only works on own properties of the object.
Solutions: Stubbing Async Methods Correctly
Here are some effective ways to address the TypeError: Cannot stub non-existent own property error:
1. Stub the Global fetch Directly:
The most common approach is to stub fetch globally using jest.spyOn:
jest.spyOn(global, 'fetch').mockImplementation(() => Promise.resolve({
json: () => Promise.resolve({ data: 'test data' })
}));
// ...test code using the fetchData function...
This overrides the fetch function globally, allowing you to control its behavior during testing.
2. Mock the Object Directly:
If fetchData() belongs to a specific object, mock that object directly:
const mockFetchData = jest.fn().mockResolvedValue({ data: 'test data' });
// Mock the object containing the fetchData method
const mockedObject = {
fetchData: mockFetchData,
// ...other properties...
};
// ...test code using the mockedObject.fetchData function...
This approach directly stubs the fetchData method within the mocked object.
3. Use the 'mockImplementationOnce' Method:
If you need to stub the method only once in a specific test, use the mockImplementationOnce method:
// ...test code...
jest.spyOn(global, 'fetch').mockImplementationOnce(() => Promise.resolve({
json: () => Promise.resolve({ data: 'test data' })
}));
// ...more test code...
This ensures that the stub is only applied once, preventing conflicts with other tests.
4. Use a Mocking Library with Prototype Support:
For advanced use cases where you need to stub methods inherited from prototypes, consider using mocking libraries that support this functionality. Libraries like jest-mock provide extensive mocking capabilities, including the ability to stub methods across prototype chains.
Best Practices for Asynchronous Method Stubbing
- Be specific: Stub the exact method you need to control, avoiding unnecessary global mocking.
- Maintain clarity: Use descriptive variable names for mocks and stubs to enhance code readability.
- Document your mocks: Add comments explaining the purpose of each mock and how it affects your test.
- Test isolated units: Focus on testing individual units of your application, keeping your mocks and stubs within the scope of the test.
Conclusion
The TypeError: Cannot stub non-existent own property error can be a common pitfall when working with asynchronous methods. By understanding the root cause and employing the correct solutions, you can confidently stub your asynchronous methods and create robust and reliable tests for your JavaScript applications.
