Jest mocking: TypeError: axios.get.mockResolvedValue is not a function

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06-10-2024

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Mastering Jest Mocking: Troubleshooting "TypeError: axios.get.mockResolvedValue is not a function"

Mocking is a vital technique in unit testing, allowing us to isolate components and control their behavior without relying on external dependencies. However, encountering "TypeError: axios.get.mockResolvedValue is not a function" can be frustrating. Let's dive into this common error and equip you with the knowledge to conquer it.

Scenario:

Imagine you're testing a component that fetches data using the popular axios library. The component might make a GET request to an API endpoint, and you want to ensure it handles the response appropriately.

import React from 'react';
import axios from 'axios';

const MyComponent = () => {
  const [data, setData] = useState([]);

  useEffect(() => {
    const fetchData = async () => {
      const response = await axios.get('https://api.example.com/data');
      setData(response.data);
    };

    fetchData();
  }, []);

  return (
    <div>
      {data.map(item => (
        <p key={item.id}>{item.name}</p>
      ))}
    </div>
  );
};

export default MyComponent;

The Problem:

The TypeError: axios.get.mockResolvedValue is not a function arises when you attempt to mock axios.get using the mockResolvedValue method, but Jest isn't able to recognize axios.get as a function. This happens because axios.get is a function within the axios object, not a standalone function itself.

Solution:

The solution lies in correctly mocking the axios.get function. Here's how:

import axios from 'axios';

jest.mock('axios');

describe('MyComponent', () => {
  it('should fetch data successfully', async () => {
    const mockData = [{ id: 1, name: 'Item 1' }];

    // Mock axios.get to return mock data
    axios.get.mockResolvedValue({ data: mockData });

    // ... rest of the test ...
  });
});

Explanation:

1. Import axios: Import the axios library as usual.
2. Mock axios: Use jest.mock('axios') to mock the entire axios object.
3. Define Mock Data: Create mock data that your component expects to receive.
4. Mock axios.get: Now that axios is mocked, you can directly access axios.get and use mockResolvedValue to specify the return value.

Key Points:

• Mock the whole object: Mocking the entire axios object allows you to mock other functions like axios.post, axios.put, etc., if needed.
• Mock before the test: Ensure you mock axios before executing your test to ensure the mocked version is used.
• Chainable Methods: mockResolvedValue and other mocking methods are often chained together to create complex mock scenarios.

Example:

axios.get.mockResolvedValueOnce({ data: mockData1 }) // Returns mockData1 on the first call
  .mockResolvedValue({ data: mockData2 }); // Returns mockData2 on subsequent calls

Additional Value:

• Error Handling: Implement test cases for error scenarios by using mockRejectedValue to simulate network errors or unexpected server responses.
• Advanced Mocking: For complex scenarios, consider using mockImplementation to define custom logic for your mocked function.

Resources:

• Jest Documentation
• Axios Documentation

By understanding how to correctly mock axios.get, you can confidently test your components that rely on external APIs and ensure they behave as expected in various scenarios.