The Problem:
You’re receiving an error “mailgunLib is not a function” when running your Jest tests for code that uses the mailgun-js library. This is because mailgun-js exports a constructor function, and your current Jest mock isn’t correctly mimicking this constructor pattern. Your tests are trying to use the library as a function, but the mock is providing an object instead.
Understanding the “Why” (The Root Cause):
The mailgun-js library uses a constructor function. This means that to create a mailgun client object, you must call it like a function, passing in your configuration:
const mailgunLib = require("mailgun-js");
const mailgunClient = mailgunLib({apiKey: process.env.MAILGUN_KEY, domain: EMAIL_DOMAIN});
Your mock, however, exports a plain object. When your test code attempts to call mailgunLib as a function, it fails because it’s not a function, it’s an object literal. Jest needs a mock function that, when called, returns the object with the necessary methods (like .messages().send()).
Step-by-Step Guide:
- Correct the Mock: Replace your existing
__mocks__/mailgun-js.js file with the following corrected mock:
module.exports = jest.fn().mockImplementation(() => ({
messages: () => ({
send: jest.fn().mockResolvedValue({})
})
}));
This creates a mock function using jest.fn().mockImplementation(). The mockImplementation allows you to define what the mock function should return when called. In this case, it returns an object with a messages method, which itself returns an object containing a send method. The send method is also mocked using jest.fn().mockResolvedValue({}). This ensures that when send is called, it returns a resolved promise (as the actual mailgun-js send method is asynchronous), preventing test failures due to asynchronous expectations.
- Verify the Mock: In your test file, you can now verify that your mock is working correctly. For example:
const mailgunLib = require("mailgun-js"); //This now uses your mock.
// ... your test code ...
expect(mailgunLib().messages().send).toHaveBeenCalledWith(emailData); //Verifying that send is called with correct arguments.
- Optional: Handle Asynchronous Operations: The above mock returns an empty object for
send. For more realistic testing you may want to return a specific value indicating success or failure:
module.exports = jest.fn().mockImplementation(() => ({
messages: () => ({
send: jest.fn().mockResolvedValue({ id: 'test-message-id', message: 'Email sent successfully!' }) //More realistic response.
})
}));
This allows you to check the id of the “sent” email (or an error message) in your tests and ensure the correct response has been handled.
Common Pitfalls & What to Check Next:
- Incorrect Mock Location: Double-check that your
__mocks__/mailgun-js.js file is in the correct directory relative to your test file. Jest automatically finds mocks in this standard directory structure.
- Module Resolution: Verify that Jest is correctly resolving your mock. Jest should prioritize this mock over the actual
mailgun-js module during testing. If you are using a custom module resolution or a very complex project setup, your tests may not be picking up the mock correctly.
- Asynchronous Testing: If your
sendRegistrationEmail function handles errors or promise rejections, ensure your tests cover these scenarios using Jest’s asynchronous testing capabilities (e.g., async/await or .then()/.catch()).
Still running into issues? Share your (sanitized) config files, the exact command you ran, and any other relevant details. The community is here to help!