Jest: Mocking Conditional Function Calls

I am trying to write a test to make sure that, when appropriate, a particular function (in this case, a sentry function) is called with a particular message. However, when I write

Solution 1:

As @balzee mentioned, you have to actually spy on the method you want to make assertions about. That causes Jest to replace the method with a special spy function that keeps track of the parameters it is called with, how many times it was called, and so on.

You should also provide a mock implementation for that function, so that you don't actually call out to Sentry when running your unit tests.

Finally, when spying on a method, you first pass the object the method is on, and then the name of the method as a string. Jest then replaces that property on the object with a spy function, that will call the original function if no mock implementation is given.

Without referencing the object the function exists on, you would just be changing what a local function variable points to, from the original/real function to a jest spy function. That won't change the function that the code you are testing invokes, so the test will fail.

So the final test should be:

handleError.test.js:

import * as sentry from'@sentry/browser'; // CHANGEDimport handleError from'../handleError';

classApiErrorextendsError {
  constructor() {
    super();
    this.name = 'ApiError';
  }
}

// added this to remove any spies/mocks after the testafterEach(() => {
  jest.restoreAllMocks();
});

test('When an ApiError is returned with no action type, sentry is notified', () => {
  const sampleError = newApiError();
  // added next line
  jest.spyOn(sentry, 'captureMessage').mockImplementation(() => {});
  handleError(sampleError);
  // note the use of `sentry.captureMessage`, which is now a jest spy fnexpect(sentry.captureMessage).toHaveBeenCalledWith('this is an error message.');
});