Now that we have a module set up with our mocked function it’s time to use it within our Jest test for findRecipe(). Remember, we want to replace the actual apiRequest() function with the mocked one we created!

Let’s take a look at the steps to use a function that we’ve created in our __mocks__/ directory in our tests.

1. First, in the test file, we import the real function. This allows the test to work as it would otherwise if no mock existed.

2. Then use the jest.mock() function provided by Jest to override the value with the mocked one defined in the __mocks__/ folder. jest.mock() accepts a string as an argument that should match the filepath to the file being mocked.

Note: This second step is only required when mocking local modules. When mocking modules installed into the node_modules directory, the module will automatically be mocked when it is imported in step 1.

// import the actual module
import apiRequest from './api-request.js';

// then tell Jest to use the mocked version!
jest.mock('./api-request.js');

Following these steps will cause any apiRequest() function calls made in the Jest tests to use the mocked version instead.

We can also further manipulate how specific methods in the mocked module behave by using special methods attached to functions mocked with jest.fn(). One of these methods is mockFunction.mockResolvedValueOnce(), which accepts a value that the next call to mockFunction() will resolve to.

// file: __tests__/recipes.js

import { findRecipe } from './recipes.js'; 

// import the actual module
import apiRequest from './api-request.js';

// then tell Jest to use the mocked version!
jest.mock('./api-request.js');

test("get the full recipe for a dish", async () => {
  // arrange  
  const dish = "Pesto";
  const expectedValue = { "Magical Deliciousness": "3 cups" };

  // set the resolved value for the next call to apiRequest  
  const mockResponse = {
    status: "mock",
    data: { "Magical Deliciousness": "3 cups" }
  }
  apiRequest.mockResolvedValueOnce(mockResponse);

  // act  
  const actualRecipe = await findRecipe(dish);

  // assertion
  expect(actualRecipe).toEqual(expectedValue);
});

In the example above, the .mockResolvedValueOnce() method is used to determine what the next call to the apiRequest() function will resolve to. This method may be called any number of times in a test and may be useful if you’d like to finely control what each call to your mocked function resolves to.

There are many other methods available for controlling mocked functions. To see a full list of these methods, check out the Jest documentation.