How to Mock ES6 Named Module Imports With Jest

Mocking ES6 named module imports with Jest can be achieved using the jest.mock() function. This function allows you to mock any module, including named imports, to control its behavior during testing. Here's a step-by-step guide on how to do it:

Let's assume you have a module called myModule.js that you want to test, and it looks like this:

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// myModule.js
export const foo = () => {
  // Some implementation
};

export const bar = () => {
  // Some implementation
};

And here's an example of how you can mock the named imports from myModule.js using Jest:

Step 1: Create a __mocks__ folder in the same directory as your test file. Inside this folder, create a file named myModule.js (the same name as the module you want to mock):

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// __mocks__/myModule.js
export const foo = jest.fn(() => "mocked foo");

export const bar = jest.fn(() => "mocked bar");

Step 2: In your test file, use jest.mock() to mock the myModule.js module:

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// myModule.test.js
import { foo, bar } from './myModule'; // This will use the mocked version of myModule.js

// ...your test code that uses foo and bar...

By calling jest.mock('./myModule'), Jest will automatically use the mock implementation from the __mocks__ folder instead of the actual module. This means that all calls to foo and bar in your tests will be using the mocked versions.

Step 3: Write your test cases:

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// myModule.test.js
import { foo, bar } from './myModule';

test('Test foo function', () => {
  // Assert and test the behavior of the foo function using the mocked version
  foo();
  expect(foo).toHaveBeenCalled();
});

test('Test bar function', () => {
  // Assert and test the behavior of the bar function using the mocked version
  const result = bar();
  expect(result).toBe("mocked bar");
});

Now, whenever you run your test suite, Jest will use the mocked versions of foo and bar from the __mocks__ folder.

Keep in mind that if you have other test files that import myModule.js, they will also use the mocked versions since Jest will replace the actual module with the mock automatically during the test run. If you need to use the original module in some test files, you can use jest.unmock() to undo the mocking for specific tests.