How to Mock ES6 Module Imports in Jest Using import *?

Mocking ES6 Module Imports in Jest

In Jasmine, mocking an ES6 module import is straightforward. However, in Jest, the process can be more challenging, especially when dealing with modules that utilize default exports. This article explores how to mock ES6 module imports in Jest using a simple import * hack.

Traditionally, mocking an import required replacing the imports with requires, and declaring them inside the tests. This approach can be undesirable.

<code class="javascript">// undesirable approach
import myModule from '../myModule';
import dependency from '../dependency';

describe('myModule', () => {
  // ...
});</code>

Instead, we can leverage import * to mutate the module's exports directly, allowing us to mock the desired function or default export. For named exports:

<code class="javascript">import * as dependency from '../dependency';

// ...

dependency.doSomething = jest.fn(); // mutate the named export
myModule(2);

expect(dependency.doSomething).toBeCalledWith(4);</code>

Similarly, for default exports:

<code class="javascript">import * as dependency from '../dependency';

// ...

dependency.default = jest.fn(); // mutate the default export
myModule(2);

expect(dependency.default).toBeCalledWith(4);</code>

While this method offers a workaround for mocking ES6 imports, it's essential to note that it involves mutating the imported module, which can introduce side effects and potential issues in testing executions. Therefore, using Jest's native mocking methods, such as jest.spyOn or jest.mock, is recommended as the preferred approach.

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