Testing dynamic import functions

[polar dove]

How can I write a jest test to test a dynamic import function that imports an external package thats in node_modules?

[thick rune]

Hi @polar dove. I'm kapa.ai, your friendly support bot. I will try to answer your question by searching through the knowledge base. :wave:

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Testing dynamic imports in Jest can be a bit tricky due to the asynchronous nature of dynamic imports and the way Jest handles module transformations. However, you can use Jest's mocking capabilities to test dynamic imports.

Here's a general approach:

1. Use jest.doMock() or jest.mock() to mock the module you're dynamically importing. This allows you to control the module's behavior in your tests.

2. In your mock, you can return a Promise that resolves to the module (or a mock of the module). This simulates the behavior of a dynamic import.

Here's an example of how you might do this, based on the knowledge sources:

jest.doMock('next/dynamic', () => ({
  __esModule: true,
  default: (loader: any) => {
    const dynamicModule = jest.requireActual('next/dynamic');
    const dynamicActualComp = dynamicModule.default;
    const RequiredComponent = dynamicActualComp(()=>
      loader().then((mod: any) => mod.default || mod)
    );
    if(RequiredComponent?.render?.displayName){
      RequiredComponent.render.displayName = loader.toString();
    }
    RequiredComponent.preload ? RequiredComponent.preload() : RequiredComponent.render.preload();
    return RequiredComponent;
  }
}));

In this example, next/dynamic is being mocked. The mock returns an object that mimics the structure of the actual next/dynamic module. The default property is a function that simulates the behavior of a dynamic import.

Please note that this is a general approach and might need to be adjusted based on the specifics of your code and what you're trying to test.

Also, keep in mind that Jest doesn't transform node_modules by default. If the module you're importing uses syntax that Jest doesn't understand (like ECMAScript Modules syntax), you might need to add a custom transformIgnorePatterns configuration to your Jest config (source (https://github.com/vercel/next.js/issues/52541)).

For more information on Jest's module mocking capabilities, see the Jest documentation (https://jestjs.io/docs/mock-functions).

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