Sergio Xalambrí
August 22, 2019

# Testing in Next.js: Dynamic Imports

Learn how to test components using dynamic imports in a Next.js application.

If you are using Next.js most probable you are also using
`next/dynamic`, their alternative to `React.lazy` which has support for Server-Side Rendering. An amazing feature bundled with Next.js actually.

And if you are trying to test your components, and you should, you will hit against an error because the dynamic import is not supported in your tests.

Let's see a way to solve this and test our code. Let's say we have the following component:

    import dynamic from "next/dynamic";
    const LazyComponent = dynamic(() => import("./lazy-component"));

    function MyComponent() {
      // some logic here
      return (
        <div>
          {/* Some JSX here */}
          {true /* replace with real condition */ && <LazyComponent />}
        </div>
      );
    }

    export default MyComponent;

## Babel Plugin for Dynamic Imports

The first thing you will need is to configure Babel to transpile dynamic imports to something Node.js can understand, to do so we can use the plugin `babel-plugin-transform-dynamic-import`.

    yarn add -D babel-plugin-transform-dynamic-import

Now let's configure it in our `.babelrc` file

    {
      "presets": ["next/babel"],
      "env": {
        "test": {
          "plugins": ["transform-dynamic-import"]
        }
      }
    }

The preset is required to don't lose the default Next.js configuration for Babel, the `env` key let us define plugins or presets based on the `NODE_ENV` environment variable value.

In our case if `NODE_ENV` is equal to `test` it will apply the plugin `babel-plugin-transform-dynamic-import`.

## Mock `next/dynamic` Implementation

Now we need to mock the `next/dynamic` implementation in our test, we can use the jest-next-dynamic package to achieve that.

    yarn add -D jest-next-dynamic

Let's write a simple test for our component.

    import { render, waitForElement } from "@testing-library/react";
    import "@testing-library/jest-dom/extend-expect";

    import MyComponent from "./my-component";

    test("MyComponent", async () => {
      const { getByText } = render(<MyComponent />);
      // fire some events here
      const lazyContent = await waitForElement(() => getByText(/I'm Lazy/));
      expect(lazyContent).toBeInTheDocument();
    });

In our test we are using @testing-library/react to render our `MyComponent`, we wait for an element with the text `I'm Lazy` to appear and, thanks to @testing-library/jest-dom we expect that element to be present in the document.

Now if we run that test it should throw an error, let's fix that too.

    import { render, waitForElement } from "@testing-library/react";
    import "@testing-library/jest-dom/extend-expect";
    import preloadAll from "jest-next-dynamic";

    import MyComponent from "./my-component";

    test("MyComponent", async () => {
      await preloadAll();
      const { getByText } = render(<MyComponent />);
      // fire some events here
      const lazyContent = await waitForElement(() => getByText(/I'm Lazy/));
      expect(lazyContent).toBeInTheDocument();
    });

We are now importing `preloadAll` from `jest-next-dynamic` and using it inside our test before everything else, this will tell `next/dynamic` to preload every dynamic component, when they are all loaded we can render our component and test for the `LazyComponent` to be there.

## Final Words

With this you could write unit and integration tests against components using `next/dynamic` without issues and be sure your application is working as supposed.