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Discover how to resolve the `ReferenceError: Document is not defined` error in Angular Universal by creating a custom type for the Document injection.
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Handling the ReferenceError: Document is not defined in Angular Universal: A Simple Solution

If you've been working with Angular Universal and come across the frustrating ReferenceError: Document is not defined error, you're not alone. This issue often arises when trying to inject the Document object in modules, particularly when using server-side rendering (SSR) in Angular applications. In this guide, we will explore the issue and present a straightforward solution to get your application running smoothly.

Understanding the Problem

You might encounter this error when attempting to import a module (like OverlayModule shown below) into your AppModule, particularly if that module has a component that leverages the Document object for functionality. Let's illustrate this with an example of an OverlayComponent:

[[See Video to Reveal this Text or Code Snippet]]

After compiling, if you try to import the OverlayModule, you might see the following error:

[[See Video to Reveal this Text or Code Snippet]]

Interestingly, if you simply inject the Document in the AppComponent, the error does not manifest. This leads to questions about whether the issue relates to the order of imports and how they are processed during server-side rendering.

The Solution: Create a Custom Type

Fortunately, there is a straightforward workaround that can help you bypass this issue. By defining a custom type that references the Document, you can avoid the complications associated with direct injection in components used across SSR and client environments. Here's how to implement this fix:

Step 1: Define a Custom Type

Create a custom type that represents the Document type in your component. Here's the modification you need to make in your OverlayComponent:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Adjust Your Component

Update your OverlayComponent to use the newly defined DocumentType. This ensures compatibility and resolves the ReferenceError during server-side execution.

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these steps and incorporating a custom type for the Document, you can effectively resolve the ReferenceError: Document is not defined error in your Angular Universal applications. This approach not only clears up the confusion but also enhances your application’s robustness in multi-environment contexts.

This fix should enable your project to run seamlessly on both the client and server sides. If you continue to experience issues, be sure to check for other potential injection or import-related configurations in your module setup.

If you've found this solution helpful, be sure to share it with your fellow developers who might be facing similar issues in Angular Universal!