Solving the Mysterious “NextJS NextRouter was not mounted in custom NPM package” Error

Are you tired of banging your head against the wall, trying to figure out why your custom NPM package isn’t playing nice with NextJS? Well, you’re not alone! The “NextRouter was not mounted” error can be frustrating, especially when you’ve triple-checked your code and still can’t find the issue. Fear not, dear developer, for today we’ll embark on a journey to diagnose and fix this pesky problem once and for all!

What’s the deal with NextRouter and custom NPM packages?

Before we dive into the solution, let’s quickly understand the context. NextJS is an incredible framework for building server-side rendered (SSR) and statically generated websites. One of its most powerful features is the built-in router, NextRouter. This router enables client-side routing, making your app feel snappy and responsive. However, when you create a custom NPM package to share functionality across multiple projects, things can get a bit hairy.

The Problem: NextRouter was not mounted in custom NPM package

When you try to use NextRouter within a custom NPM package, you might encounter the infamous “NextRouter was not mounted” error. This error occurs because NextRouter relies on the NextJS app instance to function correctly. Since your custom package isn’t part of the main NextJS app, the router doesn’t have a chance to mount properly.

Diagnostic Steps: Identify the Root Cause

Before we jump into the solution, let’s take a step back and ensure we’re tackling the right problem. Follow these diagnostic steps to confirm the issue:

1. Check your package.json file: Verify that your custom NPM package is correctly linked to your NextJS project. Make sure the package is listed as a dependency in the project’s package.json file.

2. Inspect your package’s code: Review your custom package’s code to ensure you’re not accidentally importing NextRouter from outside the NextJS app context.

3. Check for multiple NextJS instances: If you have multiple NextJS instances running (e.g., in separate packages or modules), ensure you’re not creating multiple instances of the router.

4. Verify router usage: Confirm that you’re using the NextRouter correctly within your custom package. Double-check that you’re not trying to access the router before it’s been mounted.

The Fix: Using the withRouter HOC

Now that we’ve ruled out other potential causes, it’s time to employ the trusty withRouter Higher-Order Component (HOC). This HOC is specifically designed to help you use NextRouter within a custom NPM package.

// my-custom-package/index.js
import { withRouter } from 'next/router';

export function MyComponent({ router }) {
  // Now you can use the router within your component
  console.log(router.pathname);
  // ...
}

export default withRouter(MyComponent);

The withRouter HOC injects the NextRouter instance as a prop into your component. This allows you to access the router’s methods and properties, even within a custom package.

Alternative Solution: Using the getAppContext

If, for some reason, the withRouter HOC doesn’t work for you, there’s an alternative solution using the getAppContext function from next/app. This approach requires a bit more boilerplate code, but it achieves the same result.

// my-custom-package/index.js
import { getAppContext } from 'next/app';

export function MyComponent() {
  const appContext = getAppContext();
  const router = appContext.router;

  // Now you can use the router within your component
  console.log(router.pathname);
  // ...
}

In this approach, we’re using the getAppContext function to access the NextJS app context, which includes the router instance.

Common Pitfalls and Best Practices

To avoid headaches in the future, keep the following best practices in mind:

• Avoid importing NextRouter directly: Always use the withRouter HOC or getAppContext to access the router instance.

• Keep your custom package separate: Ensure your custom package doesn’t try to create its own NextJS instance or override the main app’s router.

• Use NextRouter wisely: Only use the router within components that are part of the main NextJS app or within a custom package using the withRouter HOC or getAppContext.

Conclusion: Mastering NextRouter in Custom NPM Packages

With these solutions and best practices, you should now be able to confidently use NextRouter within your custom NPM packages. Remember to diagnose the issue carefully, use the withRouter HOC or getAppContext, and follow the guidelines to avoid common pitfalls.

By applying these strategies, you’ll unlock the full potential of NextJS and create reusable, powerful packages that enhance your development workflow. Happy coding, and may the router be with you!

Common Errors	Solutions
NextRouter was not mounted	Use withRouter HOC or getAppContext
Multiple NextJS instances	Ensure only one NextJS instance is running
Importing NextRouter directly	Avoid direct imports; use withRouter HOC or getAppContext

Final Tip: If you’re still struggling to resolve the issue, try creating a minimal reproduction of the problem and share it with the NextJS community or on platforms like GitHub or Stack Overflow. The collective wisdom of the community can help you identify and fix the problem.

Frequently Asked Question

Got stuck with NextRouter not being mounted in your custom NPM package? Don’t worry, we’ve got you covered! Here are some frequently asked questions to help you troubleshoot the issue.