Error: NextRouter was not mounted

The Problem

When using the useRouter hook in Next.js 13 or later, you may encounter the following error:

Error: NextRouter was not mounted

This error can occur in one of two ways:

1. When a component uses the useRouter hook outside of a Next.js application, or when a router is rendered outside of a Next.js application - like during unit testing.
2. When trying to use the useRouter hook from the next/router package when using the App Directory. This is because the App Router (from next/navigation) has a different behavior from the useRouter hook used in pages.

The Solution

Ensure you are using the correct useRouter hook. If you are using the App Directory, check that the useRouter hook is imported from next/navigation and not from next/router.

When using the useRouter hook from next/navigation, use the following guidelines:

• Instead of pathname, use the usePathname() hook.
• Instead of the query object, use the useSearchParams() hook.

Mock next/navigation When Writing Unit Tests

The above error can also occur when using the useRouter hook from next/navigation in unit tests. Use the next-router-mock package, which has built-in support for next/navigation:

jest.mock('next/navigation', () => require('next-router-mock/navigation'));

This provides mocks for useRouter, usePathname, useSearchParams, and useParams. You can set the URL in your tests by calling mockRouter.push() or mockRouter.setCurrentUrl() from next-router-mock.