Next.js: Return mocked router instead of actual router in useRouter

[JulioJ11]

Closes #30390

What I did

Modified the useRouter export in the Next.js navigation mock to return the mocked navigationAPI instead of the actual Next.js router. This prevents Link components in Next.js 15 from breaking out of the Storybook context when clicked.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Manual testing performed:

• Set up local Storybook development environment with Next.js framework
• Created a test story with Next.js 15 Link component pointing to non-existent route
• Configured story with appDirectory: true to use App Router
• Verified that clicking the Link component now stays within Storybook context instead of navigating to "Not Found" page
• Confirmed that the mocked router is returned when useRouter() is called

To test this fix:

• Run a sandbox for Next.js template: yarn task --task sandbox --template nextjs/default-ts
• Create a story with a Next.js Link component that navigates to a non-existent route
• Set appDirectory: true in story parameters
• Click the Link component - it should now stay within Storybook instead of showing "Not Found"

-->

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

No documentation updates required - this is an internal bug fix that restores expected behavior.

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This pull request has been released as version 0.0.0-pr-32131-sha-cac9c086. Try it out in a new sandbox by running npx storybook@0.0.0-pr-32131-sha-cac9c086 sandbox or in an existing project with npx storybook@0.0.0-pr-32131-sha-cac9c086 upgrade.

More information

Published version	0.0.0-pr-32131-sha-cac9c086
Triggered by	@valentinpalkovic
Repository	JulioJ11/storybook
Branch	fix/nextjs-15-link-component
Commit	cac9c086
Datetime	Mon Aug 11 11:34:50 UTC 2025 (1754912091)
Workflow run	16878848009

To request a new release of this pull request, mention the @storybookjs/core team.

core team members can create a new canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=32131

Greptile Summary

This PR fixes a critical issue in Next.js 15 where Link components would navigate outside of Storybook stories to actual routes, causing "Not Found" errors. The fix modifies the useRouter export in the Next.js navigation mock (code/frameworks/nextjs/src/export-mocks/navigation/index.ts) to return the mocked navigationAPI instead of the actual Next.js router.

Previously, useRouter was implemented as a passthrough mock that called the real Next.js router implementation. This worked in earlier versions but broke with Next.js 15's changes to the Link component behavior. The new implementation returns the mock navigationAPI object when available, ensuring all navigation actions (push, replace, forward, back, prefetch, refresh) stay within the Storybook context.

This change integrates seamlessly with the existing navigation mock system, which already provides the navigationAPI object through the createNavigation function and manages mock lifecycle through getRouter. The fix maintains the same error handling pattern used elsewhere in the mock system by throwing NextjsRouterMocksNotAvailable when mocks aren't initialized.

Confidence score: 4/5

• This fix addresses a clear regression and follows established patterns in the codebase
• The solution is targeted and minimal, changing only the problematic useRouter implementation
• Needs manual testing verification since automated test coverage is incomplete

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☁️ Nx Cloud last updated this comment at 2025-08-11 12:01:14 UTC

[storybook-app-bot]

Package Benchmarks

^{Commit: cac9c08, ran on 11 August 2025 at 12:54:21 UTC}

The following packages have significant changes to their size or dependencies:

@storybook/addon-docs

	Before	After	Difference
Dependency count	12	12	0
Self size	2.47 MB	2.42 MB	🎉 -53 KB 🎉
Dependency size	8.93 MB	8.93 MB	🎉 -14 B 🎉
Bundle Size Analyzer	Link	Link

storybook

	Before	After	Difference
Dependency count	51	51	0
Self size	41.75 MB	41.37 MB	🎉 -387 KB 🎉
Dependency size	18.13 MB	18.13 MB	🎉 -2 B 🎉
Bundle Size Analyzer	Link	Link

sb

	Before	After	Difference
Dependency count	52	52	0
Self size	1 KB	1 KB	🚨 +12 B 🚨
Dependency size	59.89 MB	59.50 MB	🎉 -387 KB 🎉
Bundle Size Analyzer	Link	Link

@storybook/cli

	Before	After	Difference
Dependency count	220	220	0
Self size	585 KB	585 KB	🚨 +13 B 🚨
Dependency size	104.39 MB	103.99 MB	🎉 -394 KB 🎉
Bundle Size Analyzer	Link	Link

@storybook/codemod

	Before	After	Difference
Dependency count	189	189	0
Self size	31 KB	31 KB	🎉 -2 B 🎉
Dependency size	88.46 MB	88.08 MB	🎉 -387 KB 🎉
Bundle Size Analyzer	Link	Link