Docs: Fix main.js/preset migration note

[JReinhold]

Follow up to #32641

What I did

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!

Documentation

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

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 PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

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

Summary by CodeRabbit

• Documentation
  • Updated migration guide to state that both main configuration and custom presets must be valid ESM.
  • Clarified that extensionless relative imports are no longer supported in JavaScript-based configs; explicit file extensions are required.
  • Added examples for migrating from CJS to ESM and recommended extension practices.
  • Clarified Node.js version requirements and module resolution guidance.
  • Included notes for TypeScript configurations and aligned wording across sections.

[coderabbitai]

📝 Walkthrough

Walkthrough

Documentation updates expand ESM requirements to include Storybook presets, require explicit file extensions in JS-based configs/presets, add CJS-to-ESM migration examples, clarify Node.js and tsconfig requirements, and replace require.resolve examples with import.meta.resolve. Related release notes are updated accordingly.

Changes

Cohort / File(s)	Summary of edits
Docs: Migration ESM and presets guidance MIGRATION.md, docs/releases/migration-guide.mdx	Broadened ESM requirement to include presets; added guidance on explicit extensions for JS-based config/presets; updated examples from CJS to ESM with explicit extensions; noted TS may still allow extensionless imports; clarified Node.js and tsconfig.moduleResolution; switched path resolution examples to use import.meta.resolve; minor text edits.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• Presets: Support extensionless imports in TS-based presets #32641 — Implements support and warnings around extensionless imports in TypeScript-based presets, directly related to the new docs requiring explicit extensions and clarifying TS behavior.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch jeppe/fix-preset-migration-note

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📥 Commits

Reviewing files that changed from the base of the PR and between d944892 and 631ea19.

📒 Files selected for processing (2)

• MIGRATION.md (3 hunks)
• docs/releases/migration-guide.mdx (1 hunks)

🧰 Additional context used

📓 Path-based instructions (1)

docs/**

📄 CodeRabbit inference engine (.github/copilot-instructions.md)

Update documentation under docs/ for significant changes, including migration guides for breaking changes

Files:

• docs/releases/migration-guide.mdx

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• GitHub Check: Core Unit Tests, windows-latest

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☁️ Nx Cloud last updated this comment at 2025-10-15 14:43:23 UTC

[kylegach]

Nice!