Add ESM loader support to @kbn/babel-register with regex-based allowlist and fix package.json exports resolution

[Copilot]

Summary

Adds ESM module support to @kbn/babel-register using Node.js 22's module.register() API, creates a shared regex-based ESM allowlist for configuring which packages use ESM vs CommonJS using Babel's overrides feature, and fixes @kbn/import-resolver to handle { default: string } in package.json exports field.

Changes

@kbn/babel-register

• Refactored into separate install_cjs.js and install_esm.js modules for better separation of concerns
• Main install() function now calls both installCJS() and installESM() automatically
• ESM loader resolves imports by appending .ts|.tsx|.js|.jsx extensions and checking for index files
• Transforms loaded files with Babel, respecting existing ignore patterns (node_modules except @kbn)
• Created esm_allowlist.js containing regex patterns for packages that should use ESM (e.g., /kbn-streams-ai-cli/)

// Enable both CJS and ESM transformation
require('@kbn/babel-register').install();

@kbn/babel-preset (node_preset)

• Uses Babel's overrides feature to apply ESM configuration based on regex pattern matching
• Shares the ESM allowlist from @kbn/babel-register/esm_allowlist to ensure consistency
• Automatically configures packages matching allowlist patterns (like @kbn/streams-ai-cli) to output ESM (module: false) instead of CJS (module: 'cjs')
• Leverages Babel's built-in file matching instead of custom logic for cleaner, more maintainable code

@kbn/import-resolver

• Fixed tryExportsResolve() to handle package root exports with { ".": { "default": "./dist/index.js" } } pattern
• When subPathParts is empty, entry is now "." instead of early exit null
• Enables resolution of packages using modern conditional exports syntax

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

Identify risks

• ESM loader requires Node.js 22+ - throws clear error on older versions
• Import resolver change only affects packages using conditional exports with default field - existing behavior preserved for all other cases
• ESM allowlist architecture allows gradual migration - packages are explicitly opted in via regex patterns, default behavior unchanged
• Babel preset changes use standard overrides feature, scoped to allowlisted packages only via regex matching, no impact on existing packages

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• ci-stats.kibana.dev
  • Triggering command: /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node --no-experimental-require-module scripts/kbn bootstrap --quiet (dns block)
  • Triggering command: /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node --no-experimental-require-module scripts/kbn bootstrap (dns block)
  • Triggering command: /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node node scripts/eslint --fix src/platform/packages/private/kbn-import-resolver/src/import_resolver.ts src/platform/packages/shared/kbn-babel-register/index.js src/platform/packages/shared/kbn-babel-register/esm_loader.js (dns block)
• download.cypress.io
  • Triggering command: /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node index.js --exec install (dns block)
• googlechromelabs.github.io
  • Triggering command: /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node /home/REDACTED/.nvm/versions/node/v22.17.1/bin/node install.js (dns block)
• iojs.org
  • Triggering command: /usr/bin/curl curl -q --fail --compressed -L -s REDACTED -o - (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[dgieselaar]

@copilot:

• create two separate files, install_esm.js and install_cjs.js
• make sure kbn-babel-register exports only install(), which calls both installCJS and installESM
• do not add READMEs or tests for existing functionality. Only add tests for new changes. Undo any changes in this PR that do not meet these requirements
• in node_preset, which contains the Babel config, add an override that sets module: false instead of module: cjs, based on an includelist. Add @kbn/streams-ai-cli to this list, this package should use ESM and not CJS.
• consider whether this allowlist should be shared with kbn-babel-register. If so, figure out the right way to do this, without copying files.

[Copilot]

@copilot:

• create two separate files, install_esm.js and install_cjs.js
• make sure kbn-babel-register exports only install(), which calls both installCJS and installESM
  ...

Completed all requested changes in commit 5319e20:

Structural changes:

• Created install_cjs.js and install_esm.js with separate installation logic
• Updated index.js to export only install() which calls both installCJS() and installESM()

ESM allowlist:

• Created esm_allowlist.js with @kbn/streams-ai-cli in the list
• Updated node_preset.js to check the allowlist and set module: false (instead of module: 'cjs') for packages in the list
• The allowlist is shared between kbn-babel-register and kbn-babel-preset via require

Cleanup:

• Removed USAGE_EXAMPLES.md and index.test.js (tests for existing functionality)
• Removed import_resolver.exports.test.ts (test for existing functionality)
• Reverted README changes to original state
• Kept only esm_loader.test.js (tests for new ESM loader functionality)

[dgieselaar]

@copilot Use regexes (e.g. /kbn-streams-ai-cli/) and overrides for the ESM allowlist.

[dgieselaar]

@copilot Use regexes (e.g. /kbn-streams-ai-cli/) and overrides for the ESM allowlist.

[Copilot]

@copilot Use regexes (e.g. /kbn-streams-ai-cli/) and overrides for the ESM allowlist.

Implemented in commit c55b165:

• Changed ESM_ALLOWLIST to use regex patterns: /kbn-streams-ai-cli/
• Updated node_preset.js to use Babel's overrides feature with the regex patterns as the test property
• Removed custom filename checking logic - now leveraging Babel's built-in pattern matching

@copilot Use regexes (e.g. /kbn-streams-ai-cli/) and overrides for the ESM allowlist.

Implemented in commit c55b165:

• Changed ESM_ALLOWLIST to use regex patterns: /kbn-streams-ai-cli/
• Updated node_preset.js to use Babel's overrides feature with the regex patterns as the test property
• Removed custom filename checking logic - now leveraging Babel's built-in pattern matching