feat: support tsx as a fallback loader for TypeScript and JSX configs

[alexander-akait]

Summary

Adds tsx as a fallback loader for TypeScript and JSX configuration files (.ts, .tsx, .cts, .mts, .jsx), used when none of the loaders known to interpret (ts-node, sucrase, @babel/register, esbuild-register, @swc/register) are installed.

Closes #4790

The approach in #4790 — appending "tsx/cjs" to interpret's loader tables — cannot work: rechoir resolves loader modules with the legacy resolve@1.x package, which does not support package exports maps — and tsx/cjs exists only in tsx's exports map (there is no physical cjs.js file). rechoir.prepare() therefore always fails to resolve the real package (Cannot find module 'tsx/cjs'); the tests in #4790 only passed because they committed a hand-written fake tsx stub into a fixture node_modules.

Implementation

• The interpret/rechoir flow is untouched and its loaders keep priority.
• When rechoir.prepare() fails for a config with a tsx-loadable extension, the CLI registers tsx itself via createRequire(configFilePath)("tsx/cjs") — Node's own resolver, which understands exports maps and resolves from the user's project. Requiring tsx/cjs installs its require hook, then config loading proceeds as usual.
• .mts, which has no interpret entry at all, gets a best-effort tsx registration; when tsx is missing, the error output is identical to main today (the original import() failure is reported).
• When nothing works, the tsx resolution failure is appended to the existing loader failure list before Please install one of them (first line only, to avoid the noisy require stack).

The tsx registration has no dependency on interpret/rechoir, so it survives their planned future removal unchanged.

Test plan

• New fixture test/build/config-format/typescript-tsx/ exercises the real tsx package (added as a root devDependency) for .ts, .mts and .jsx configs. Because the repository has working interpret loaders installed, the test's beforeAll generates throwing stubs for all of them into the fixture's node_modules (already gitignored — nothing fake is committed), so the assertions can only pass through tsx.
• Negative paths verified manually: with tsx also unavailable, .ts exits 2 listing all loader failures including tsx; .mts reports the original import error exactly as before.
• All 17 config-format suites (37 tests) and all 69 config/extends suites (152 tests) pass; tsc --build, ESLint, Prettier and cspell are clean.
• Changeset included (webpack-cli: minor).

🤖 Generated with Claude Code

https://claude.ai/code/session_01Hv8piVkEGYdWzWoWCtMcif

[changeset-bot]

🦋 Changeset detected

Latest commit: 58a215d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
webpack-cli	Minor

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Click here if you're a maintainer who wants to add another changeset to this PR

[linux-foundation-easycla]

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[codecov]

Codecov Report

❌ Patch coverage is 62.74510% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.30%. Comparing base (3cd7423) to head (58a215d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/webpack-cli/src/webpack-cli.ts	62.74%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4796      +/-   ##
==========================================
- Coverage   93.43%   93.30%   -0.14%     
==========================================
  Files          14       14              
  Lines        5420     5463      +43     
  Branches      788      792       +4     
==========================================
+ Hits         5064     5097      +33     
- Misses        354      364      +10     
  Partials        2        2              

Files with missing lines	Coverage Δ
packages/webpack-cli/src/webpack-cli.ts	96.71% <62.74%> (-0.25%)	⬇️

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[Copilot]

Pull request overview

Adds a tsx-based fallback loader path for TypeScript/JSX webpack config files when interpret/rechoir can’t register any of their known loaders, and introduces a fixture that validates the fallback against the real tsx package.

Changes:

• Add tsx/cjs registration (via createRequire(configFilePath)) as a fallback when rechoir.prepare() fails for TS/JSX config extensions, and as best-effort support for .mts.
• Add a new test/build/config-format/typescript-tsx/ fixture + Jest suite that disables interpret loaders via throwing stubs to force exercising the tsx fallback.
• Add tsx as a root devDependency and include a changeset entry for the feature.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/webpack-cli/src/webpack-cli.ts	Implements the tsx fallback loader registration and integrates it into the config loading flow.
test/build/config-format/typescript-tsx/typescript.test.mjs	New integration tests forcing the tsx fallback by shadowing interpret loaders with throwing stubs.
test/build/config-format/typescript-tsx/webpack.config.ts	TS config fixture consumed by the new test.
test/build/config-format/typescript-tsx/webpack.config.mts	MTS config fixture consumed by the new test.
test/build/config-format/typescript-tsx/webpack.config.jsx	JSX config fixture consumed by the new test.
test/build/config-format/typescript-tsx/main.ts	Minimal entry file for the build output assertions.
test/build/config-format/typescript-tsx/package.json	Fixture-local package settings (CommonJS package type + engine constraint).
package.json	Adds tsx devDependency for running the new fixture against the real package.
package-lock.json	Locks tsx (and its deps) into the dependency graph.
eslint.config.mjs	Ignores the JSX fixture config file for ESLint parsing.
.changeset/tsx-config-loader.md	Declares a minor bump for webpack-cli describing the new fallback behavior.

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