Turbopack: support importing .ts/.tsx via .js extension (parity with webpack resolve.extensionAlias)

Link to the code that reproduces this issue

https://github.com/jeremytenjo/startup-template/tree/convert-to-turbopack

To Reproduce

To Reproduce

Clone the repo above and switch to the convert-to-turbopack branch.

Install and start in dev:

npm i && npm run dev:nextjs

In the app, a file imports a module using a .js extension, e.g.:

// foo.js (import site-wide)
import { something } from "./bar.js";

where bar.ts (or bar.tsx) exists on disk, but no bar.js file.

With Turbopack enabled, the import fails to resolve to bar.ts/bar.tsx.

Note: With webpack, the equivalent works using:

// next.config.js -> webpack config
config.resolve.extensionAlias = {
  ".js": [".ts", ".tsx", ".js"],
};

but I can’t achieve the same behavior with Turbopack.

Current vs. Expected behavior

Current vs. Expected behavior

Current (Turbopack):

• Importing ./bar.js does not resolve to bar.ts/bar.tsx (only attempts real .js), resulting in a module resolution error during development.

Expected:

• Parity with webpack’s resolve.extensionAlias so that imports written with .js seamlessly resolve to .ts/.tsx (and .js) in order:

  • When a file on disk is bar.ts or bar.tsx, import "./bar.js" should resolve successfully to that file.

This is useful for mixed codebases, gradual TS adoption, or when libraries/app code keep .js import specifiers but the implementation is in TS.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8132
  Available memory (MB): 24576
  Available CPU cores: 10
Binaries:
  Node: 20.18.2
  npm: 11.5.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.5.0 // Latest available version is detected (15.5.0).
  eslint-config-next: 13.1.0
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack, Module Resolution

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

• Goal is feature parity with webpack’s config.resolve.extensionAlias = { ".js": [".ts", ".tsx", ".js"] } so codebases can keep .js import specifiers while the implementation lives in TS.

• I tried adjusting resolution-related settings per Turbopack docs (e.g., resolve extensions), but couldn’t achieve aliasing of an extension to multiple extensions.

• Repro is local; no special hosting needed. Tested on macOS (Apple Silicon).

• Happy to try an experimental flag or canary if there’s a hidden knob that enables this in Turbopack.

Thank you!

_PACK-5449

Start with the linked startup-template reproduction, switch to the convert-to-turbopack branch, and run npm i && npm run dev:nextjs. Compare the failing ./bar.js import with the next.config.js webpack resolve.extensionAlias example. Done means Turbopack resolves that specifier to bar.ts, bar.tsx, or bar.js in the stated order.