Module resolution behaves differently when typeRoots is specified

[wjhsf]

🔎 Search Terms

typeRoots, module resolution, moduleResolution, bundler

🕗 Version & Regression Information

• This changed between versions 5.0.4 and 5.1.3 (variants with Preserve since 5.4.2)

⏯ Playground Link

No response

💻 Code

No response

🙁 Actual behavior

Setting typeRoots to ./node_modules/@types changes how modules are resolved when using the following compiler options:

• moduleResolution: Node16, module: Node16
• moduleResolution: Node10, module: Preserve
• moduleResolution: Bundler, module: ES2015 | ES2020 | ES2022 | ESNext | Preserve

🙂 Expected behavior

Setting typeRoots to ./node_modules/@types should not change how modules are resolved.

Additional information about the issue

Clone this gist and execute setup.sh to see a minimal reproduction. It sets up a package with various export types, and tries to compile a file with corresponding imports using various module/moduleResolution combinations. Not all imports are expected to work for all combinations, but the errors are expected to be the same regardless of whether typeRoots is specified. If the errors are different, the script will log the errors along with the compiler options used. (No output from the script means there were no mismatched errors.)

Sample output:

{ module: 'Preserve', moduleResolution: 'Bundler' }
=== no typeRoots ===
index.ts(1,22): error TS2307: Cannot find module 'pkg' or its corresponding type declarations.
index.ts(2,30): error TS2307: Cannot find module 'pkg/index' or its corresponding type declarations.
index.ts(3,32): error TS2307: Cannot find module 'pkg/index.js' or its corresponding type declarations.
index.ts(4,32): error TS2307: Cannot find module 'pkg/internal' or its corresponding type declarations.
index.ts(5,34): error TS2307: Cannot find module 'pkg/internal.js' or its corresponding type declarations.
index.ts(6,32): error TS2307: Cannot find module 'pkg/external' or its corresponding type declarations.
index.ts(7,34): error TS2307: Cannot find module 'pkg/external.js' or its corresponding type declarations.
index.ts(8,31): error TS2307: Cannot find module 'pkg/exports' or its corresponding type declarations.

== with typeRoots ==
index.ts(8,31): error TS2307: Cannot find module 'pkg/exports' or its corresponding type declarations.

{ module: 'Preserve', moduleResolution: 'Node10' }
=== no typeRoots ===
index.ts(1,22): error TS2307: Cannot find module 'pkg' or its corresponding type declarations.
index.ts(2,30): error TS2307: Cannot find module 'pkg/index' or its corresponding type declarations.
index.ts(3,32): error TS2307: Cannot find module 'pkg/index.js' or its corresponding type declarations.
index.ts(4,32): error TS2307: Cannot find module 'pkg/internal' or its corresponding type declarations.
index.ts(5,34): error TS2307: Cannot find module 'pkg/internal.js' or its corresponding type declarations.
index.ts(6,32): error TS2307: Cannot find module 'pkg/external' or its corresponding type declarations.
index.ts(7,34): error TS2307: Cannot find module 'pkg/external.js' or its corresponding type declarations.
index.ts(8,31): error TS2307: Cannot find module 'pkg/exports' or its corresponding type declarations.

== with typeRoots ==
index.ts(8,31): error TS2307: Cannot find module 'pkg/exports' or its corresponding type declarations.

[RyanCavanaugh]

Can you specify exactly one config which you think should behave differently? We're also not super keen on running .sh files; a concrete set of flat files which we can copy locally would be preferred.

[wjhsf]

Here's a proper repo you can clone. It's not quite flat because typeRoots needs a directory structure.

This is the tsconfig.json from the repo:

{
  "files": ["index.ts"],
  "compilerOptions": {
    "noEmit": true,

    // When "typeRoots" is not specified, this project has 8 compiler errors.
    // When "typeRoots" is specified, this project has 1 compiler error.
    // "typeRoots": ["./node_modules/@types"],

    "module": "Node16",
    "moduleResolution": "Node16",
    // Preserve/Node10, Preserve/Bundler, and ES2015+/Bundler all have
    // differences in behavior depending on the presence of "typeRoots".
    // "moduleResolution": "Node10",
    // "moduleResolution": "Bundler"
    // "module": "Preserve",
    // "module": "ES2015",
    // "module": "ES2020",
    // "module": "ES2022",
    // "module": "ESNext",
  }
}

---

I also noticed that different combinations of "type": "module" or "type": "commonjs" in the root package.json and in typeRoots/pkg/package.json result in different errors being reported when you have typeRoots set in the tsconfig. That's expected behavior. However, if you don't have typeRoots set, then the errors are always the same. Which seems weird.

Root package	typeRoots package	Errors
cjs	cjs	TS2307
cjs	esm	TS1479, TS1479, TS1479, TS1479, TS1479, TS1479, TS1479, TS2307
esm	cjs	TS2307, TS2307, TS2307, TS2307
esm	esm	TS2307, TS2307, TS2307, TS2307

When typeRoots is not specified, the set of errors is "TS2307, TS2307, TS2307, TS2307, TS2307, TS2307, TS2307, TS2307" in all four scenarios.

---

Perhaps unsurprisingly, I've found that it's the presence of "exports" in the package.json that is causing the issue. When that key is removed, then the behavior of tsc is the same regardless of whether typeRoots is specified.

[andrewbranch]

I think this is a duplicate of #47444. I tried to fix it at #58638 and the breaks looked untenable. It may be worth experimenting with resolving "exports" first and falling back to resolving "types", but I held off since non-type-reference module resolution does not continue with fallbacks if "exports" resolution fails, and keeping that consistency with the spec is a plus.