Support .js → .ts conversion in local export map

[remcohaszing]

Feature request

Using tsx it’s possible to write import … from './index.js'. It’s ok if index.js doesn’t exist, but only index.ts. It will just resolve to that.

If package.json contains an exports field, it’s valid to import the package by name instead of by a path reference.

For example, after compilation of https://github.com/remcohaszing/remark-mdx-frontmatter, it’s valid to rewrite the compiled test to use import remarkMdxFrontmatter from 'remark-mdx-frontmatter' in test.ts. This won’t work without compiling first.

I suggest to make tsx use the same logic for aliasing .js → .ts after resolving package.json exports as it does for relative imports.

Why?

1. When testing the public interface, tests closer match how a user would use a package.
2. This allows to use tsx for importing other local packages inside a mono repo.

Alternatives

N/A

Additional context

No response

[privatenumber]

Does TypeScript follow this resolution logic?

[remcohaszing]

Does TypeScript follow this resolution logic?

Yes. (I think as of version 4.7)

[privatenumber]

Confirmed this is TypeScript behavior:

index.ts

import test from 'test-package';
console.log(test);

node_modules/test-package/package.json

{ "exports": "./file.js" }

node_modules/test-package/file.ts

export default 1234;

Output

$ npx tsc --noEmit --traceResolution

...

======== Resolving module 'test-package' from '/test-js-ext-ts/index.ts'. ========
Explicitly specified module resolution kind: 'Node16'.
File '/test-js-ext-ts/package.json' exists according to earlier cached lookups.
Loading module 'test-package' from 'node_modules' folder, target file type 'TypeScript'.
Found 'package.json' at '/test-js-ext-ts/node_modules/test-package/package.json'.
'package.json' does not have a 'typesVersions' field.
File name '/test-js-ext-ts/node_modules/test-package/file.js' has a '.js' extension - stripping it.
File '/test-js-ext-ts/node_modules/test-package/file.ts' exist - use it as a name resolution result.
Resolving real path for '/test-js-ext-ts/node_modules/test-package/file.ts', result '/test-js-ext-ts/node_modules/test-package/file.ts'.
======== Module name 'test-package' was successfully resolved to '/test-js-ext-ts/node_modules/test-package/file.ts'. ========

...

[privatenumber]

Duplicate of #341 and fixed via 4503421