transpileModule does not account for package.json type when ran with node16/nodenext module setting

[romkhub]

Bug Report

🔎 Search Terms

transpileModule node16 nodenext

🕗 Version & Regression Information

Checked with

@4.9.4
@beta - 5.0.0-beta
@next - 5.1.0-dev.20230227

It's only about node16/nodenext and therefore doesn't affect TS versions before 4.7

⏯ Playground Link

https://github.com/romkhub/ts-transpile-node16

The repo illustrates how tsc provides ESM, while transpileModule gives CJS upon the same setup.
After debugging I see it related to this code which, as I understand, essentially hides all the files except the one transpiled.

My real-world scenario is an attempt to use node16 TS setup in a repo built with webpack and ts-loader in transpileOnly mode. With transpileOnly: true I get CJS, transpileOnly: false - ESM

🙁 Actual behavior

transpileModule does not recognize "type": "module" in package.json and emit CJS, while tsc emits ESM

🙂 Expected behavior

JS emitted by transpileModule is the same as of tsc

[mrazauskas]

Hitting the same problem. Seems like the cause of it is the following:

• a custom compilerHost is created inside of transpileModule()
• the fileExists() method of the host is only able to check existence of the input file path:

  TypeScript/src/services/transpile.ts

  Lines 108 to 109 in 27d3454

  	fileExists: (fileName): boolean => fileName === inputFileName,
  	readFile: () => "",

• the host is passed to getImpliedNodeFormatForFile() function, which is responsible to find nearest package.json and to look at its the "type" field
• this job is done by getPackageJsonInfo() function, which calls host.fileExists() on each of potential packageJsonPaths:

  TypeScript/src/compiler/moduleNameResolver.ts

  Line 2263 in 27d3454

  if (directoryExists && host.fileExists(packageJsonPath)) {

Unfortunately the host cannot know about package.json files, because it is only able to check the existence of the input file path.

---

Might be this is expected behaviour. For instance, if the transpileModule() function is intended to work with virtual files only and it shouldn’t know anything about the real file system.

[mrazauskas]

Here is a patch of the compilerHost which makes "module": "node16" compiler option work as expected:

- fileExists: (fileName): boolean => fileName === inputFileName,
- readFile: () => "",
+ fileExists: (fileName) => fileName.endsWith("package.json") ? sys.fileExists(fileName) : fileName === inputFileName,
+ readFile: (fileName) => fileName.endsWith("package.json") ? sys.readFile(fileName) : "",

[RyanCavanaugh]

I'll wait for @rbuckton to make a final call, but my impression is that the entire point of transpileModule is to be a text-only transform process. It shouldn't attempt to hit the disk at all; it especially cannot call sys functions.

[rbuckton]

I concur with @RyanCavanaugh. transpileModule shouldn't interact with the file system at all. If you want it to treat the input file as ESM, you would need to provide a fileName to the transpileOptions argument with a .mts (or .mtsx) extension. If that isn't sufficient, we could consider adding an option to TranspileOptions to override the implied node format.