Bug: An error occurs when invoking `ESLint#lintFiles()` in Jest about eslint

[mdjermanovic]

I can reproduce this locally. Thanks for the detailed analysis of the cause!

https://github.com/StewEucen/eslint-bug-with-jest?tab=readme-ov-file#cause

• Retrier#retry() uses Promise declared in lib.es2015.iterable.d.ts
• fs.readFile() uses Promise declared in lib.es5.d.ts

So it seems that in Node.js different operations may use different Promise constructors?

@nzakas perhaps @humanwhocodes/retry could check if the result is a thenable instead of checking result instanceof Promise?

[nzakas]

I can update retry. Never occurred to me that there might be a promise from another realm involved.

[nzakas]

A new version of @humanwhocodes/retry was released as a patch version that should fix this.

[mdjermanovic]

Here's a PR to update the dependency: #18416

[fasttime]

The Promise object returned by fs.readFile is from a different realm:

This only occurs when the Node.js process is launched with the --experimental-vm-modules flag. It may have something to do with Jest's experimental ESM support, which is enabled by the flag.

[StewEucen]

@mdjermanovic

• Thank you for your confirmation :)

---

So it seems that in Node.js different operations may use different Promise constructors?

• I think that since fs is old package, it uses polyfill Promise class.

---

• I have already found two ways to fix now.

  1. Change argument of #retry() function to async in ESLint repository.

     -                return retrier.retry(() => fs.readFile(...)
     +                return retrier.retry(async () => fs.readFile(...)

  2. Change the logic to confirm Promise in #retry() of @humanwhocodes/retry repository.

     -        if (!(result instanceof Promise)) {
     +        if (
     +          !(result instanceof Promise)
     +          && (!result || typeof result.then !== 'function')
     +        ) {

• I think that 2. is a fixing ad-hoc, thus we can select 1.

[StewEucen]

@fasttime

• Thank you for your information

---

• I used --experimental-vm-modules on run Jest, it was due to the following reasons.

1. I had used ESLint#lintFiles() in Jest with [email protected]. As we know, Jest works as CommonJS. But It did not require to use --experimental-vm-modules flag.

2. I changed the code to fit Flat Config with 9.1.1, but it was not work. Because new code of ESLint#lintFiles() uses dynamic import.

   https://github.com/eslint/eslint/blob/v9.1.1/lib/eslint/eslint.js#L317

   async function loadFlatConfigFile(filePath) {
       ...
   
       const config = (await import(fileURL)).default;
   
       ...
   }

   https://github.com/eslint/eslint/blob/v9.1.1/lib/eslint/eslint.js#L369

   async function calculateConfigArray(eslint, { ... }) {
       ...
   
       if (configFilePath) {
           const fileConfig = await loadFlatConfigFile(configFilePath);
   
           ...
       }
   
       ...
   }

   https://github.com/eslint/eslint/blob/v9.1.1/lib/eslint/eslint.js#L815

   class ESLint {
       ...
   
       async lintFiles(patterns) {
           ...
   
           const configs = await calculateConfigArray(this, eslintOptions);
   
           ...
       }
   
       ...
   }

3. To solve the problem, I added a --experimental-vm-modules flag for Jest script.

   https://github.com/StewEucen/eslint-bug-with-jest/blob/main/package.json#L7

   {
     "scripts": {
       "test": "export NODE_OPTIONS=\"$NODE_OPTIONS --experimental-vm-modules\"; jest --forceExit"
     },
   }

4. And then, I got the behavior that I reported in this issue.

[StewEucen]

@nzakas @mdjermanovic @fasttime

• Thank you for your work related to this issue.
• I am going to use ESLint patched version for my repository.