Latest change from 1.1.1 breaks Koa 2.x completely.

[rochdev]

More specifically, this change: 5477ff1

Reproduction code:

package.json

{
  "dependencies": {
    "koa": "^2.16.2"
  }
}

index.js

'use strict'

const Koa = require('koa')

const app = new Koa()

app.use(function * handle (next) {
  this.body = 'test'
  yield next
})

app.listen(3000)

This results in a 404.

To fix it, we can add the following to package.json:

{
  "resolutions": {
    "is-generator-function": "1.1.0"
  }
}

Then we get a 200 with the test output.

cc @ljharb

[ljharb]

Can you elaborate on the cause of the 404? Presumably there's an error in an error log somewhere?

[rochdev]

There is no error, it seems like the middleware is just ignored (so effectively the same as not even calling app.use())

[ljharb]

That's very confusing. I assume it's something about koa determining whether the callback is a generator or not. Can you elaborate on your node version, and if you're transpiling, and if so with what version of what?

[rochdev]

No transpilation, the above is the entirety of the code with a simple package.json and a single JS file. I used Node 22.15.0 for my test by it doesn't seem to change anything if I switch to a different version (tried Node 18 and 24 for good measure).

[rochdev]

It looks like this is the only place where the function is used: https://github.com/koajs/koa/blob/v2.x/lib/application.js#L130

Interestingly, the warning is not even displayed to it looks like the function passed to app.use() returns false as-is 🤔

[ljharb]

If isGeneratorFunction is returning a different result for the same function in a different version, that's certainly a bug here.

Can you console.log(fn+'', isGeneratorFunction(fn)) right before that line, with both v1.1.0 and v1.1.1 of this package? That would help me reproduce the issue.

[cayter]

This seems to only happen with ESM projects. I'm on NodeJS 24.8.0.

var getGeneratorFunction = require('generator-function'); // { default: [Getter], 'module.exports': [Getter] }

...

var GeneratorFunction = getGeneratorFunction(); // Throw `TypeError: getGeneratorFunction is not a function`
var GeneratorFunction = getGeneratorFunction.default(); // Works

[ljharb]

What is an "ESM project"?

[cayter]

What is an "ESM project"?

When the project's package.json is using "type": "module",.

[ljharb]

ok, that should only affect that project's parsing of .js files, though - it's not supposed to affect anything else. What extension is the file that has the require call?

[cayter]

ok, that should only affect that project's parsing of .js files, though - it's not supposed to affect anything else. What extension is the file that has the require call?

I'm not sure about OP, but here's the error stack I'm seeing from Koa 2.16 source code:

TypeError: getGeneratorFunction is not a function
      at isGeneratorFunction (node_modules/.pnpm/is-generator-function@1.1.1/node_modules/is-generator-function/index.js:29:26)
      at Application.use (node_modules/.pnpm/koa@2.16.2/node_modules/koa/lib/application.js:130:9)
      at DBOSHttpServer.setupAdminApp (node_modules/.pnpm/@dbos-inc+dbos-sdk@3.5.8/node_modules/@dbos-inc/dbos-sdk/src/httpServer/server.ts:64:14)
      at DBOS.launch (node_modules/.pnpm/@dbos-inc+dbos-sdk@3.5.8/node_modules/@dbos-inc/dbos-sdk/src/dbos.ts:315:39)
      at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
      at async Worker.start (<project>/app/.server/primitives/workflow/worker.ts:51:4)
      at async Promise.all (index 1)
      at async Object.run (<project>/app/.server/commands/start.ts:62:5)
      at async CLI.run (<project>/app/.server/primitives/cli.ts:105:4)
      at async main (<project>/app/.server/main.ts:265:2)

The root cause is ESM/CJS interop issue where require('generator-function') doesn't properly resolve in an ESM project context.

Ideally, for a library to support both CJS/ESM, this is what the library's package.json needs to do (tedious as we'd need to use transpiler):

{
    "type": "module",
    "exports": {
      ".": {
        "import": "./dist/index.mjs",
        "require": "./dist/index.cjs"
      }
    }
}

A not-so-good workaround though:

let getGeneratorFunction;
try {
  getGeneratorFunction = require('generator-function');
} catch {
  getGeneratorFunction = await import('generator-function').then(m => m.default);
}

[rochdev]

My reproduction snippet above isn't ESM though, and Koa is also not using ESM.

[rochdev]

Since Koa simply calls the function directly with the middleware, I figured I'd try without Koa and get the same result.

'use strict'

const isGeneratorFunction = require('is-generator-function')

console.log(isGeneratorFunction(function * handle (next) {
  this.body = 'test'
  yield next
}))

This outputs false in 1.1.1 but true in 1.1.0.

[cayter]

My reproduction snippet above isn't ESM though, and Koa is also not using ESM.

Oh yeah, just realized we're 2 different issues. The latest commit updates to use require('generator-function') package which breaks for:

1. ESM projects due to the CJS/ESM interop issue.
2. Koa 2.x projects due to generator-function is probably buggy which returns false when it should return true.

[ljharb]

The first one is confusing, because there shouldn't be any interop issue that I'm aware of, but a repro where require and import differ across node versions would be helpful.

For the second one, in which environment does it return false when it should return true?