TypeError when constructing subclass of Promise

[chajath]

• Version: 6.5.0
• Platform: Linux
• Subsystem: Promise

I'm writing a library that extends Promise class. In version 6.5.0, I've started to see the errors being thrown when trying to construct the subclass:

     TypeError: #<ChainedPromise> is not a promise
      at new ChainedPromise (src/ChainedPromise.js:68:72)

Here is the source code for ChainedPromise

[bnoordhuis]

You're using import/export syntax so you must have a transpile step like babel. Can you post an example using just built-in syntax with no third-party dependencies?

[chajath]

Actually, when I test with vanilla node 6.5.0 code, it works as expected (as with previous versions)

"use strict";

class subPromise extends Promise {
  constructor(executor) {
    super(executor);
  }
}

const testPromise = new subPromise((resolve, reject) => {
  resolve(42);
});

testPromise.then(console.log);

Even with babel polyfill imported, it works fine. The problem is when I transpile with babel

# with es2015 preset
$ babel subPromise.js > subPromise.babel.js 
$ node subPromise.babel.js

    return _possibleConstructorReturn(this, (subPromise.__proto__ || Object.getPrototypeOf(subPromise)).call(this, executor));

        ^

TypeError: #<subPromise> is not a promise

Curiously this is only happening with 6.5.0 and not previous versions of node. I will also open an issue with babel and see what they think.

[chajath]

It was a known issue from babel when trying to extend built-in classes, with a plugin provided as a workaround. I guess the change was that from Node 6.5.0 Promise starts to behaves a lot more like a built-in class.