Separating constructor and new?

[domenic]

I'm trying to implement whatwg/streams#1035 and running into the issue that we don't have a way to invoke the Web IDL new operation without also invoking the impl constructor.

The impl constructor is what backs the wrapper constructor. But in specs we have two different paths for creating platform objects:

• new PlatformObject() from JS code runs these steps
• Specs will do "new PlatformObject" and then initialize them directly.

We don't currently have a way of doing the latter.

I'll probably investigate with some patches locally in the streams repo, but this issue can track upstreaming them.

[domenic]

This is what I came up with. It does not require any impl-side changes.

exports.webidlNew = (globalObject, typeName, implModule) => {
  if (globalObject[utils.ctorRegistrySymbol] === undefined) {
    throw new Error('Internal error: invalid global object');
  }

  const ctor = globalObject[utils.ctorRegistrySymbol][typeName];
  if (ctor === undefined) {
    throw new Error(`Internal error: constructor ${typeName} is not installed on the passed global object`);
  }

  const obj = Object.create(ctor.prototype);
  Object.defineProperty(obj, utils.implSymbol, {
    value: Object.create(implModule.implementation.prototype),
    configurable: true
  });
  obj[utils.implSymbol][utils.wrapperSymbol] = obj;
  return obj[utils.implSymbol];
};

The arguments would change (to just globalObject, I think) if we exported this from the generated file. And we'd want to do some work to deduplicate the contents of this function with create/createImpl/setup. But it seems to work.

Naming it new is probably reasonable if it's an export, so that you'd use it as MyClass.new(globalObject). I named mine webidlNew because it's just in a utils file, and I wanted to be able to do const { webidlNew } = require("./utils.js").

[TimothyGu]

How do you call the impl constructor after creating an object with webidlNew? Just wondering how this is useful.

[domenic]

You don't; you fill in the slots manually. See for example https://whatpr.org/streams/1035.html#acquire-readable-stream-byob-reader vs. https://whatpr.org/streams/1035.html#default-reader-constructor .