The async-iterator-helpers shim makes its own AsyncIteratorPrototype rather that enhancing the real one.

[erights]

For the sync iterator-helpers shim, it correctly finds and enhances the existing IteratorPrototype if there.

However, for the async-iterator-helpers shim, it does not succeed at finding the existing one. It therefore makes its own, rather than enhancing the existing one.

This bug causes the SES shim to fail to initialize during lockdown with

TypeError('Conflicting bindings of %AsyncIteratorPrototype%')

(Thus SES does correctly fails safe.) See endojs/endo#1655 (comment)

import 'core-js/actual/iterator/index.js';
import 'core-js/actual/async-iterator/index.js';

const { getPrototypeOf } = Object;

// /////////////////////////////////////////////////////////////////
// See that the synchronous case works, i.e., that it reuses the
// existing IteratorPrototype if there is one.

// Normal JS that also works without core-js

const ArrayIterator = [][Symbol.iterator]();

const ArrayIteratorPrototype = getPrototypeOf(ArrayIterator);

// This is the real one.
const IteratorPrototype = getPrototypeOf(ArrayIteratorPrototype);

// End normal JS

console.log(Iterator.prototype === IteratorPrototype);
// Should return true, and does


// /////////////////////////////////////////////////////////////////
// See that the asynchronous case does not reuse the existing
// AsyncIteratorPrototype if there is one.

// Normal JS that also works without core-js

async function* AsyncGeneratorFunctionInstance() {}
const AsyncGeneratorFunction = getPrototypeOf(
  AsyncGeneratorFunctionInstance,
).constructor;

const AsyncGenerator = AsyncGeneratorFunction.prototype;
const AsyncGeneratorPrototype = AsyncGenerator.prototype;

// This is the real one.
const AsyncIteratorPrototype = getPrototypeOf(AsyncGeneratorPrototype);

// End normal JS

console.log(AsyncIterator.prototype === AsyncIteratorPrototype);
// Should return true, but does return false

[zloirock]

It's not a bug, see this note.

[mhofman]

I've confirmed that USE_FUNCTION_CONSTRUCTOR does work as intended. Thanks for pointing that out.

[zloirock]

I think that it can be closed.