Re: [whatwg/streams] add @@asyncIterator to ReadableStream (#954)

ricea commented on this pull request.



> +    if (IsReadableStreamAsyncIterator(this) === false) {
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
+    }
+    const reader = this._asyncIteratorReader;
+    if (reader._ownerReadableStream === undefined) {
+      return Promise.reject(readerLockException('next'));
+    }
+    return ReadableStreamDefaultReaderRead(reader, true);
+  },
+
+  return(value) {
+    if (IsReadableStreamAsyncIterator(this) === false) {
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
+    }
+    const reader = this._asyncIteratorReader;
+    if (this.preventCancel === false) {

Should be `this._preventCancel`.

>  }
 
+const AsyncIteratorPrototype = Object.getPrototypeOf(Object.getPrototypeOf(async function* () {}).prototype);
+const ReadableStreamAsyncIteratorPrototype = Object.setPrototypeOf({
+  next() {
+    if (IsReadableStreamAsyncIterator(this) === false) {
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
+    }
+    const reader = this._asyncIteratorReader;
+    if (reader._ownerReadableStream === undefined) {
+      return Promise.reject(readerLockException('next'));

Maybe this should be `readerLockException('iterate')`?

"Cannot next a stream using a released reader" doesn't make much sense.

> +    }
+    const reader = this._asyncIteratorReader;
+    if (reader._ownerReadableStream === undefined) {
+      return Promise.reject(readerLockException('next'));
+    }
+    return ReadableStreamDefaultReaderRead(reader, true);
+  },
+
+  return(value) {
+    if (IsReadableStreamAsyncIterator(this) === false) {
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
+    }
+    const reader = this._asyncIteratorReader;
+    if (this.preventCancel === false) {
+      if (reader._ownerReadableStream === undefined) {
+        return Promise.reject(readerLockException('next'));

This should not be `'next'`. Maybe `'finish iterating'` makes sense?

> +
+<emu-alg>
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, return <a>a promise rejected with</a> *TypeError* exception.
+  1. Let _reader_ be *this*.[[asyncIteratorReader]].
+  1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError* exception.
+  1. Return ! ReadableStreamDefaultReaderRead(_reader_, *true*).
+</emu-alg>
+
+<h4 id="rs-asynciterator-prototype-return" method
+for="ReadableStreamAsyncIteratorPrototype">return( <var>value</var> )</h4>
+
+<emu-alg>
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, return <a>a promise rejected with</a> *TypeError* exception.
+  1. Let _reader_ be *this*.[[asyncIteratorReader]].
+  1. If *this*.[[preventCancel]] is *false*, then:
+    1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError*

These two steps are in both branches of the if statement and so should be moved up above it.

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