stream: improve webstream readable async iterator performance

lib/internal/webstreams/readablestream.js

@@ -477,9 +477,13 @@ class ReadableStream {
 
     // eslint-disable-next-line no-use-before-define
     const reader = new ReadableStreamDefaultReader(this);
-    let done = false;
+
+    // No __proto__ here to avoid the performance hit.
+    const state = {
+      done: false,
+      current: undefined,
+    };
     let started = false;
-    let current;
 
     // The nextSteps function is not an async function in order
     // to make it more efficient. Because nextSteps explicitly
@@ -488,7 +492,7 @@ class ReadableStream {
     // unnecessary Promise allocations to occur, which just add
     // cost.
     function nextSteps() {
-      if (done)
+      if (state.done)
         return PromiseResolve({ done: true, value: undefined });
 
       if (reader[kState].stream === undefined) {
@@ -498,31 +502,15 @@ class ReadableStream {
       }
       const promise = createDeferredPromise();
 
-      readableStreamDefaultReaderRead(reader, {
-        [kChunk](chunk) {
-          current = undefined;
-          promise.resolve({ value: chunk, done: false });
-        },
-        [kClose]() {
-          current = undefined;
-          done = true;
-          readableStreamReaderGenericRelease(reader);
-          promise.resolve({ done: true, value: undefined });
-        },
-        [kError](error) {
-          current = undefined;
-          done = true;
-          readableStreamReaderGenericRelease(reader);
-          promise.reject(error);
-        },
-      });
+      // eslint-disable-next-line no-use-before-define
+      readableStreamDefaultReaderRead(reader, new ReadableStreamAsyncIteratorReadRequest(reader, state, promise));
       return promise.promise;
     }
 
     async function returnSteps(value) {
-      if (done)
+      if (state.done)
         return { done: true, value }; // eslint-disable-line node-core/avoid-prototype-pollution
-      done = true;
+      state.done = true;
 
       if (reader[kState].stream === undefined) {
         throw new ERR_INVALID_STATE.TypeError(
@@ -559,19 +547,19 @@ class ReadableStream {
         // need to investigate if it's a bug in our impl or
         // the spec.
         if (!started) {
-          current = PromiseResolve();
+          state.current = PromiseResolve();
           started = true;
         }
-        current = current !== undefined ?
-          PromisePrototypeThen(current, nextSteps, nextSteps) :
+        state.current = state.current !== undefined ?
+          PromisePrototypeThen(state.current, nextSteps, nextSteps) :
           nextSteps();
-        return current;
+        return state.current;
       },
 
       return(error) {
-        return current ?
+        return state.current ?
           PromisePrototypeThen(
-            current,
+            state.current,
             () => returnSteps(error),
             () => returnSteps(error)) :
           returnSteps(error);
@@ -773,6 +761,33 @@ function createReadableStreamBYOBRequest(controller, view) {
   return stream;
 }
 
+class ReadableStreamAsyncIteratorReadRequest {
+  constructor(reader, state, promise) {
+    this.reader = reader;
+    this.state = state;
+    this.promise = promise;
+  }
+
+  [kChunk](chunk) {
+    this.state.current = undefined;
+    this.promise.resolve({ value: chunk, done: false });
+  }
+
+  [kClose]() {
+    this.state.current = undefined;
+    this.state.done = true;
+    readableStreamReaderGenericRelease(this.reader);
+    this.promise.resolve({ done: true, value: undefined });
+  }
+
+  [kError](error) {
+    this.state.current = undefined;
+    this.state.done = true;
+    readableStreamReaderGenericRelease(this.reader);
+    this.promise.reject(error);
+  }
+}
+
 class DefaultReadRequest {
   constructor() {
     this[kState] = createDeferredPromise();