stream: speed up async iteration of Readable#64447

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Merged
nodejs-github-bot merged 4 commits into from
Aug 14, 2026
+351 −35
Merged

stream: speed up async iteration of Readable #64447

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6f9a2f3
stream: speed up async iteration of Readable
mcollina Jul 11, 2026
14505b2
fixup: address review comments
mcollina Jul 13, 2026
9eb1e60
fixup: remove [SymbolAsyncIterator]
mcollina Jul 15, 2026
873a47e
stream: fix lint in readable async iterator
mcollina Aug 2, 2026
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243 changes: 215 additions & 28 deletions lib/internal/streams/readable.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
ArrayPrototypeIndexOf,
AsyncIteratorPrototype,
FunctionPrototypeCall,
NumberIsInteger,
NumberIsNaN,
NumberParseInt,
ObjectDefineProperties,
ObjectKeys,
ObjectSetPrototypeOf,
Promise,
PromisePrototypeThen,
PromiseReject,
PromiseResolve,
ReflectApply,
SafeSet,
Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
let callback = nop;

function next(resolve) {
let error; // undefined: active, null: ended cleanly, else: Error
let started = false;
let completed = false;
let inFlight = false; // An asynchronous request is outstanding
let queue = null; // Requests received while inFlight
let draining = false;
let cleanup;

// Used both as the 'readable' listener (where `this === stream`) and
// as a promise executor storing the resolver that wakes up a pending
// pump().
function wakeup(resolve) {
if (this === stream) {
callback();
callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
}
}

stream.on('readable', next);
function start() {
started = true;

let error;
const cleanup = eos(stream, { writable: false }, (err) => {
error = err ? aggregateTwoErrors(error, err) : null;
callback();
callback = nop;
});
stream.on('readable', wakeup);

cleanup = eos(stream, { writable: false }, (err) => {
error = err ? aggregateTwoErrors(error, err) : null;
callback();
callback = nop;
});
}

// Complete the iterator and either destroy the stream or detach
// from it.
function finalize() {
completed = true;

try {
while (true) {
const chunk = stream.destroyed ? null : stream.read();
if (chunk !== null) {
yield chunk;
} else if (error) {
throw error;
} else if (error === null) {
return;
} else {
await new Promise(next);
}
}
} catch (err) {
error = aggregateTwoErrors(error, err);
throw error;
} finally {
const preserveHalfOpenDuplex =
error === null &&
stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
) {
destroyImpl.destroyer(stream, null);
} else {
stream.off('readable', next);
stream.off('readable', wakeup);
cleanup();
}
}

function settleError(err, reject) {
error = aggregateTwoErrors(error, err);
finalize();
reject(error);
}

function drain() {
// Requests settled synchronously call back into drain(); the guard
// keeps a single loop going instead of recursing once per request.
if (draining) {
return;
}
draining = true;
try {
while (!inFlight && !queue.isEmpty()) {
const req = queue.shift();
if (req.type === 'next') {
processNext(req.resolve, req.reject);
} else if (req.type === 'return') {
processReturn(req.value, req.resolve);
} else {
processThrow(req.value, req.reject);
}
}
} finally {
draining = false;
}
}

// Thenable chunks are unwrapped before delivery; a rejection tears
// down the iterator and the stream.
function onChunkFulfilled(value) {
inFlight = false;
if (queue !== null) drain();
return { done: false, value };
}

function onChunkRejected(err) {
inFlight = false;
error = aggregateTwoErrors(error, err);
finalize();
if (queue !== null) drain();
throw error;
}

// Runs with inFlight === true; settles the request and hands over to
// any requests that queued up behind it.
function pump(resolve, reject) {
const chunk = stream.destroyed ? null : stream.read();
if (chunk !== null) {
// Read `then` only once so that a getter cannot observe (or throw
// on) a second access.
const then = chunk.then;
if (typeof then === 'function') {
FunctionPrototypeCall(then, chunk, (value) => {
inFlight = false;
resolve({ done: false, value });
if (queue !== null) drain();
}, (err) => {
inFlight = false;
settleError(err, reject);
if (queue !== null) drain();
});
return;
}
inFlight = false;
resolve({ done: false, value: chunk });
if (queue !== null) drain();
} else if (error) {
inFlight = false;
settleError(error, reject);
if (queue !== null) drain();
} else if (error === null) {
inFlight = false;
finalize();
resolve({ done: true, value: undefined });
if (queue !== null) drain();
} else {
// No data buffered yet; wait for 'readable' or end-of-stream and
// retry.
PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
}
}

function processNext(resolve, reject) {
if (completed) {
resolve({ done: true, value: undefined });
return;
}
if (!started) start();
inFlight = true;
pump(resolve, reject);
}

function processReturn(value, resolve) {
if (!completed) {
if (started) {
finalize();
} else {
// Never started: complete without touching the stream.
completed = true;
}
}
resolve({ done: true, value });
}

function processThrow(err, reject) {
if (completed || !started) {
completed = true;
reject(err);
return;
}
settleError(err, reject);
}

return {

@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.

@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)

@Renegade334Renegade334Jul 15, 2026

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The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)

__proto__: AsyncIteratorPrototype,
next() {
if (!inFlight && !completed) {
if (!started) start();
// Fast path: a chunk is already buffered.
const chunk = stream.destroyed ? null : stream.read();
if (chunk !== null) {
// Read `then` only once so that a getter cannot observe (or
// throw on) a second access.
const then = chunk.then;
if (typeof then === 'function') {
inFlight = true;
return FunctionPrototypeCall(
then, chunk, onChunkFulfilled, onChunkRejected);
}
return PromiseResolve({ done: false, value: chunk });
}
if (error) {
finalize();
return PromiseReject(error);
}
if (error === null) {
finalize();
return PromiseResolve({ done: true, value: undefined });
}
// No data buffered yet; wait for 'readable' or end-of-stream.
inFlight = true;
return new Promise((resolve, reject) => {
PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
});
}
return new Promise((resolve, reject) => {
if (inFlight) {
queue ??= new FixedQueue();
queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
} else {
resolve({ done: true, value: undefined });
}
});
},
return(value) {
return new Promise((resolve, reject) => {
if (inFlight) {
queue ??= new FixedQueue();
queue.push({ __proto__: null, type: 'return', value, resolve, reject });
} else {
processReturn(value, resolve);
}
});
},
throw(err) {
return new Promise((resolve, reject) => {
if (inFlight) {
queue ??= new FixedQueue();
queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
} else {
processThrow(err, reject);
}
});
},
};
}

let composeImpl;
Expand Down
23 changes: 16 additions & 7 deletions test/parallel/test-stream-flatMap.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
// Concurrency + AbortSignal
// Two mappers are started concurrently and block until their signal
// is aborted. Aborting while both are in flight must cancel them and
// reject the iteration, without ever starting a third mapper.
const ac = new AbortController();
const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
await setTimeout(100, { signal });
}), { signal: ac.signal, concurrency: 2 });
const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
if (x === 2) {
// Both mappers allowed by `concurrency` are now in flight.
ac.abort();
}
const { promise, reject } = Promise.withResolvers();
if (signal.aborted) {
reject(signal.reason);
}
signal.addEventListener('abort', () => reject(signal.reason), { once: true });
// Promise is expected to reject.
await promise;
}, 2), { signal: ac.signal, concurrency: 2 });
// pump
assert.rejects(async () => {
for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
}, {
name: 'AbortError',
}).then(common.mustCall());

queueMicrotask(() => {
ac.abort();
});
}

{
Expand Down
Loading
Loading
, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

Skip to content
stream: speed up async iteration of Readable#64447

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to your account

 Merged

nodejs-github-bot
merged 4 commits into

from

Aug 14, 2026

+351

−35

 Merged


stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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Contributor
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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
stream: speed up async iteration of Readable#64447

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
Sign in
to your account

 Merged

nodejs-github-bot
merged 4 commits into

from

Aug 14, 2026

+351

−35

 Merged


stream: speed up async iteration of Readable
#64447
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Commits

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4 commits


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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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stream: speed up async iteration of Readable#64447

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merged 4 commits into

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Aug 14, 2026

+351

−35

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stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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Choose a reason for hiding this comment

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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Member
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

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stream: speed up async iteration of Readable#64447

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merged 4 commits into

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Aug 14, 2026

+351

−35

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stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
stream: speed up async iteration of Readable#64447

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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 Merged

nodejs-github-bot
merged 4 commits into

from

Aug 14, 2026

+351

−35

 Merged


stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

Copy link


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Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
stream: speed up async iteration of Readable#64447

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
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to your account

 Merged

nodejs-github-bot
merged 4 commits into

from

Aug 14, 2026

+351

−35

 Merged


stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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merged 4 commits into

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Aug 14, 2026

+351

−35

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stream: speed up async iteration of Readable
#64447
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6f9a2f3

stream: speed up async iteration of Readable


mcollina Jul 11, 2026
14505b2

fixup: address review comments


mcollina Jul 13, 2026
9eb1e60

fixup: remove [SymbolAsyncIterator]


mcollina Jul 15, 2026
873a47e

stream: fix lint in readable async iterator


mcollina Aug 2, 2026

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243 changes: 215 additions & 28 deletions
lib/internal/streams/readable.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -23,13 +23,18 @@

const {
 ArrayPrototypeIndexOf,
 AsyncIteratorPrototype,
 FunctionPrototypeCall,
 NumberIsInteger,
 NumberIsNaN,
 NumberParseInt,
 ObjectDefineProperties,
 ObjectKeys,
 ObjectSetPrototypeOf,
 Promise,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 ReflectApply,
 SafeSet,
 Symbol,
Expand DownExpand Up@@ -100,6 +105,7 @@ const FastBuffer = Buffer[SymbolSpecies];

const { StringDecoder } = require('string_decoder');
const from = require('internal/streams/from');
const FixedQueue = require('internal/fixed_queue');

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
Expand DownExpand Up@@ -1382,10 +1388,22 @@ function streamToAsyncIterator(stream, options) {
 return iter;
}

async function* createAsyncIterator(stream, options) {
// Async iterator over a Readable. Requests received while another is
// outstanding are queued and processed in order.
function createAsyncIterator(stream, options) {
 let callback = nop;

 function next(resolve) {
 let error; // undefined: active, null: ended cleanly, else: Error
 let started = false;
 let completed = false;
 let inFlight = false; // An asynchronous request is outstanding
 let queue = null; // Requests received while inFlight
 let draining = false;
 let cleanup;

 // Used both as the 'readable' listener (where `this === stream`) and
 // as a promise executor storing the resolver that wakes up a pending
 // pump().
 function wakeup(resolve) {
 if (this === stream) {
 callback();
 callback = nop;
Expand All@@ -1394,32 +1412,23 @@ async function* createAsyncIterator(stream, options) {
 }
 }

 stream.on('readable', next);
 function start() {
 started = true;

 let error;
 const cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 stream.on('readable', wakeup);

 cleanup = eos(stream, { writable: false }, (err) => {
 error = err ? aggregateTwoErrors(error, err) : null;
 callback();
 callback = nop;
 });
 }

 // Complete the iterator and either destroy the stream or detach
 // from it.
 function finalize() {
 completed = true;

 try {
 while (true) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 yield chunk;
 } else if (error) {
 throw error;
 } else if (error === null) {
 return;
 } else {
 await new Promise(next);
 }
 }
 } catch (err) {
 error = aggregateTwoErrors(error, err);
 throw error;
 } finally {
 const preserveHalfOpenDuplex =
 error === null &&
 stream.allowHalfOpen === true &&
Expand All@@ -1433,10 +1442,188 @@ async function* createAsyncIterator(stream, options) {
 ) {
 destroyImpl.destroyer(stream, null);
 } else {
 stream.off('readable', next);
 stream.off('readable', wakeup);
 cleanup();
 }
 }

 function settleError(err, reject) {
 error = aggregateTwoErrors(error, err);
 finalize();
 reject(error);
 }

 function drain() {
 // Requests settled synchronously call back into drain(); the guard
 // keeps a single loop going instead of recursing once per request.
 if (draining) {
 return;
 }
 draining = true;
 try {
 while (!inFlight && !queue.isEmpty()) {
 const req = queue.shift();
 if (req.type === 'next') {
 processNext(req.resolve, req.reject);
 } else if (req.type === 'return') {
 processReturn(req.value, req.resolve);
 } else {
 processThrow(req.value, req.reject);
 }
 }
 } finally {
 draining = false;
 }
 }

 // Thenable chunks are unwrapped before delivery; a rejection tears
 // down the iterator and the stream.
 function onChunkFulfilled(value) {
 inFlight = false;
 if (queue !== null) drain();
 return { done: false, value };
 }

 function onChunkRejected(err) {
 inFlight = false;
 error = aggregateTwoErrors(error, err);
 finalize();
 if (queue !== null) drain();
 throw error;
 }

 // Runs with inFlight === true; settles the request and hands over to
 // any requests that queued up behind it.
 function pump(resolve, reject) {
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or throw
 // on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 FunctionPrototypeCall(then, chunk, (value) => {
 inFlight = false;
 resolve({ done: false, value });
 if (queue !== null) drain();
 }, (err) => {
 inFlight = false;
 settleError(err, reject);
 if (queue !== null) drain();
 });
 return;
 }
 inFlight = false;
 resolve({ done: false, value: chunk });
 if (queue !== null) drain();
 } else if (error) {
 inFlight = false;
 settleError(error, reject);
 if (queue !== null) drain();
 } else if (error === null) {
 inFlight = false;
 finalize();
 resolve({ done: true, value: undefined });
 if (queue !== null) drain();
 } else {
 // No data buffered yet; wait for 'readable' or end-of-stream and
 // retry.
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 }
 }

 function processNext(resolve, reject) {
 if (completed) {
 resolve({ done: true, value: undefined });
 return;
 }
 if (!started) start();
 inFlight = true;
 pump(resolve, reject);
 }

 function processReturn(value, resolve) {
 if (!completed) {
 if (started) {
 finalize();
 } else {
 // Never started: complete without touching the stream.
 completed = true;
 }
 }
 resolve({ done: true, value });
 }

 function processThrow(err, reject) {
 if (completed || !started) {
 completed = true;
 reject(err);
 return;
 }
 settleError(err, reject);
 }

 return {
@Renegade334Renegade334Jul 12, 2026

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This object should ideally have a prototype of AsyncIteratorPrototype.
@aduh95aduh95Jul 13, 2026

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I'm not sure it should if the sprecs doesn't require it – it does add a @@asyncDispose method which may or may not be desirable – it also adds a @@asyncIterator, which begs the question whether we should re-implement the method (re-implementing it ourselves means we're not subject to prototype tampering; otherwise, letting the built-in method be inherited would saves a bit of memory maybe?)
@Renegade334Renegade334Jul 15, 2026

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Member
Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

The current implementation derives from AsyncIteratorPrototype (as a generator) so if we're trying to minimise observability then this is a fairly free move. (While the language currently only provides for the two well-known symbol methods, once we get into async iterator helpers territory, there will be significant advantages to keeping this inheritance.)
 __proto__: AsyncIteratorPrototype,
 next() {
 if (!inFlight && !completed) {
 if (!started) start();
 // Fast path: a chunk is already buffered.
 const chunk = stream.destroyed ? null : stream.read();
 if (chunk !== null) {
 // Read `then` only once so that a getter cannot observe (or
 // throw on) a second access.
 const then = chunk.then;
 if (typeof then === 'function') {
 inFlight = true;
 return FunctionPrototypeCall(
 then, chunk, onChunkFulfilled, onChunkRejected);
 }
 return PromiseResolve({ done: false, value: chunk });
 }
 if (error) {
 finalize();
 return PromiseReject(error);
 }
 if (error === null) {
 finalize();
 return PromiseResolve({ done: true, value: undefined });
 }
 // No data buffered yet; wait for 'readable' or end-of-stream.
 inFlight = true;
 return new Promise((resolve, reject) => {
 PromisePrototypeThen(new Promise(wakeup), () => pump(resolve, reject));
 });
 }
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'next', value: undefined, resolve, reject });
 } else {
 resolve({ done: true, value: undefined });
 }
 });
 },
 return(value) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'return', value, resolve, reject });
 } else {
 processReturn(value, resolve);
 }
 });
 },
 throw(err) {
 return new Promise((resolve, reject) => {
 if (inFlight) {
 queue ??= new FixedQueue();
 queue.push({ __proto__: null, type: 'throw', value: err, resolve, reject });
 } else {
 processThrow(err, reject);
 }
 });
 },
 };
}

let composeImpl;
Expand Down

23 changes: 16 additions & 7 deletions
test/parallel/test-stream-flatMap.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -72,10 +72,23 @@ function oneTo5() {

{
 // Concurrency + AbortSignal
 // Two mappers are started concurrently and block until their signal
 // is aborted. Aborting while both are in flight must cancel them and
 // reject the iteration, without ever starting a third mapper.
 const ac = new AbortController();
 const stream = oneTo5().flatMap(common.mustNotCall(async (_, { signal }) => {
 await setTimeout(100, { signal });
 }), { signal: ac.signal, concurrency: 2 });
 const stream = oneTo5().flatMap(common.mustCall(async (x, { signal }) => {
 if (x === 2) {
 // Both mappers allowed by `concurrency` are now in flight.
 ac.abort();
 }
 const { promise, reject } = Promise.withResolvers();
 if (signal.aborted) {
 reject(signal.reason);
 }
 signal.addEventListener('abort', () => reject(signal.reason), { once: true });
 // Promise is expected to reject.
 await promise;
 }, 2), { signal: ac.signal, concurrency: 2 });
 // pump
 assert.rejects(async () => {
 for await (const item of stream) {
Expand All@@ -85,10 +98,6 @@ function oneTo5() {
 }, {
 name: 'AbortError',
 }).then(common.mustCall());

 queueMicrotask(() => {
 ac.abort();
 });
}

{
Expand Down
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