doc: piping from async generators using pipeline()

PR-URL: https://github.com/nodejs/node/pull/33992
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

1 files changed, 20 insertions, 67 deletions

diff --git a/doc/api/stream.md b/doc/api/stream.md
index b195458cec9..16252e13ec3 100644
--- a/doc/api/stream.md
+++ b/doc/api/stream.md
@@ -2886,80 +2886,33 @@ readable.on('data', (chunk) => {
 
 #### Piping to writable streams from async iterators
 
-In the scenario of writing to a writable stream from an async iterator, ensure
-the correct handling of backpressure and errors.
+When writing to a writable stream from an async iterator, ensure correct
+handling of backpressure and errors. [`stream.pipeline()`][] abstracts away
+the handling of backpressure and backpressure-related errors:
 
 ```js
-const { once } = require('events');
-const finished = util.promisify(stream.finished);
+const { pipeline } = require('stream');
+const util = require('util');
+const fs = require('fs');
 
 const writable = fs.createWriteStream('./file');
 
-function drain(writable) {
-  if (writable.destroyed) {
-    return Promise.reject(new Error('premature close'));
-  }
-  return Promise.race([
-    once(writable, 'drain'),
-    once(writable, 'close')
-      .then(() => Promise.reject(new Error('premature close')))
-  ]);
-}
-
-async function pump(iterable, writable) {
-  for await (const chunk of iterable) {
-    // Handle backpressure on write().
-    if (!writable.write(chunk)) {
-      await drain(writable);
-    }
+// Callback Pattern
+pipeline(iterator, writable, (err, value) => {
+  if (err) {
+    console.error(err);
+  } else {
+    console.log(value, 'value returned');
   }
-  writable.end();
-}
-
-(async function() {
-  // Ensure completion without errors.
-  await Promise.all([
-    pump(iterable, writable),
-    finished(writable)
-  ]);
-})();
-```
-
-In the above, errors on `write()` would be caught and thrown by the
-`once()` listener for the `'drain'` event, since `once()` will also handle the
-`'error'` event. To ensure completion of the write stream without errors,
-it is safer to use the `finished()` method as above, instead of using the
-`once()` listener for the `'finish'` event. Under certain cases, an `'error'`
-event could be emitted by the writable stream after `'finish'` and as `once()`
-will release the `'error'` handler on handling the `'finish'` event, it could
-result in an unhandled error.
-
-Alternatively, the readable stream could be wrapped with `Readable.from()` and
-then piped via `.pipe()`:
-
-```js
-const finished = util.promisify(stream.finished);
-
-const writable = fs.createWriteStream('./file');
-
-(async function() {
-  const readable = Readable.from(iterable);
-  readable.pipe(writable);
-  // Ensure completion without errors.
-  await finished(writable);
-})();
-```
-
-Or, using `stream.pipeline()` to pipe streams:
-
-```js
-const pipeline = util.promisify(stream.pipeline);
-
-const writable = fs.createWriteStream('./file');
+});
 
-(async function() {
-  await pipeline(iterable, writable);
-})();
+// Promise Pattern
+const pipelinePromise = util.promisify(pipeline);
+pipelinePromise(iterator, writable)
+  .then((value) => {
+    console.log(value, 'value returned');
+  })
+  .catch(console.error);
 ```
 
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