stream: add toArray

Add the toArray method from the TC39 iterator helper proposal to
Readable streams. This also enables a common-use case of converting a
stream to an array.

Co-Authored-By: Robert Nagy <[email protected]>
PR-URL: #41553
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

doc/api/stream.md

@@ -1835,6 +1835,46 @@ await dnsResults.forEach((result) => {
 console.log('done'); // Stream has finished
 ```
 
+### `readable.toArray([options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `options` {Object}
+  * `signal` {AbortSignal} allows cancelling the toArray operation if the
+    signal is aborted.
+* Returns: {Promise} a promise containing an array (if the stream is in
+  object mode) or Buffer with the contents of the stream.
+
+This method allows easily obtaining the contents of a stream. If the
+stream is in [object mode][object-mode] an array of its contents is returned.
+If the stream is not in object mode a Buffer containing its data is returned.
+
+As this method reads the entire stream into memory, it negates the benefits of
+streams. It's intended for interoperability and convenience, not as the primary
+way to consume streams.
+
+```mjs
+import { Readable } from 'stream';
+import { Resolver } from 'dns/promises';
+
+await Readable.from([1, 2, 3, 4]).toArray(); // [1, 2, 3, 4]
+
+// Make dns queries concurrently using .map and collect
+// the results into an aray using toArray
+const dnsResults = await Readable.from([
+  'nodejs.org',
+  'openjsf.org',
+  'www.linuxfoundation.org',
+]).map(async (domain) => {
+  const { address } = await resolver.resolve4(domain, { ttl: true });
+  return address;
+}, { concurrency: 2 }).toArray();
+```
+
 ### Duplex and transform streams
 
 #### Class: `stream.Duplex`

lib/internal/streams/operators.js

@@ -1,6 +1,8 @@
 'use strict';
 
 const { AbortController } = require('internal/abort_controller');
+const { Buffer } = require('buffer');
+
 const {
   codes: {
     ERR_INVALID_ARG_TYPE,
@@ -10,6 +12,7 @@ const {
 const { validateInteger } = require('internal/validators');
 
 const {
+  ArrayPrototypePush,
   MathFloor,
   Promise,
   PromiseReject,
@@ -174,11 +177,25 @@ async function * filter(fn, options) {
   yield* this.map(filterFn, options);
 }
 
+async function toArray(options) {
+  const result = [];
+  for await (const val of this) {
+    if (options?.signal?.aborted) {
+      throw new AbortError({ cause: options.signal.reason });
+    }
+    ArrayPrototypePush(result, val);
+  }
+  if (!this.readableObjectMode) {
+    return Buffer.concat(result);
+  }
+  return result;
+}
 module.exports.streamReturningOperators = {
   filter,
   map,
 };
 
 module.exports.promiseReturningOperators = {
   forEach,
+  toArray,
 };

test/parallel/test-stream-toArray.js

@@ -0,0 +1,79 @@
+'use strict';
+
+const common = require('../common');
+const {
+  Readable,
+} = require('stream');
+const assert = require('assert');
+
+{
+  // Works on a synchronous stream
+  (async () => {
+    const tests = [
+      [],
+      [1],
+      [1, 2, 3],
+      Array(100).fill().map((_, i) => i),
+    ];
+    for (const test of tests) {
+      const stream = Readable.from(test);
+      const result = await stream.toArray();
+      assert.deepStrictEqual(result, test);
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Works on a non-object-mode stream and flattens it
+  (async () => {
+    const stream = Readable.from(
+      [Buffer.from([1, 2, 3]), Buffer.from([4, 5, 6])]
+      , { objectMode: false });
+    const result = await stream.toArray();
+    assert.strictEqual(Buffer.isBuffer(result), true);
+    assert.deepStrictEqual(Array.from(result), [1, 2, 3, 4, 5, 6]);
+  })().then(common.mustCall());
+}
+
+{
+  // Works on an asynchronous stream
+  (async () => {
+    const tests = [
+      [],
+      [1],
+      [1, 2, 3],
+      Array(100).fill().map((_, i) => i),
+    ];
+    for (const test of tests) {
+      const stream = Readable.from(test).map((x) => Promise.resolve(x));
+      const result = await stream.toArray();
+      assert.deepStrictEqual(result, test);
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Support for AbortSignal
+  const ac = new AbortController();
+  let stream;
+  assert.rejects(async () => {
+    stream = Readable.from([1, 2, 3]).map(async (x) => {
+      if (x === 3) {
+        await new Promise(() => {}); // Explicitly do not pass signal here
+      }
+      return Promise.resolve(x);
+    });
+    await stream.toArray({ signal: ac.signal });
+  }, {
+    name: 'AbortError',
+  }).then(common.mustCall(() => {
+    // Only stops toArray, does not destory the stream
+    assert(stream.destroyed, false);
+  }));
+  ac.abort();
+}
+{
+  // Test result is a Promise
+  const result = Readable.from([1, 2, 3, 4, 5]).toArray();
+  assert.strictEqual(result instanceof Promise, true);
+}