stream: add map method to Readable

Implement the map method on readable stream. This starts the alignment
with the tc39-iterator-helpers proposal and adds a `.map` method to
every Node.js readable stream.

Co-Authored-By: Robert Nagy <[email protected]>

PR-URL: #40815
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

doc/api/stream.md

@@ -1682,6 +1682,50 @@ async function showBoth() {
 showBoth();
 ```
 
+### `readable.map(fn[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `fn` {Function|AsyncFunction} a function to map over every item in the stream.
+  * `data` {any} a chunk of data from the stream.
+  * `options` {Object}
+    * `signal` {AbortSignal} aborted if the stream is destroyed allowing to
+      abort the `fn` call early.
+* `options` {Object}
+  * `concurrency` {number} the maximal concurrent invocation of `fn` to call
+    on the stream at once. **Default:** `1`.
+  * `signal` {AbortSignal} allows destroying the stream if the signal is
+    aborted.
+* Returns: {Readable} a stream mapped with the function `fn`.
+
+This method allows mapping over the stream. The `fn` function will be called
+for every item in the stream. If the `fn` function returns a promise - that
+promise will be `await`ed before being passed to the result stream.
+
+```mjs
+import { Readable } from 'stream';
+import { Resolver } from 'dns/promises';
+
+// With a synchronous mapper.
+for await (const item of Readable.from([1, 2, 3, 4]).map((x) => x * 2)) {
+  console.log(item); // 2, 4, 6, 8
+}
+// With an asynchronous mapper, making at most 2 queries at a time.
+const resolver = new Resolver();
+const dnsResults = await Readable.from([
+  'nodejs.org',
+  'openjsf.org',
+  'www.linuxfoundation.org',
+]).map((domain) => resolver.resolve4(domain), { concurrency: 2 });
+for await (const result of dnsResults) {
+  console.log(result); // Logs the DNS result of resolver.resolve4.
+}
+```
+
 ### Duplex and transform streams
 
 #### Class: `stream.Duplex`

lib/internal/streams/operators.js

@@ -0,0 +1,152 @@
+'use strict';
+
+const { AbortController } = require('internal/abort_controller');
+const {
+  codes: {
+    ERR_INVALID_ARG_TYPE,
+  },
+  AbortError,
+} = require('internal/errors');
+const { validateInteger } = require('internal/validators');
+
+const {
+  MathFloor,
+  Promise,
+  PromiseReject,
+  PromisePrototypeCatch,
+  Symbol,
+} = primordials;
+
+const kEmpty = Symbol('kEmpty');
+const kEof = Symbol('kEof');
+
+async function * map(fn, options) {
+  if (typeof fn !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE(
+      'fn', ['Function', 'AsyncFunction'], this);
+  }
+
+  if (options != null && typeof options !== 'object') {
+    throw new ERR_INVALID_ARG_TYPE('options', ['Object']);
+  }
+
+  let concurrency = 1;
+  if (options?.concurrency != null) {
+    concurrency = MathFloor(options.concurrency);
+  }
+
+  validateInteger(concurrency, 'concurrency', 1);
+
+  const ac = new AbortController();
+  const stream = this;
+  const queue = [];
+  const signal = ac.signal;
+  const signalOpt = { signal };
+
+  const abort = () => ac.abort();
+  options?.signal?.addEventListener('abort', abort);
+
+  let next;
+  let resume;
+  let done = false;
+
+  function onDone() {
+    done = true;
+  }
+
+  async function pump() {
+    try {
+      for await (let val of stream) {
+        if (done) {
+          return;
+        }
+
+        if (signal.aborted) {
+          throw new AbortError();
+        }
+
+        try {
+          val = fn(val, signalOpt);
+        } catch (err) {
+          val = PromiseReject(err);
+        }
+
+        if (val === kEmpty) {
+          continue;
+        }
+
+        if (typeof val?.catch === 'function') {
+          val.catch(onDone);
+        }
+
+        queue.push(val);
+        if (next) {
+          next();
+          next = null;
+        }
+
+        if (!done && queue.length && queue.length >= concurrency) {
+          await new Promise((resolve) => {
+            resume = resolve;
+          });
+        }
+      }
+      queue.push(kEof);
+    } catch (err) {
+      const val = PromiseReject(err);
+      PromisePrototypeCatch(val, onDone);
+      queue.push(val);
+    } finally {
+      done = true;
+      if (next) {
+        next();
+        next = null;
+      }
+      options?.signal?.removeEventListener('abort', abort);
+    }
+  }
+
+  pump();
+
+  try {
+    while (true) {
+      while (queue.length > 0) {
+        const val = await queue[0];
+
+        if (val === kEof) {
+          return;
+        }
+
+        if (signal.aborted) {
+          throw new AbortError();
+        }
+
+        if (val !== kEmpty) {
+          yield val;
+        }
+
+        queue.shift();
+        if (resume) {
+          resume();
+          resume = null;
+        }
+      }
+
+      await new Promise((resolve) => {
+        next = resolve;
+      });
+    }
+  } finally {
+    ac.abort();
+
+    done = true;
+    if (resume) {
+      resume();
+      resume = null;
+    }
+  }
+}
+
+module.exports = {
+  map,
+};

lib/stream.js

@@ -23,12 +23,15 @@
 
 const {
   ObjectDefineProperty,
+  ObjectKeys,
+  ReflectApply,
 } = primordials;
 
 const {
   promisify: { custom: customPromisify },
 } = require('internal/util');
 
+const operators = require('internal/streams/operators');
 const compose = require('internal/streams/compose');
 const { pipeline } = require('internal/streams/pipeline');
 const { destroyer } = require('internal/streams/destroy');
@@ -42,6 +45,12 @@ const Stream = module.exports = require('internal/streams/legacy').Stream;
 Stream.isDisturbed = utils.isDisturbed;
 Stream.isErrored = utils.isErrored;
 Stream.Readable = require('internal/streams/readable');
+for (const key of ObjectKeys(operators)) {
+  const op = operators[key];
+  Stream.Readable.prototype[key] = function(...args) {
+    return Stream.Readable.from(ReflectApply(op, this, args));
+  };
+}
 Stream.Writable = require('internal/streams/writable');
 Stream.Duplex = require('internal/streams/duplex');
 Stream.Transform = require('internal/streams/transform');

test/parallel/test-bootstrap-modules.js

@@ -110,6 +110,7 @@ const expectedModules = new Set([
   'NativeModule internal/streams/end-of-stream',
   'NativeModule internal/streams/from',
   'NativeModule internal/streams/legacy',
+  'NativeModule internal/streams/operators',
   'NativeModule internal/streams/passthrough',
   'NativeModule internal/streams/pipeline',
   'NativeModule internal/streams/readable',

test/parallel/test-stream-map.js

@@ -0,0 +1,108 @@
+'use strict';
+
+const common = require('../common');
+const {
+  Readable,
+} = require('stream');
+const assert = require('assert');
+const { setTimeout } = require('timers/promises');
+
+{
+  // Map works on synchronous streams with a synchronous mapper
+  const stream = Readable.from([1, 2, 3, 4, 5]).map((x) => x + x);
+  const result = [2, 4, 6, 8, 10];
+  (async () => {
+    for await (const item of stream) {
+      assert.strictEqual(item, result.shift());
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Map works on synchronous streams with an asynchronous mapper
+  const stream = Readable.from([1, 2, 3, 4, 5]).map(async (x) => {
+    await Promise.resolve();
+    return x + x;
+  });
+  const result = [2, 4, 6, 8, 10];
+  (async () => {
+    for await (const item of stream) {
+      assert.strictEqual(item, result.shift());
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Map works on asynchronous streams with a asynchronous mapper
+  const stream = Readable.from([1, 2, 3, 4, 5]).map(async (x) => {
+    return x + x;
+  }).map((x) => x + x);
+  const result = [4, 8, 12, 16, 20];
+  (async () => {
+    for await (const item of stream) {
+      assert.strictEqual(item, result.shift());
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Concurrency + AbortSignal
+  const ac = new AbortController();
+  let calls = 0;
+  const stream = Readable.from([1, 2, 3, 4, 5]).map(async (_, { signal }) => {
+    calls++;
+    await setTimeout(100, { signal });
+  }, { signal: ac.signal, concurrency: 2 });
+  // pump
+  assert.rejects(async () => {
+    for await (const item of stream) {
+      // nope
+      console.log(item);
+    }
+  }, {
+    name: 'AbortError',
+  }).then(common.mustCall());
+
+  setImmediate(() => {
+    ac.abort();
+    assert.strictEqual(calls, 2);
+  });
+}
+
+{
+  // Concurrency result order
+  const stream = Readable.from([1, 2]).map(async (item, { signal }) => {
+    await setTimeout(10 - item, { signal });
+    return item;
+  }, { concurrency: 2 });
+
+  (async () => {
+    const expected = [1, 2];
+    for await (const item of stream) {
+      assert.strictEqual(item, expected.shift());
+    }
+  })().then(common.mustCall());
+}
+
+{
+  // Error cases
+  assert.rejects(async () => {
+    // eslint-disable-next-line no-unused-vars
+    for await (const unused of Readable.from([1]).map(1));
+  }, /ERR_INVALID_ARG_TYPE/).then(common.mustCall());
+  assert.rejects(async () => {
+    // eslint-disable-next-line no-unused-vars
+    for await (const _ of Readable.from([1]).map((x) => x, {
+      concurrency: 'Foo'
+    }));
+  }, /ERR_OUT_OF_RANGE/).then(common.mustCall());
+  assert.rejects(async () => {
+    // eslint-disable-next-line no-unused-vars
+    for await (const _ of Readable.from([1]).map((x) => x, 1));
+  }, /ERR_INVALID_ARG_TYPE/).then(common.mustCall());
+}
+{
+  // Test result is a Readable
+  const stream = Readable.from([1, 2, 3, 4, 5]).map((x) => x);
+  assert.strictEqual(stream.readable, true);
+}

tools/doc/type-parser.mjs

@@ -203,12 +203,12 @@ const customTypesMap = {
 
   'Stream': 'stream.html#stream',
   'stream.Duplex': 'stream.html#class-streamduplex',
-  'stream.Readable': 'stream.html#class-streamreadable',
-  'stream.Transform': 'stream.html#class-streamtransform',
-  'stream.Writable': 'stream.html#class-streamwritable',
   'Duplex': 'stream.html#class-streamduplex',
+  'stream.Readable': 'stream.html#class-streamreadable',
   'Readable': 'stream.html#class-streamreadable',
+  'stream.Transform': 'stream.html#class-streamtransform',
   'Transform': 'stream.html#class-streamtransform',
+  'stream.Writable': 'stream.html#class-streamwritable',
   'Writable': 'stream.html#class-streamwritable',
 
   'Immediate': 'timers.html#class-immediate',