stream: add reduce

doc/api/stream.md

@@ -2142,6 +2142,49 @@ const pairs = await Readable.from(['a', 'b', 'c']).asIndexedPairs().toArray();
 console.log(pairs); // [[0, 'a'], [1, 'b'], [2, 'c']]
 ```
 
+### `readable.reduce(fn[, initial[, options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+* `fn` {Function|AsyncFunction} a reducer function to call over every chunk
+  in the stream.
+  * `previous` {any} the value obtained from the last call to `fn` or the
+    `initial` value if specified or the first chunk of the stream otherwise.
+  * `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.
+* `initial` {any} the initial value to use in the reduction.
+* `options` {Object}
+  * `signal` {AbortSignal} allows destroying the stream if the signal is
+    aborted.
+* Returns: {Promise} a promise for the final value of the reduction.
+
+This method calls `fn` on each chunk of the stream in order, passing it the
+result from the calculation on the previous element. It returns a promise for
+the final value of the reduction.
+
+The reducer function iterates the stream element-by-element which means that
+there is no `concurrency` parameter or parallism. To perform a `reduce`
+concurrently, it can be chained to the [`readable.map`][] method.
+
+If no `initial` value is supplied the first chunk of the stream is used as the
+initial value. If the stream is empty, the promise is rejected with a
+`TypeError` with the `ERR_INVALID_ARGS` code property.
+
+```mjs
+import { Readable } from 'stream';
+
+const ten = await Readable.from([1, 2, 3, 4]).reduce((previous, data) => {
+  return previous + data;
+});
+console.log(ten); // 10
+```
+
 ### Duplex and transform streams
 
 #### Class: `stream.Duplex`

lib/internal/streams/operators.js

@@ -6,6 +6,7 @@ const {
   codes: {
     ERR_INVALID_ARG_TYPE,
     ERR_OUT_OF_RANGE,
+    ERR_MISSING_ARGS,
   },
   AbortError,
 } = require('internal/errors');
@@ -196,8 +197,8 @@ async function every(fn, options) {
       'fn', ['Function', 'AsyncFunction'], fn);
   }
   // https://en.wikipedia.org/wiki/De_Morgan%27s_laws
-  return !(await some.call(this, async (x) => {
-    return !(await fn(x));
+  return !(await some.call(this, async (...args) => {
+    return !(await fn(...args));
   }, options));
 }
 
@@ -228,11 +229,57 @@ async function * filter(fn, options) {
   yield* this.map(filterFn, options);
 }
 
+// Specific to provide better error to reduce since the argument is only
+// missing if the stream has no items in it - but the code is still appropriate
+class ReduceAwareErrMissingArgs extends ERR_MISSING_ARGS {
+  constructor() {
+    super('reduce');
+    this.message = 'Reduce of an empty stream requires an initial value';
+  }
+}
+
+async function reduce(reducer, initialValue, options) {
+  if (typeof reducer !== 'function') {
+    throw new ERR_INVALID_ARG_TYPE(
+      'reducer', ['Function', 'AsyncFunction'], reducer);
+  }
+  let hasInitialValue = arguments.length > 1;
+  if (options?.signal?.aborted) {
+    const err = new AbortError(undefined, { cause: options.signal.reason });
+    this.once('error', () => {}); // The error is already propagated
+    this.destroy(err);
+    throw err;
+  }
+  const ac = new AbortController();
+  const signal = ac.signal;
+  let gotAnyItemFromStream = false;
+  try {
+    for await (const value of this) {
+      gotAnyItemFromStream = true;
+      if (options?.signal?.aborted) {
+        throw new AbortError();
+      }
+      if (!hasInitialValue) {
+        initialValue = value;
+        hasInitialValue = true;
+      } else {
+        initialValue = await reducer(initialValue, value, { signal });
+      }
+    }
+    if (!gotAnyItemFromStream && !hasInitialValue) {
+      throw new ReduceAwareErrMissingArgs();
+    }
+  } finally {
+    ac.abort();
+  }
+  return initialValue;
+}
+
 async function toArray(options) {
   const result = [];
   for await (const val of this) {
     if (options?.signal?.aborted) {
-      throw new AbortError({ cause: options.signal.reason });
+      throw new AbortError(undefined, { cause: options.signal.reason });
     }
     ArrayPrototypePush(result, val);
   }
@@ -307,6 +354,7 @@ module.exports.streamReturningOperators = {
 module.exports.promiseReturningOperators = {
   every,
   forEach,
+  reduce,
   toArray,
   some,
 };

test/parallel/test-stream-reduce.js

@@ -0,0 +1,112 @@
+'use strict';
+
+const common = require('../common');
+const {
+  Readable,
+} = require('stream');
+const assert = require('assert');
+
+function sum(p, c) {
+  return p + c;
+}
+
+{
+  // Does the same thing as `(await stream.toArray()).reduce(...)`
+  (async () => {
+    const tests = [
+      [[], sum, 0],
+      [[1], sum, 0],
+      [[1, 2, 3, 4, 5], sum, 0],
+      [[...Array(100).keys()], sum, 0],
+      [['a', 'b', 'c'], sum, ''],
+      [[1, 2], sum],
+      [[1, 2, 3], (x, y) => y],
+    ];
+    for (const [values, fn, initial] of tests) {
+      const streamReduce = await Readable.from(values)
+                                         .reduce(fn, initial);
+      const arrayReduce = values.reduce(fn, initial);
+      assert.deepStrictEqual(streamReduce, arrayReduce);
+    }
+    // Does the same thing as `(await stream.toArray()).reduce(...)` with an
+    // asynchronous reducer
+    for (const [values, fn, initial] of tests) {
+      const streamReduce = await Readable.from(values)
+                                         .map(async (x) => x)
+                                         .reduce(fn, initial);
+      const arrayReduce = values.reduce(fn, initial);
+      assert.deepStrictEqual(streamReduce, arrayReduce);
+    }
+  })().then(common.mustCall());
+}
+{
+  // Works with an async reducer, with or without initial value
+  (async () => {
+    const six = await Readable.from([1, 2, 3]).reduce(async (p, c) => p + c, 0);
+    assert.strictEqual(six, 6);
+  })().then(common.mustCall());
+  (async () => {
+    const six = await Readable.from([1, 2, 3]).reduce(async (p, c) => p + c);
+    assert.strictEqual(six, 6);
+  })().then(common.mustCall());
+}
+{
+  // Works lazily
+  assert.rejects(Readable.from([1, 2, 3, 4, 5, 6])
+    .map(common.mustCall((x) => {
+      return x;
+    }, 3)) // Two consumed and one buffered by `map` due to default concurrency
+    .reduce(async (p, c) => {
+      if (p === 1) {
+        throw new Error('boom');
+      }
+      return c;
+    }, 0)
+  , /boom/).then(common.mustCall());
+}
+
+{
+  // Support for AbortSignal
+  const ac = new AbortController();
+  assert.rejects(async () => {
+    await Readable.from([1, 2, 3]).reduce(async (p, c) => {
+      if (c === 3) {
+        await new Promise(() => {}); // Explicitly do not pass signal here
+      }
+      return Promise.resolve();
+    }, 0, { signal: ac.signal });
+  }, {
+    name: 'AbortError',
+  }).then(common.mustCall());
+  ac.abort();
+}
+
+
+{
+  // Support for AbortSignal - pre aborted
+  const stream = Readable.from([1, 2, 3]);
+  assert.rejects(async () => {
+    await stream.reduce(async (p, c) => {
+      if (c === 3) {
+        await new Promise(() => {}); // Explicitly do not pass signal here
+      }
+      return Promise.resolve();
+    }, 0, { signal: AbortSignal.abort() });
+  }, {
+    name: 'AbortError',
+  }).then(common.mustCall(() => {
+    assert.strictEqual(stream.destroyed, true);
+  }));
+}
+
+{
+  // Error cases
+  assert.rejects(() => Readable.from([]).reduce(1), /TypeError/);
+  assert.rejects(() => Readable.from([]).reduce('5'), /TypeError/);
+}
+
+{
+  // Test result is a Promise
+  const result = Readable.from([1, 2, 3, 4, 5]).reduce(sum, 0);
+  assert.ok(result instanceof Promise);
+}