re-use IteratorResult objects when possible in Iterator.concat

tc39/proposal-iterator-sequencing#17
tc39/proposal-iterator-sequencing#18

Author: zloirock

CHANGELOG.md

@@ -4,6 +4,8 @@
   - Added built-ins:
     - `Error.isError`
   - We have no bulletproof way to polyfill this method / check if the object is an error, so it's an enough naive implementation that is marked as `.sham`
+- Added [`Iterator` sequencing stage 2.7 proposal](https://github.com/tc39/proposal-iterator-sequencing):
+  - Re-use `IteratorResult` objects when possible, [tc39/proposal-iterator-sequencing/17](https://github.com/tc39/proposal-iterator-sequencing/issues/17), [tc39/proposal-iterator-sequencing/18](https://github.com/tc39/proposal-iterator-sequencing/pull/18), December 2024 TC39 meeting
 - Optimized `DataView.prototype.{ getFloat16, setFloat16 }` performance, [#1379](https://github.com/zloirock/core-js/pull/1379), thanks [**@LeviPesin**](https://github.com/LeviPesin)
 - Dropped unneeded feature detection of non-standard `%TypedArray%.prototype.toSpliced`
 - Dropped possible re-usage of some non-standard / early stage features (like `Math.scale`) available on global

packages/core-js/internals/iterator-create-proxy.js

@@ -22,12 +22,13 @@ var createIteratorProxyPrototype = function (IS_ITERATOR) {
     next: function next() {
       var state = getInternalState(this);
       // for simplification:
-      //   for `%WrapForValidIteratorPrototype%.next` our `nextHandler` returns `IterResultObject`
+      //   for `%WrapForValidIteratorPrototype%.next` or with `state.returnHandlerResult` our `nextHandler` returns `IterResultObject`
       //   for `%IteratorHelperPrototype%.next` - just a value
       if (IS_ITERATOR) return state.nextHandler();
+      if (state.done) return createIterResultObject(undefined, true);
       try {
-        var result = state.done ? undefined : state.nextHandler();
-        return createIterResultObject(result, state.done);
+        var result = state.nextHandler();
+        return state.returnHandlerResult ? result : createIterResultObject(result, state.done);
       } catch (error) {
         state.done = true;
         throw error;
@@ -57,13 +58,14 @@ var IteratorHelperPrototype = createIteratorProxyPrototype(false);
 
 createNonEnumerableProperty(IteratorHelperPrototype, TO_STRING_TAG, 'Iterator Helper');
 
-module.exports = function (nextHandler, IS_ITERATOR) {
+module.exports = function (nextHandler, IS_ITERATOR, RETURN_HANDLER_RESULT) {
   var IteratorProxy = function Iterator(record, state) {
     if (state) {
       state.iterator = record.iterator;
       state.next = record.next;
     } else state = record;
     state.type = IS_ITERATOR ? WRAP_FOR_VALID_ITERATOR : ITERATOR_HELPER;
+    state.returnHandlerResult = !!RETURN_HANDLER_RESULT;
     state.nextHandler = nextHandler;
     state.counter = 0;
     state.done = false;

packages/core-js/modules/esnext.iterator.concat.js

@@ -5,6 +5,7 @@ var aCallable = require('../internals/a-callable');
 var anObject = require('../internals/an-object');
 var getIteratorMethod = require('../internals/get-iterator-method');
 var createIteratorProxy = require('../internals/iterator-create-proxy');
+var createIterResultObject = require('../internals/create-iter-result-object');
 
 var $Array = Array;
 
@@ -16,7 +17,7 @@ var IteratorProxy = createIteratorProxy(function () {
       var iterables = this.iterables;
       if (iterableIndex >= iterables.length) {
         this.done = true;
-        return;
+        return createIterResultObject(undefined, true);
       }
       var entry = iterables[iterableIndex];
       this.iterables[iterableIndex] = null;
@@ -29,9 +30,9 @@ var IteratorProxy = createIteratorProxy(function () {
       this.next = null;
       continue;
     }
-    return result.value;
+    return result;
   }
-});
+}, false, true);
 
 // `Iterator.concat` method
 // https://github.com/tc39/proposal-iterator-sequencing

tests/unit-global/esnext.iterator.concat.js

@@ -62,6 +62,27 @@ QUnit.test('Iterator.concat', assert => {
   assert.deepEqual(iterator.return(), { done: true, value: undefined }, '.return with active inner iterator with return result');
   assert.true(called, 'inner .return called');
 
+  // https://github.com/tc39/proposal-iterator-sequencing/issues/17
+  const oldIterResult = {
+    done: false,
+    value: 123,
+  };
+  const testIterator = {
+    next() {
+      return oldIterResult;
+    },
+  };
+  const iterable = {
+    [Symbol.iterator]() {
+      return testIterator;
+    },
+  };
+  iterator = concat(iterable);
+  const iterResult = iterator.next();
+  assert.same(iterResult.done, false);
+  assert.same(iterResult.value, 123);
+  assert.same(iterResult, oldIterResult);
+
   assert.throws(() => concat(createIterator([1, 2, 3])), TypeError, 'non-iterable iterator #1');
   assert.throws(() => concat([], createIterator([1, 2, 3])), TypeError, 'non-iterable iterator #2');
   assert.throws(() => concat(''), TypeError, 'iterable non-object argument #1');

tests/unit-pure/esnext.iterator.concat.js

@@ -1,7 +1,8 @@
 import { createIterable, createIterator } from '../helpers/helpers.js';
 
 import concat from 'core-js-pure/full/iterator/concat';
-import Iterator from 'core-js-pure/full/iterator';
+import Iterator from 'core-js-pure/es/iterator';
+import Symbol from 'core-js-pure/es/symbol';
 import from from 'core-js-pure/es/array/from';
 
 QUnit.test('Iterator.concat', assert => {
@@ -60,6 +61,27 @@ QUnit.test('Iterator.concat', assert => {
   assert.deepEqual(iterator.return(), { done: true, value: undefined }, '.return with active inner iterator with return result');
   assert.true(called, 'inner .return called');
 
+  // https://github.com/tc39/proposal-iterator-sequencing/issues/17
+  const oldIterResult = {
+    done: false,
+    value: 123,
+  };
+  const testIterator = {
+    next() {
+      return oldIterResult;
+    },
+  };
+  const iterable = {
+    [Symbol.iterator]() {
+      return testIterator;
+    },
+  };
+  iterator = concat(iterable);
+  const iterResult = iterator.next();
+  assert.same(iterResult.done, false);
+  assert.same(iterResult.value, 123);
+  assert.same(iterResult, oldIterResult);
+
   assert.throws(() => concat(createIterator([1, 2, 3])), TypeError, 'non-iterable iterator #1');
   assert.throws(() => concat([], createIterator([1, 2, 3])), TypeError, 'non-iterable iterator #2');
   assert.throws(() => concat(''), TypeError, 'iterable non-object argument #1');