Added detection for throwing on undefined initial parameter in Iterator.prototype.reduce

Author: slowcheetah

CHANGELOG.md

@@ -28,6 +28,7 @@
   - `Array.prototype.fromAsync` (due to the lack of async feature detection capability - temporarily, only in own core-js implementation)
   - `AsyncIterator.prototype.from`
   - `Iterator.prototype.toAsync`
+- Added detection for throwing on `undefined` initial parameter in `Iterator.prototype.reduce` (see [WebKit bug](https://bugs.webkit.org/show_bug.cgi?id=291651))
 - Compat data improvements:
   - [Explicit Resource Management](https://github.com/tc39/proposal-explicit-resource-management) features disabled (again) in V8 ~ Chromium 135 and re-added in 136
   - [`RegExp.escape`](https://github.com/tc39/proposal-regex-escaping) marked as [shipped from V8 ~ Chromium 136](https://issues.chromium.org/issues/353856236#comment17)

packages/core-js-compat/src/data.mjs

@@ -711,7 +711,9 @@ export const data = {
   'es.iterator.reduce': {
     // with changes related to the new iteration closing approach on early error
     // https://github.com/tc39/ecma262/pull/3467
-    bun: '1.2.4', // '1.1.31',
+    // due to bug that causes a throw when the initial parameter is `undefined`
+    // https://bugs.webkit.org/show_bug.cgi?id=291651
+    // bun: '1.2.4', // '1.1.31',
     chrome: '135', // '122',
     deno: '2.2.5', // '1.38.1',
     // firefox: '131',

packages/core-js/modules/es.iterator.reduce.js

@@ -12,11 +12,29 @@ var aCallable = require('../internals/a-callable');
 var $TypeError = TypeError;
 var Iterator = globalThis.Iterator;
 var nativeReduce = Iterator && Iterator.prototype && Iterator.prototype.reduce;
-var NATIVE_METHOD_WITHOUT_CLOSING_ON_EARLY_ERROR = nativeReduce && !checkIteratorClosingOnEarlyError(TypeError, nativeReduce, null);
+
+// https://bugs.webkit.org/show_bug.cgi?id=291651
+var nativeMethodFailsOnUndefinedInitialParam = function () {
+  try {
+    // eslint-disable-next-line es/no-iterator-prototype-reduce, es/no-array-prototype-keys -- required for testing
+    [].keys().reduce(function () { return false; }, undefined);
+    return false;
+  } catch (err) {
+    return true;
+  }
+};
+
+var NATIVE_METHOD_WITHOUT_CLOSING_ON_EARLY_ERROR = false;
+var nativeMethodWithoutClosingOnEarlyError = function () {
+  NATIVE_METHOD_WITHOUT_CLOSING_ON_EARLY_ERROR = !checkIteratorClosingOnEarlyError(TypeError, nativeReduce, null);
+  return NATIVE_METHOD_WITHOUT_CLOSING_ON_EARLY_ERROR;
+};
+
+var FORCED = !nativeReduce || nativeMethodFailsOnUndefinedInitialParam() || nativeMethodWithoutClosingOnEarlyError();
 
 // `Iterator.prototype.reduce` method
 // https://tc39.es/ecma262/#sec-iterator.prototype.reduce
-$({ target: 'Iterator', proto: true, real: true, forced: NATIVE_METHOD_WITHOUT_CLOSING_ON_EARLY_ERROR }, {
+$({ target: 'Iterator', proto: true, real: true, forced: FORCED }, {
   reduce: function reduce(reducer /* , initialValue */) {
     anObject(this);
     try {

tests/compat/tests.js

@@ -744,7 +744,12 @@ GLOBAL.tests = {
     return Iterator.from;
   },
   'es.iterator.map': checkIteratorClosingOnEarlyError(TypeError, 'map', null),
-  'es.iterator.reduce': checkIteratorClosingOnEarlyError(TypeError, 'reduce', null),
+  'es.iterator.reduce': [checkIteratorClosingOnEarlyError(TypeError, 'reduce', null), function () {
+    // fails on undefined initial parameter
+    // https://bugs.webkit.org/show_bug.cgi?id=291651
+    [].keys().reduce(function () { /* empty */ }, undefined);
+    return true;
+  }],
   'es.iterator.some': checkIteratorClosingOnEarlyError(TypeError, 'some', null),
   'es.iterator.take': checkIteratorClosingOnEarlyError(RangeError, 'take', -1),
   'es.iterator.to-array': function () {

tests/unit-global/es.iterator.reduce.js

@@ -32,4 +32,6 @@ QUnit.test('Iterator#reduce', assert => {
   const it = createIterator([1], { return() { this.closed = true; } });
   assert.throws(() => reduce.call(it, {}, 1), TypeError);
   assert.true(it.closed, "reduce doesn't close iterator on validation error");
+  assert.notThrows(() => reduce.call(createIterator([]), () => false, undefined), 'fails on undefined initial parameter');
+  assert.same(reduce.call(createIterator([]), () => false, undefined), undefined, 'incorrect result on undefined initial parameter');
 });

tests/unit-pure/es.iterator.reduce.js

@@ -32,4 +32,6 @@ QUnit.test('Iterator#reduce', assert => {
   const it = createIterator([1], { return() { this.closed = true; } });
   assert.throws(() => reduce.call(it, {}, 1), TypeError);
   assert.true(it.closed, "reduce doesn't close iterator on validation error");
+  assert.notThrows(() => reduce.call(createIterator([]), () => false, undefined), 'fails on undefined initial parameter');
+  assert.same(reduce.call(createIterator([]), () => false, undefined), undefined, 'incorrect result on undefined initial parameter');
 });