[Fix] avoid creating string wrapper objects with sloppy mode flatMap …

…mappers

See tc39/proposal-iterator-helpers#281

Iterator.from/implementation.js

@@ -6,8 +6,6 @@ var $TypeError = GetIntrinsic('%TypeError%');
 
 var OrdinaryHasInstance = require('es-abstract/2022/OrdinaryHasInstance');
 var OrdinaryObjectCreate = require('es-abstract/2022/OrdinaryObjectCreate');
-var ToObject = require('es-abstract/2022/ToObject');
-var Type = require('es-abstract/2022/Type');
 
 var GetIteratorFlattenable = require('../aos/GetIteratorFlattenable');
 
@@ -21,22 +19,17 @@ module.exports = function from(O) {
 		throw new $TypeError('`Iterator.from` is not a constructor');
 	}
 
-	if (Type(O) === 'String') {
-		// eslint-disable-next-line no-param-reassign
-		O = ToObject(O); // step 1
-	}
-
-	var iteratorRecord = GetIteratorFlattenable(O); // step 2
+	var iteratorRecord = GetIteratorFlattenable(O, 'iterate-strings'); // step 1
 
-	var hasInstance = OrdinaryHasInstance($Iterator, iteratorRecord['[[Iterator]]']); // step 3
+	var hasInstance = OrdinaryHasInstance($Iterator, iteratorRecord['[[Iterator]]']); // step 2
 
-	if (hasInstance) { // step 4
-		return iteratorRecord['[[Iterator]]']; // step 4.a
+	if (hasInstance) { // step 3
+		return iteratorRecord['[[Iterator]]']; // step 3.a
 	}
 
-	var wrapper = OrdinaryObjectCreate($WrapForValidIteratorPrototype); // , ['[[Iterated]]']); // step 5
+	var wrapper = OrdinaryObjectCreate($WrapForValidIteratorPrototype); // , ['[[Iterated]]']); // step 4
 
-	SLOT.set(wrapper, '[[Iterated]]', iteratorRecord); // step 6
+	SLOT.set(wrapper, '[[Iterated]]', iteratorRecord); // step 5
 
-	return wrapper; // step 7
+	return wrapper; // step 6
 };

Iterator.prototype.flatMap/implementation.js

@@ -80,7 +80,7 @@ module.exports = function flatMap(mapper) {
 			try {
 				var mapped = Call(mapper, void undefined, [value, counter]); // step 5.b.iv
 				// yield mapped // step 5.b.vi
-				innerIterator = GetIteratorFlattenable(mapped); // step 5.b.vi
+				innerIterator = GetIteratorFlattenable(mapped, 'reject-strings'); // step 5.b.vi
 			} catch (e) {
 				innerAlive = false;
 				innerIterator = sentinel;

aos/GetIteratorFlattenable.js

@@ -13,9 +13,11 @@ var Type = require('es-abstract/2022/Type');
 
 var getIteratorMethod = require('es-abstract/helpers/getIteratorMethod');
 
-module.exports = function GetIteratorFlattenable(obj) {
+module.exports = function GetIteratorFlattenable(obj, stringHandling) {
 	if (Type(obj) !== 'Object') {
-		throw new $TypeError('obj must be an Object'); // step 1
+		if (stringHandling === 'reject-strings' || typeof obj !== 'string') {
+			throw new $TypeError('obj must be an Object'); // step 1.a
+		}
 	}
 
 	var method = void undefined; // step 2

package.json

@@ -150,6 +150,7 @@
 		"has-strict-mode": "^1.0.1",
 		"in-publish": "^2.0.1",
 		"iterate-iterator": "^1.0.2",
+		"mock-property": "^1.0.0",
 		"npmignore": "^0.3.0",
 		"nyc": "^10.3.2",
 		"object-inspect": "^1.12.3",

test/Iterator.from.js

@@ -9,6 +9,7 @@ var forEach = require('for-each');
 var debug = require('object-inspect');
 var v = require('es-value-fixtures');
 var hasSymbols = require('has-symbols/shams')();
+var mockProperty = require('mock-property');
 
 var index = require('../Iterator.from');
 var impl = require('../Iterator.from/implementation');
@@ -104,6 +105,27 @@ module.exports = {
 				s2t.end();
 			});
 
+			st.test('observability in a replaced String iterator', function (s2t) {
+				var originalStringIterator = String.prototype[Symbol.iterator];
+				var observedType;
+				s2t.teardown(mockProperty(String.prototype, Symbol.iterator, {
+					get: function () {
+						'use strict'; // eslint-disable-line strict, lines-around-directive
+
+						console.log(this);
+						observedType = typeof this;
+						return originalStringIterator;
+					}
+				}));
+
+				from('');
+				s2t.equal(observedType, 'string', 'string primitive -> primitive receiver in Symbol.iterator getter');
+				from(Object(''));
+				s2t.equal(observedType, 'object', 'boxed string -> boxed string in Symbol.iterator getter');
+
+				s2t.end();
+			});
+
 			st.end();
 		});
 	},