Ensure _.isEqual works with maps and sets with circular references.

lodash.js

@@ -2895,8 +2895,9 @@
           othIsObj = othTag == objectTag && !isHostObject(other),
           isSameTag = objTag == othTag;
 
+      stack || (stack = new Stack);
       if (isSameTag && !(objIsArr || objIsObj)) {
-        return equalByTag(object, other, objTag, equalFunc, customizer, bitmask);
+        return equalByTag(object, other, objTag, equalFunc, customizer, bitmask, stack);
       }
       var isPartial = bitmask & PARTIAL_COMPARE_FLAG;
       if (!isPartial) {
@@ -2910,7 +2911,6 @@
       if (!isSameTag) {
         return false;
       }
-      stack || (stack = new Stack);
       return (objIsArr ? equalArrays : equalObjects)(object, other, equalFunc, customizer, bitmask, stack);
     }
 
@@ -4704,9 +4704,9 @@
      * @param {Array} array The array to compare.
      * @param {Array} other The other array to compare.
      * @param {Function} equalFunc The function to determine equivalents of values.
-     * @param {Function} [customizer] The function to customize comparisons.
-     * @param {number} [bitmask] The bitmask of comparison flags. See `baseIsEqual` for more details.
-     * @param {Object} [stack] Tracks traversed `array` and `other` objects.
+     * @param {Function} customizer The function to customize comparisons.
+     * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual` for more details.
+     * @param {Object} stack Tracks traversed `array` and `other` objects.
      * @returns {boolean} Returns `true` if the arrays are equivalent, else `false`.
      */
     function equalArrays(array, other, equalFunc, customizer, bitmask, stack) {
@@ -4773,11 +4773,12 @@
      * @param {Object} other The other object to compare.
      * @param {string} tag The `toStringTag` of the objects to compare.
      * @param {Function} equalFunc The function to determine equivalents of values.
-     * @param {Function} [customizer] The function to customize comparisons.
-     * @param {number} [bitmask] The bitmask of comparison flags. See `baseIsEqual` for more details.
+     * @param {Function} customizer The function to customize comparisons.
+     * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual` for more details.
+     * @param {Object} stack Tracks traversed `object` and `other` objects.
      * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
      */
-    function equalByTag(object, other, tag, equalFunc, customizer, bitmask) {
+    function equalByTag(object, other, tag, equalFunc, customizer, bitmask, stack) {
       switch (tag) {
         case arrayBufferTag:
           if ((object.byteLength != other.byteLength) ||
@@ -4812,9 +4813,18 @@
           var isPartial = bitmask & PARTIAL_COMPARE_FLAG;
           convert || (convert = setToArray);
 
+          if (object.size != other.size && !isPartial) {
+            return false;
+          }
+          // Assume cyclic values are equal.
+          var stacked = stack.get(object);
+          if (stacked) {
+            return stacked == other;
+          }
+          stack.set(object, other);
+
           // Recursively compare objects (susceptible to call stack limits).
-          return (isPartial || object.size == other.size) &&
-            equalFunc(convert(object), convert(other), customizer, bitmask | UNORDERED_COMPARE_FLAG);
+          return equalArrays(convert(object), convert(other), equalFunc, customizer, bitmask | UNORDERED_COMPARE_FLAG, stack);
 
         case symbolTag:
           return !!Symbol && (symbolValueOf.call(object) == symbolValueOf.call(other));
@@ -4830,9 +4840,9 @@
      * @param {Object} object The object to compare.
      * @param {Object} other The other object to compare.
      * @param {Function} equalFunc The function to determine equivalents of values.
-     * @param {Function} [customizer] The function to customize comparisons.
-     * @param {number} [bitmask] The bitmask of comparison flags. See `baseIsEqual` for more details.
-     * @param {Object} [stack] Tracks traversed `object` and `other` objects.
+     * @param {Function} customizer The function to customize comparisons.
+     * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual` for more details.
+     * @param {Object} stack Tracks traversed `object` and `other` objects.
      * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
      */
     function equalObjects(object, other, equalFunc, customizer, bitmask, stack) {

test/test.js

@@ -9090,6 +9090,26 @@
       }
     });
 
+    QUnit.test('should compare maps with circular references', function(assert) {
+      assert.expect(2);
+
+      if (Map) {
+        var map1 = new Map,
+            map2 = new Map;
+
+        map1.set('a', map1);
+        map2.set('a', map2);
+        assert.strictEqual(_.isEqual(map1, map2), true);
+
+        map1.set('b', 1);
+        map2.set('b', 2);
+        assert.strictEqual(_.isEqual(map1, map2), false);
+      }
+      else {
+        skipAssert(assert, 2);
+      }
+    });
+
     QUnit.test('should compare regexes', function(assert) {
       assert.expect(5);
 
@@ -9127,6 +9147,26 @@
       }
     });
 
+    QUnit.test('should compare sets with circular references', function(assert) {
+      assert.expect(2);
+
+      if (Set) {
+        var set1 = new Set,
+            set2 = new Set;
+
+        set1.add(set1);
+        set2.add(set2);
+        assert.strictEqual(_.isEqual(set1, set2), true);
+
+        set1.add(1);
+        set2.add(2);
+        assert.strictEqual(_.isEqual(set1, set2), false);
+      }
+      else {
+        skipAssert(assert, 2);
+      }
+    });
+
     QUnit.test('should compare typed arrays', function(assert) {
       assert.expect(1);