Ensure _.flattenDeep works on arrays with circular references. [closes #1664]

lodash.js

@@ -2489,9 +2489,10 @@
      * @param {boolean} [isDeep] Specify a deep flatten.
      * @param {boolean} [isStrict] Restrict flattening to arrays-like objects.
      * @param {Array} [result=[]] The initial result value.
+     * @param {Object} [stack] Tracks traversed arrays.
      * @returns {Array} Returns the new flattened array.
      */
-    function baseFlatten(array, isDeep, isStrict, result) {
+    function baseFlatten(array, isDeep, isStrict, result, stack) {
       result || (result = []);
 
       var index = -1,
@@ -2502,9 +2503,18 @@
         if (isArrayLikeObject(value) &&
             (isStrict || isArray(value) || isArguments(value))) {
           if (isDeep) {
+            stack || (stack = new Stack);
+            if (stack.get(array)) {
+              return result;
+            }
+            stack.set(array, true);
+
             // Recursively flatten arrays (susceptible to call stack limits).
-            baseFlatten(value, isDeep, isStrict, result);
+            baseFlatten(value, isDeep, isStrict, result, stack);
-          } else {
+
+            stack['delete'](array);
+          }
+          else {
             arrayPush(result, value);
           }
         } else if (!isStrict) {

test/test.js

@@ -5081,6 +5081,32 @@
 
   /*--------------------------------------------------------------------------*/
 
+  QUnit.module('lodash.flattenDeep');
+
+  (function() {
+    QUnit.test('should flatten arrays with circular references', function(assert) {
+      assert.expect(2);
+
+      var array = [1, 2];
+      array.push(array);
+
+      var expected = [1, 2, 1, 2];
+
+      try {
+        var actual = _.flattenDeep(array);
+      } catch (e) {}
+
+      assert.deepEqual(actual, expected);
+
+      array = [1, 2];
+      array = [array, [array]];
+
+      assert.deepEqual(_.flattenDeep(array), expected);
+    });
+  }());
+
+  /*--------------------------------------------------------------------------*/
+
   QUnit.module('flatten methods');
 
   (function() {