Avoid object mutation in getRawTag. [closes #2755]

lodash.js

@@ -5956,15 +5956,21 @@
      * @returns {string} Returns the raw `toStringTag`.
      */
     function getRawTag(value) {
+      var isOwn = hasOwnProperty.call(value, symToStringTag),
+          tag = value[symToStringTag];
+
       try {
-        var symbol = value[symToStringTag];
         value[symToStringTag] = undefined;
-      } catch (e) {
+        var isSet = true;
-        symbol = undefined;
+      } catch (e) {}
-      }
+
       var result = nativeObjectToString.call(value);
-      if (symbol) {
+      if (isSet) {
-        value[symToStringTag] = symbol;
+        if (isOwn) {
+          value[symToStringTag] = tag;
+        } else {
+          delete value[symToStringTag];
+        }
       }
       return result;
     }

test/test.js

@@ -11360,7 +11360,13 @@
       assert.strictEqual(_.isPlainObject(object), true);
     });
 
-    QUnit.test('should return `true` for objects with a `Symbol.toStringTag` property', function(assert) {
+    QUnit.test('should return `true` for objects with a `valueOf` property', function(assert) {
+      assert.expect(1);
+
+      assert.strictEqual(_.isPlainObject({ 'valueOf': 0 }), true);
+    });
+
+    QUnit.test('should return `true` for objects with a writable `Symbol.toStringTag` property', function(assert) {
       assert.expect(1);
 
       if (Symbol && Symbol.toStringTag) {
@@ -11374,12 +11380,6 @@
       }
     });
 
-    QUnit.test('should return `true` for objects with a `valueOf` property', function(assert) {
-      assert.expect(1);
-
-      assert.strictEqual(_.isPlainObject({ 'valueOf': 0 }), true);
-    });
-
     QUnit.test('should return `false` for objects with a custom `[[Prototype]]`', function(assert) {
       assert.expect(1);
 
@@ -11387,25 +11387,6 @@
       assert.strictEqual(_.isPlainObject(object), false);
     });
 
-    QUnit.test('should return `false` for objects with a read-only `Symbol.toStringTag` property', function(assert) {
-      assert.expect(1);
-
-      if (Symbol && Symbol.toStringTag) {
-        var object = {};
-        defineProperty(object, Symbol.toStringTag, {
-          'configurable': true,
-          'enumerable': false,
-          'writable': false,
-          'value': 'X'
-        });
-
-        assert.deepEqual(_.isPlainObject(object), false);
-      }
-      else {
-        skipAssert(assert);
-      }
-    });
-
     QUnit.test('should return `false` for DOM elements', function(assert) {
       assert.expect(1);
 
@@ -11440,6 +11421,41 @@
       assert.strictEqual(_.isPlainObject(symbol), false);
     });
 
+    QUnit.test('should return `false` for objects with a read-only `Symbol.toStringTag` property', function(assert) {
+      assert.expect(1);
+
+      if (Symbol && Symbol.toStringTag) {
+        var object = {};
+        defineProperty(object, Symbol.toStringTag, {
+          'configurable': true,
+          'enumerable': false,
+          'writable': false,
+          'value': 'X'
+        });
+
+        assert.deepEqual(_.isPlainObject(object), false);
+      }
+      else {
+        skipAssert(assert);
+      }
+    });
+
+    QUnit.test('should not mutate `value`', function(assert) {
+      assert.expect(2);
+
+      if (Symbol && Symbol.toStringTag) {
+        var proto = {};
+        proto[Symbol.toStringTag] = undefined;
+        var object = create(proto);
+
+        assert.strictEqual(_.isPlainObject(object), false);
+        assert.notOk(lodashStable.has(object, Symbol.toStringTag));
+      }
+      else {
+        skipAssert(assert, 2);
+      }
+    });
+
     QUnit.test('should work with objects from another realm', function(assert) {
       assert.expect(1);