diff --git a/xpcom/glue/nsTArray.h b/xpcom/glue/nsTArray.h
index 1e9d0e0e5eb4..5d58ae5849a6 100644
--- a/xpcom/glue/nsTArray.h
+++ b/xpcom/glue/nsTArray.h
@@ -1175,6 +1175,31 @@ public:
   //
   // Mutation methods
   //
+
+  template<class Allocator, typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType Assign(
+      const nsTArray_Impl<E, Allocator>& aOther)
+  {
+    return ActualAlloc::ConvertBoolToResultType(
+      !!ReplaceElementsAt<E, ActualAlloc>(0, Length(),
+                                          aOther.Elements(), aOther.Length()));
+  }
+
+  template<class Allocator>
+  /* MOZ_WARN_UNUSED_RESULT */
+  bool Assign(const nsTArray_Impl<E, Allocator>& aOther,
+              const mozilla::fallible_t&)
+  {
+    return Assign<Allocator, FallibleAlloc>(aOther);
+  }
+
+  template<class Allocator>
+  void Assign(nsTArray_Impl<E, Allocator>&& aOther)
+  {
+    Clear();
+    SwapElements(aOther);
+  }
+
   // This method call the destructor on each element of the array, empties it,
   // but does not shrink the array's capacity.
   // See also SetLengthAndRetainStorage.
diff --git a/xpcom/tests/gtest/TestTArray.cpp b/xpcom/tests/gtest/TestTArray.cpp
new file mode 100644
index 000000000000..86364de3e6b2
--- /dev/null
+++ b/xpcom/tests/gtest/TestTArray.cpp
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsTArray.h"
+#include "gtest/gtest.h"
+
+using namespace mozilla;
+
+namespace TestTArray {
+
+const nsTArray<int>& DummyArray()
+{
+  static nsTArray<int> sArray;
+  if (sArray.IsEmpty()) {
+    const int data[] = {4, 1, 2, 8};
+    sArray.AppendElements(data, ArrayLength(data));
+  }
+  return sArray;
+}
+
+// This returns an invalid nsTArray with a huge length in order to test that
+// fallible operations actually fail.
+#ifdef DEBUG
+const nsTArray<int>& FakeHugeArray()
+{
+  static nsTArray<int> sArray;
+  if (sArray.IsEmpty()) {
+    sArray.AppendElement();
+    ((nsTArrayHeader*)sArray.DebugGetHeader())->mLength = UINT32_MAX;
+  }
+  return sArray;
+}
+#endif
+
+TEST(TArray, assign)
+{
+  nsTArray<int> array;
+  array.Assign(DummyArray());
+  ASSERT_EQ(DummyArray(), array);
+
+  ASSERT_TRUE(array.Assign(DummyArray(), fallible));
+  ASSERT_EQ(DummyArray(), array);
+
+#ifdef DEBUG
+  ASSERT_FALSE(array.Assign(FakeHugeArray(), fallible));
+#endif
+
+  nsTArray<int> array2;
+  array2.Assign(Move(array));
+  ASSERT_TRUE(array.IsEmpty());
+  ASSERT_EQ(DummyArray(), array2);
+}
+
+}
diff --git a/xpcom/tests/gtest/moz.build b/xpcom/tests/gtest/moz.build
index 32bd5d117471..6c47bcee91c1 100644
--- a/xpcom/tests/gtest/moz.build
+++ b/xpcom/tests/gtest/moz.build
@@ -17,6 +17,7 @@ UNIFIED_SOURCES += [
     'TestStrings.cpp',
     'TestStringStream.cpp',
     'TestSynchronization.cpp',
+    'TestTArray.cpp',
     'TestThreadPool.cpp',
     'TestThreads.cpp',
     'TestTimeStamp.cpp',