diff -r 000000000000 -r 6474c204b198 xpcom/tests/unit/test_nsIMutableArray.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/tests/unit/test_nsIMutableArray.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,122 @@
+/* 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/. */
+
+const Ci = Components.interfaces;
+const Cr = Components.results;
+const Cc = Components.classes;
+const CC = Components.Constructor;
+
+var MutableArray = CC("@mozilla.org/array;1", "nsIMutableArray");
+var SupportsString = CC("@mozilla.org/supports-string;1", "nsISupportsString");
+
+function create_n_element_array(n)
+{
+  var arr = new MutableArray();
+  for (let i=0; i<n; i++) {
+    let str = new SupportsString();
+    str.data = "element " + i;
+    arr.appendElement(str, false);
+  }
+  return arr;
+}
+
+function test_appending_null_actually_inserts()
+{
+  var arr = new MutableArray();
+  do_check_eq(0, arr.length);
+  arr.appendElement(null, false);
+  do_check_eq(1, arr.length);
+}
+
+function test_object_gets_appended()
+{
+  var arr = new MutableArray();
+  var str = new SupportsString();
+  str.data = "hello";
+  arr.appendElement(str, false);
+  do_check_eq(1, arr.length);
+  var obj = arr.queryElementAt(0, Ci.nsISupportsString);
+  do_check_eq(str, obj);
+}
+
+function test_insert_at_beginning()
+{
+  var arr = create_n_element_array(5);
+  // just a sanity check
+  do_check_eq(5, arr.length);
+  var str = new SupportsString();
+  str.data = "hello";
+  arr.insertElementAt(str, 0, false);
+  do_check_eq(6, arr.length);
+  var obj = arr.queryElementAt(0, Ci.nsISupportsString);
+  do_check_eq(str, obj);
+  // check the data of all the other objects
+  for (let i=1; i<arr.length; i++) {
+    let obj = arr.queryElementAt(i, Ci.nsISupportsString);
+    do_check_eq("element " + (i-1), obj.data);
+  }
+}
+
+function test_replace_element()
+{
+  var arr = create_n_element_array(5);
+  // just a sanity check
+  do_check_eq(5, arr.length);
+  var str = new SupportsString();
+  str.data = "hello";
+  // replace first element
+  arr.replaceElementAt(str, 0, false);
+  do_check_eq(5, arr.length);
+  var obj = arr.queryElementAt(0, Ci.nsISupportsString);
+  do_check_eq(str, obj);
+  // replace last element
+  arr.replaceElementAt(str, arr.length - 1, false);
+  do_check_eq(5, arr.length);
+  obj = arr.queryElementAt(arr.length - 1, Ci.nsISupportsString);
+  do_check_eq(str, obj);
+  // replace after last element, should insert empty elements
+  arr.replaceElementAt(str, 9, false);
+  do_check_eq(10, arr.length);
+  obj = arr.queryElementAt(9, Ci.nsISupportsString);
+  do_check_eq(str, obj);
+  // AFAIK there's no way to check the empty elements, since you can't QI them.
+}
+
+function test_clear()
+{
+  var arr = create_n_element_array(5);
+  // just a sanity check
+  do_check_eq(5, arr.length);
+  arr.clear();
+  do_check_eq(0, arr.length);
+}
+
+function test_enumerate()
+{
+  var arr = create_n_element_array(5);
+  do_check_eq(5, arr.length);
+  var en = arr.enumerate();
+  var i = 0;
+  while (en.hasMoreElements()) {
+    let str = en.getNext();
+    do_check_true(str instanceof Ci.nsISupportsString);
+    do_check_eq(str.data, "element " + i);
+    i++;
+  }
+  do_check_eq(arr.length, i);
+}
+
+var tests = [
+  test_appending_null_actually_inserts,
+  test_object_gets_appended,
+  test_insert_at_beginning,
+  test_replace_element,
+  test_clear,
+  test_enumerate,
+];
+
+function run_test() {
+  for (var i = 0; i < tests.length; i++)
+    tests[i]();
+}