diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/collections/Map-delete-size.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/collections/Map-delete-size.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,14 @@
+// map.delete(k) decrements the map size iff an entry was actually removed.
+
+var m = Map();
+m.delete(3);
+assertEq(m.size, 0);
+m.set({}, 'ok');
+m.set(Math, 'ok');
+assertEq(m.size, 2);
+m.delete({});
+assertEq(m.size, 2);
+m.delete(Math);
+assertEq(m.size, 1);
+m.delete(Math);
+assertEq(m.size, 1);