diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/collections/Map-set-undefined.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/collections/Map-set-undefined.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,15 @@
+// Setting a Map key to undefined, or a missing argument, isn't the same as deleting it.
+
+var m = new Map;
+m.set(42, undefined);
+assertEq(m.has(42), true);
+assertEq(m.get(42), undefined);
+
+m.set(42, "wrong");
+m.set(42);
+assertEq(m.has(42), true);
+assertEq(m.get(42), undefined);
+
+m.set();
+assertEq(m.has(undefined), true);
+assertEq(m.get(undefined), undefined);