diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/jaeger/bug639808.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/jaeger/bug639808.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,16 @@
+function f() {
+    var x = 1.23;
+    var y = [].length;
+    x = ++y;
+    y - 1;
+}
+f();
+
+function g(q) {
+    var x = 1.23;
+    var y = [].length;
+    x = ++y;
+    if (q)
+      assertEq(y + 5, 6);
+}
+g(1);