diff --git a/js2py/constructors/jsmath.py b/js2py/constructors/jsmath.py
index 06751a3b..cef5f230 100644
--- a/js2py/constructors/jsmath.py
+++ b/js2py/constructors/jsmath.py
@@ -152,6 +152,12 @@ def max():
 
     def random():
         return random.random()
+        
+    def sign(x):
+        a = x.to_number().value
+        if a != a:  # it must be a nan
+            return NaN
+        return math.copysign(1, a)
 
 
 fill_prototype(Math, MathFunctions, default_attrs)