diff --git a/js/public/Equality.h b/js/public/Equality.h
index 03181074de..6d2db50fa9 100644
--- a/js/public/Equality.h
+++ b/js/public/Equality.h
@@ -8,6 +8,8 @@
 #ifndef js_Equality_h
 #define js_Equality_h
 
+#include "mozilla/FloatingPoint.h"
+
 #include "jstypes.h"  // JS_PUBLIC_API
 
 #include "js/RootingAPI.h" // JS::Handle
@@ -50,6 +52,17 @@ LooselyEqual(JSContext* cx, JS::Handle<JS::Value> v1, JS::Handle<JS::Value> v2,
 extern JS_PUBLIC_API(bool)
 SameValue(JSContext* cx, JS::Handle<JS::Value> v1, JS::Handle<JS::Value> v2, bool* same);
 
+/**
+ * Implements |SameValueZero(v1, v2)| for Number values |v1| and |v2|.
+ * SameValueZero equates NaNs, equal nonzero values, and zeroes without respect
+ * to their signs.
+ */
+static inline bool
+SameValueZero(double v1, double v2)
+{
+    return mozilla::EqualOrBothNaN(v1, v2);
+}
+
 }  // namespace JS
 
 #endif /* js_Equality_h */
diff --git a/mfbt/FloatingPoint.h b/mfbt/FloatingPoint.h
index 6a0e454ae7..7d73d7e848 100644
--- a/mfbt/FloatingPoint.h
+++ b/mfbt/FloatingPoint.h
@@ -396,6 +396,20 @@ NumbersAreIdentical(T aValue1, T aValue2)
   return BitwiseCast<Bits>(aValue1) == BitwiseCast<Bits>(aValue2);
 }
 
+/**
+ * Return true if |aValue| and |aValue2| are equal (ignoring sign if both are
+ * zero) or both NaN.
+ */
+template <typename T>
+static inline bool
+EqualOrBothNaN(T aValue1, T aValue2)
+{
+  if (IsNaN(aValue1)) {
+    return IsNaN(aValue2);
+  }
+  return aValue1 == aValue2;
+}
+
 namespace detail {
 
 template<typename T>