js-on-sx: bind returns dict-with-__callable__ for property mutation + length

lib/js/runtime.sx

@@ -373,9 +373,19 @@
                     (< (len args) 1)
                     (list)
                     (js-list-slice args 1 (len args)))))
-              (fn
-                (&rest more)
-                (js-call-with-this this-arg recv (js-list-concat bound more)))))))
+              (let
+                ((target-len (js-fn-length recv)))
+                (let
+                  ((bound-len
+                     (let ((d (- target-len (len bound))))
+                       (if (< d 0) 0 d))))
+                  {:__callable__
+                    (fn
+                      (&rest more)
+                      (js-call-with-this this-arg recv (js-list-concat bound more)))
+                    :length bound-len
+                    :name "bound"
+                    :__js_bound_target__ recv}))))))
       ((= key "toString")
         (let
           ((override (js-dict-get-walk (get js-function-global "prototype") "toString")))