Clarified that instrumenting window.nonExisting object properties is not supported

automation/Extension/webext-instrumentation/src/lib/js-instruments.ts

@@ -301,6 +301,9 @@ export function jsInstruments(event_id, sendMessagesToLogger) {
   // Rough implementations of Object.getPropertyDescriptor and Object.getPropertyNames
   // See http://wiki.ecmascript.org/doku.php?id=harmony:extended_object_api
   Object.getPropertyDescriptor = function(subject, name) {
+    if (subject === undefined) {
+      throw new Error("Can't get property descriptor for undefined");
+    }
     let pd = Object.getOwnPropertyDescriptor(subject, name);
     let proto = Object.getPrototypeOf(subject);
     while (pd === undefined && proto !== null) {

test/test_pages/js_instrument/instrument_non_existent_properties.html

@@ -45,6 +45,13 @@
           console.log("nonExistingProp1 after set - caught exception: ", e);
         }
 
+        // call non-existing object method
+        try {
+          window.nonExisting.nonExistingMethod1('hello', {'world': true});
+        } catch (e) {
+          console.log("call object method - caught exception: ", e);
+        }
+
         /*
          * Interact with partially existing object instrumented non-recursively
          */
@@ -111,9 +118,18 @@
 
       // Instrument non-existing object non-recursively
       console.log("Instrumenting window.nonExisting!");
+      // Instrument the window.nonExisting property itself (not its object properties)
       window.instrumentObject(window, "window", {
         propertiesToInstrument: ["nonExisting"],
       });
+      // Instrument window.nonExisting object properties - currently not supported
+      // window.instrumentObject(window.nonExisting, "window.nonExisting", {
+      //   propertiesToInstrument: [
+      //     "existingProp",
+      //     "nonExistingProp1",
+      //     "nonExistingMethod1",
+      //   ],
+      // });
 
       // Instrument partially existing object non-recursively
       console.log("Instrumenting window.partiallyExisting!");