Allow an old onerror function to tell TraceKit to not notify handlers

README.md

@@ -72,6 +72,11 @@ You can also tell TraceKit to ignore global window errors with:
 TraceKit.collectWindowErrors = false;
 ```
 
+### Existing `window.onerror` function?
+
+TraceKit installs it's own window.onerror function, but yours will still get called normally.
+If you don't want TraceKit to notify subscribers, then `return {notifyHandlers: false};` from your window.onerror function.
+
 View the source for more details and examples.
 
 ![Stacktrace or GTFO](http://i.imgur.com/jacoj.jpg)

tracekit.js

@@ -180,10 +180,17 @@ TraceKit.report = (function reportModuleWrapper() {
             };
         }
 
-        notifyHandlers(stack, 'from window.onerror');
-
         if (_oldOnerrorHandler) {
-            return _oldOnerrorHandler.apply(this, arguments);
+            var _oldOnerrorReturn = _oldOnerrorHandler.apply(this, arguments);
+
+            // if there is no return value or that return value has notifyHandlers set to something truthy, notifyHandlers
+            if (!_oldOnerrorReturn || _oldOnerrorReturn.notifyHandlers) {
+                notifyHandlers(stack, 'from window.onerror');
+            }
+
+            return _oldOnerrorReturn; // preserve any potential return behavior of the old onerror function.
+        } else {
+            notifyHandlers(stack, 'from window.onerror');
         }
 
         return false;