Issue #2304 - Part 4 - Add xray support for the Error cause property. https://bugzilla.mozilla.org/show_bug.cgi?id=1679653

js/xpconnect/src/XPCJSContext.cpp

@@ -111,6 +111,7 @@ const char* const XPCJSContext::mStrings[] = {
     "columnNumber",         // IDX_COLUMNNUMBER
     "stack",                // IDX_STACK
     "message",              // IDX_MESSAGE
+    "cause",                // IDX_CAUSE
     "errors",               // IDX_ERRORS
     "lastIndex"             // IDX_LASTINDEX
 };

js/xpconnect/src/xpcprivate.h

@@ -495,6 +495,7 @@ public:
         IDX_COLUMNNUMBER            ,
         IDX_STACK                   ,
         IDX_MESSAGE                 ,
+        IDX_CAUSE                   ,
         IDX_ERRORS                  ,
         IDX_LASTINDEX               ,
         IDX_TOTAL_COUNT // just a count of the above

js/xpconnect/wrappers/XrayWrapper.cpp

@@ -609,6 +609,11 @@ JSXrayTraits::resolveOwnProperty(JSContext* cx, const Wrapper& jsWrapper,
                 return true;
             }
 
+            // The optional .cause property can have any value.
+            if (id == GetJSIDByIndex(cx, XPCJSContext::IDX_CAUSE)) {
+                return getOwnPropertyFromWrapperIfSafe(cx, wrapper, id, desc);
+            }
+
             if (key == JSProto_AggregateError &&
                 id == GetJSIDByIndex(cx, XPCJSContext::IDX_ERRORS)) {
                 return getOwnPropertyFromWrapperIfSafe(cx, wrapper, id, desc);