I intended to write something with more substance tonight, but I'm exhausted from wrasslin' with Gecko/XULRunner/SpiderMonkey in a days-long marathon debugging session. None of you will understand this entry, because its intent is to contain enough keywords and content that others don't have to go through the pain that I did.

If you're embedding Gecko/XULRunner/SpiderMonkey into your application, and you want to evaluate some JavaScript in the context of an nsIDOMWindow or nsIWebBrowser, you'd think you'd have many approaches. You could call JS_EvaluateScript or JS_EvaluateUCScript directly, getting the JSContext from the nsIScriptContext and the JSObject* global from the nsIScriptGlobalObject... However, I simply could not get this to work: I kept running into crazy errors inside of JS_InitArrayClass. I still don't understand those errors.

People suggested using EvaluateString and EvaluateStringWithValue on nsIScriptContext, but that failed in an empty window (I define empty as not having called nsIWebNavigation::LoadURI) because it did not have a security principal (nsIPrincipal). Eventually I learned that you can grab the system principal from the nsIScriptSecurityManager service and pass that directly to EvaluateStringWithValue. With a few more minor details, this approach worked in all cases that we care about so far!

Here is the final magic incantation:

typedef std::map<jsval, boost::python::object> ReferenceMap;

boost::python::object GeckoWindow::evalJavaScript(const std::wstring& js) {
    nsresult rv;

    nsCOMPtr<nsIPrincipal> principal;
    nsCOMPtr<nsIScriptSecurityManager> secMan = do_GetService(
    rv = secMan->GetSystemPrincipal(getter_AddRefs(principal));
    if (NS_FAILED(rv)) {
        throw GeckoError("Failed to get system principal");

    nsCOMPtr<nsIScriptGlobalObject> sgo = do_GetInterface(webBrowser);
    nsCOMPtr<nsIScriptContext> ctx = sgo->GetContext();

    JSContext* cx = reinterpret_cast<JSContext*>(ctx->GetNativeContext());
    uint32 previous = JS_SetOptions(

    jsval out;
    rv = ctx->EvaluateStringWithValue(
        nsString(js.data(), js.size()),

    JS_SetOptions(cx, previous);

    JSAutoRequest ar(cx);
    JSAutoLocalRootScope alrs(cx);


    if (NS_SUCCEEDED(rv)) {
        ReferenceMap references;
        return buildPythonObjectFromJsval(references, cx, out);
    } else {
        throw GeckoEvalUnknownError("eval failed with no exception set");

void GeckoWindow::maybeThrowPythonExceptionFromJsContext(JSContext* cx) {
    jsval exception;
    if (JS_GetPendingException(cx, &exception)) {
        ReferenceMap references;
        boost::python::object py_exc_value(buildPythonObjectFromJsval(
        throw GeckoEvalError(py_exc_value.ptr());

boost::python::object GeckoWindow::buildPythonObjectFromJsval(
    ReferenceMap& references,
    JSContext* cx,
    const jsval v
) {
    using namespace boost::python;

    if (v == JSVAL_TRUE) {
        return object(handle<>(Py_True));
    } else if (v == JSVAL_FALSE) {
        return object(handle<>(Py_False));
    } else if (v == JSVAL_NULL) {
        return object(handle<>(Py_None));
    } else if (v == JSVAL_VOID) {
        return object(handle<>(Py_None));
    } else if (JSVAL_IS_INT(v)) {
        return object(handle<>(PyInt_FromLong(JSVAL_TO_INT(v))));
    } else if (JSVAL_IS_NUMBER(v)) {
        return object(handle<>(PyFloat_FromDouble(*JSVAL_TO_DOUBLE(v))));
    // } else if (JSVAL_IS_STRING(v)) {
    } else if (JSVAL_IS_OBJECT(v)) {
        JSObject* obj = JSVAL_TO_OBJECT(v);

        if (references.count(v)) {
            return references[v];

        if (JS_IsArrayObject(cx, obj)) {
            list rv;
            references[v] = rv;
            jsuint length;
            if (JS_GetArrayLength(cx, obj, &length)) {
                jsval element;
                for (jsuint i = 0; i < length; ++i) {
                    if (JS_GetElement(cx, obj, i, &element)) {
                        rv.append(buildPythonObjectFromJsval(references, cx, element));
            return rv;
        } else {
            dict rv;
            references[v] = rv;

            JSObject* iterator = JS_NewPropertyIterator(cx, obj);
            if (!iterator) {
                throw GeckoEvalUnknownError("Error creating object property iterator while marshalling");
            for (;;) {
                jsid propertyName;
                if (!JS_NextProperty(cx, iterator, &propertyName)) {
                    throw GeckoEvalUnknownError("Error enumerating property list of object while marshalling");

                if (propertyName == JSVAL_VOID) {

                jsval propertyNameValue;
                jsval propertyValue;
                object k;

                if (!JS_IdToValue(cx, propertyName, &propertyNameValue)) {
                    throw GeckoEvalUnknownError("Error converting property name to jsval while marshalling");
                if (JSVAL_IS_INT(propertyNameValue)) {
                    jsint propertyIndex = JSVAL_TO_INT(propertyNameValue);
                    k = long_(propertyIndex);

                    if (!JS_LookupElement(cx, obj, propertyIndex, &propertyValue)) {
                        throw GeckoEvalUnknownError("Error looking up property value by index");
                } else if (JSVAL_IS_STRING(propertyNameValue)) {
                    JSString* kjsstr = JSVAL_TO_STRING(propertyNameValue);
                    std::wstring kstr(JS_GetStringChars(kjsstr), JS_GetStringLength(kjsstr));
                    k = object(kstr);

                    if (!JS_LookupUCProperty(cx, obj, kstr.c_str(), kstr.size(), &propertyValue)) {
                        throw GeckoEvalUnknownError("Error looking up property value by name");
                } else {
                    throw GeckoEvalUnknownError("Unknown property name type while marshalling");

                rv[k] = buildPythonObjectFromJsval(references, cx, propertyValue);
            return rv;
    } else {
        // We don't know what type it is, or we can't marshal it,
        // so convert it to a string and hope for the best...
        JSString* string = JS_ValueToString(cx, v);
        return str(std::wstring(JS_GetStringChars(string), JS_GetStringLength(string)));

Hope that helps, and Godspeed.