static Callable getCallable(Scriptable thisObj)
    {
        Callable function;
        if (thisObj instanceof Callable) {
            function = (Callable)thisObj;
        } else {
            Object value = thisObj.getDefaultValue(ScriptRuntime.FunctionClass);
            if (!(value instanceof Callable)) {
                throw ScriptRuntime.notFunctionError(value, thisObj);
            }
            function = (Callable)value;
        }
        return function;
    }