How to check if a property value is readonly using extendscript?

I'm writing a script for After Effects that collects all properties from a layer and write them into an XML file. When I retrieve the values from the XML, some values are readOnly and the toolkit throws an error.

Is there any way to check it, like readonly attribute of File object? ie: layer.property().(readonly||readOnly)

If not, someone can tell me wich aproach can I take to go in the right direction?

Solution

Given that the first item in the project is a comp with a solid in it, this works but it is arguably kludgey, and you'd need to be able to build the (each) string in order to do this -- but maybe you are already set up to do that:

var r;
r = testForReadability("app.project.items[1].layers[1].enabled");
alert(r);
r = testForReadability("app.project.items[1].layers[1].width");//a solid's width is NOT writable
alert(r);

function testForReadability(thisProperty) {
    var x;
    try {
        x = eval(thisProperty);
        eval(thisProperty + " = x;");
        return true;
    } catch(e) {
        return false;
    }
}

However, there is a small can of worms opening up here, in that "false"s will not work if the "Enable Script Debugger" option is set. So you need to do a workaround in order to check for this setting and temporarily reset it (see http://aenhancers.com/viewtopic.php?f=8&t=189&p=554&hilit=debugger#p554 )