Bug with Extendscript Reflection object

I had noticed this too and it is probably not a bug.

All properties of objects that belong to the After Effects Object Model seem to have type "readwrite", even though some can only be read.

An explanation can be (not sure)  that all properties are created with that type, and then are being watched (with Object.prototype.watch).

Then when you try to set a property that is not supposed to be set by you (us), the watch function will throw an error telling that it is readonly, while on the javascript level it is actually readwrite.

Note that the type of core javascript properties is correct. For instance:

var r = new RegExp("");

alert(r.reflect.find("source").type);    // alerts : "readonly" (the source of a regexp is readonly)

So if you want to know if a property is readonly, i think that the only way is to try to set the value:

function checkReadonlyValues(objToCheck) {  
    var readOnlyArr = [];
    var name;
    for (var i = 0; i < objToCheck.reflect.properties.length; i++) {
        name = objToCheck.reflect.properties.name;

        if (objToCheck.reflect.properties.type === "readonly"){

            readOnlyArr.push(name);
            }
        else{
            try{
                objToCheck[name] = objToCheck[name];
                }
            catch(e){
                readOnlyArr.push(name);
                };
            };
        };
    return readOnlyArr;
    };
checkReadonlyValues(app.project.activeItem)

The problem is that it creates a bunch of history entries, so you'd have to wrap the whole thing into an undoGroup and use app.executeCommand(app.findMenuCommendId("Undo")) at the end to clean things. Not very nice...

Xavier