Necroing a thread a bit, but figure I'd throw in a solution for this for anyone looking. In JavaScript there's a few options for changing a string to an integer. Number('1') and +'1' both work pretty much identically.
As for actually grabbing the durability values, RegEx is the way to go.
function DurabilityTest()
    {
    Orion.WaitForAddObject('myTarget');
    Orion.Print(GetDurability('myTarget'));
    }
function GetDurability(objSerial)
    {
    var obj = Orion.FindObject(objSerial);
    var durabilityArr = obj.Properties().match(/Durability (\d+) \/ (\d+)/) //evaluates to e.g., ['Durability 250 / 255', '250', '255'] OR null if match isn't found
    if(!durabilityArr) //if a match isn't found returns false
        return false;
    var minDurability = +durabilityArr[1]; //convert string to int
    var maxDurability = Number(durabilityArr[2]); //convert string to int
    return [minDurability, maxDurability]; //e.g. [250, 255]
    }