You can try following in your command js.

Extensions.YourExnName.prototype.isAvailable = function YourExnName$isAvailable(target)
{

//check here for publication
    return true/false;;
};

Extensions.YourExnName.prototype.isEnabled = function YourExnName$isEnabled(target) {
//check here for publication
    return true/false;
};

You can try following in your command js.

Extensions.YourExnName.prototype.isAvailable = function YourExnName$isAvailable(target)
{

//check here for publication
    return true/false;;
};

Extensions.YourExnName.prototype.isEnabled = function YourExnName$isEnabled(target) {
//check here for publication
    return true/false;
};

2. Validation can be done using javaScript also in your onclick or other events, did you try?