basic logic of lock - mutual exclusion

I'm struggling with the logic of lock for mutual exclusion; Here Im checking if the key is taken or not, if not we take it and when done release it; but could you please help me how I can have a loop to check the key until it becomes available?

rdb.setnx(lockkey, 'taken', function (err, lockvalue) {
    if (err) {
        console.error(err);
    } else if (lockvalue == 1) {
        // You have the lock; process it
        //Release the key when you are done:
        rdb.del(lockkey);
    } else if (lockvalue == 0) {
        // Key is taken by someone else; so how can I have a loop to check until key is available to take? 
    }
});

Your help is appreciated :-)