NodeJS failure to concatenate string and number

I use this code to get the user's ID saved in the key user:their_email_id.

redis.get("user:" + email, function(err, reply) {
    if (reply != null) {
        id = reply;
        console.log(reply);
    }
});

This gives me the user's ID in the id variable.

Now I use this ID to query the user's hash:

redis.hgetall("user:" + id, function (err, reply) {
    console.log(reply);
});

Here I get the reply as null. The reason is instead of the key becoming something like:

    user:0

I'm getting this:

   user:undefined

Now I know that it successfully returns the ID, and everything is working, except that when concatenating the ID with the "user:" string, it fails.

I also tried using id.toString(), but Express shows method not found. Why could this be happening? Do I have to require some library?

And how do I solve this issue?

Thanks