NodeJS passing variable to already defined callback function

Question

Consider the following code:

    if(connections.hasOwnProperty(id)){
        console.log('client '+id+' is connected. querying redis.');
        id = id;
        redisClientActor.lrange("NOTIFICATIONS_"+id, 0, 0, function(err, reply) {
            if(reply !== null && reply !== undefined){
                console.log(JSON.stringify(Object.keys(connections)));
                connections[id].sendUTF(reply);
                console.log('Forwarded notification to client '+id);
            }else{
                console.log(err);
            }
        });
    }else{
        console.log('Received notification, but client '+id+' not connected.')
    }

it's part of a very basic notification server written in NodeJS. It's using the redis npm package. Because of the asynchronous nature of Node I understand why the code is unable to work currently (id is out-of-scope which leads to sendUTF failing which leads to the script crashing).

If lrange was a self-defined function I'd simply add a third parameter here and be done with it. But since it isn't I struggle to find the solution as to how access "id" inside of the lrange callback (l5 and following)

I'd be very grateful for a quick hint in the right direction.

actually id is not out of scope, it should be there. Are you using this snippet within a loop that iterates through a series of values which are assigned to the variable "id"? At the end of the loop the value of id would be fixed to the last value of the iteration. Every callback would then see that last value of id. — Tobias Gassmann
– Tobias Gassmann, Commented Nov 14, 2015 at 20:33
Where does id come from? Shouldn't the check if(connections.hasOwnProperty(id)) { ... } happen inside the callback to lrange? Also, what is the purpose of the line id = id; ? — kliron
– kliron, Commented Nov 14, 2015 at 20:35

Tobias Gassmann · Accepted Answer · 2015-11-14 20:39:05Z

In case you are iterating through a loop that alters the value of "id", the callbacks would all see the last value of "id" which was assigned to it during the last iteration.

In that case you need to capture the value of id with a closure:

var produceClosureForId = function(id){
    return function(err, reply) {
        if(reply !== null && reply !== undefined){
            console.log(JSON.stringify(Object.keys(connections)));
            connections[id].sendUTF(reply);
            console.log('Forwarded notification to client '+id);
        }else{
            console.log(err);
        }
    };
}
redisClientActor.lrange("NOTIFICATIONS_"+id, 0, 0, produceClosureForId(id) );

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