Question

In my app I have the following:

client.on('test', function(req, fn) {
        var returnArr = [];
        redis.hkeys(req, function (err, replies) {
            replies.forEach(function(reply, i) {
                if (reply.indexOf('list.') > -1) {
                    redis.hgetall(reply.substring(5), function(err, r) {
                        returnArr.push({name:r['name'],index:i});
                        console.log(returnArr);
                    });
                }
            });
            console.log(returnArr);
        });
        console.log(returnArr);
    });

For some reason, the second and third logs contain a blank array even though the array is declared once at the beginnning of the event. Any ideas?

EDIT: Sorry, I changed the variable name when I posted it here without thinking. This happens when it’s named anything.

Answer 1

Those redis calls are asynchronous. That’s why you provide them with callbacks. The code won’t work even if you fix the variable name for that reason.

To elaborate: the code in the callback to “hkeys” will be invoked when the data is available. The call will return immediately, however, so your array will have nothing in it at that point.

You cannot wrap asynchronous calls in a function and expect to return a value. It simply won’t work.

Instead, the general pattern is to do exactly what the redis API (and virtually everything else in the node.js world; that’s kind-of the whole point in fact): give your own function a callback argument to be invoked when appropriate. In your case, it’ll be inside the “hgetall” callback that’s the last one to be invoked. It should figure out that your results array has as many values in it as there are keys, and so it’s time to call the callback passed in to your function.

(I should note that it’s not clear what you’re trying to do, given that the overall function appears to be a callback to something.)

Another approach would be to use some sort of “promise” pattern, though that’s really just a restructuring of the same idea.

edit — the general pattern for an API with a callback would be something like this:

function yourAPI( param1, param2, callback ) {
  // ...
  some.asynchronousFunction( whatever, function( result ) {
    callback( result );
  }
}

Now in your case you’re making multiple asynchronous service requests, and you’d need to figure out when it’s time to invoke the callback. I think you’d probably want to iterate through the “replies” from the call to get the keys and extract the list of ones you want to fetch:

    redis.hkeys(req, function (err, replies) {
        var keys = [];
        replies.forEach(function(reply, i) {
            if (reply.indexOf('list.') > -1) {
                keys.push( reply.substring(5) );
            }
        });
        keys.forEach( function( key ) {
                redis.hgetall(key, function(err, r) {
                    returnArr.push({name:r['name'],index:i});

                    if (returnArr.length === keys.length) {
                      // all values ready
                      callback( returnArr );
                    }
                });

        });