node.js async waterfall module callbacks

Question

I have my node.js code like this

async.waterfall([
    function(callback){
        category = [{"id":1,"name":xxx},{"id":2,"name":yyy}];
        callback(null,category);
    },
    function(category,callback){
        var categoryData ={};
        _.each(category,function(item){
            categoryData[item.id] = item;        
            SolrClient.select(query,function(data){
                //data is the response from SolrClient
                categoryData[item.id]["data"] = data;
                log(categoryData);   //log1 
            },callback);
            log(categoryData);  //log2
        });
        log(categoryData); //log3
    }
]);

log1 => the data that I have added to categoryData is logged correctly
log2 => I cant get the data that is logged in the callback function to SolrClient
log3 => same as log2

I understand that this has something to do with variable scope OR that the callback to SolrClient cannot access the same categoryData initialized before _.each .

I have wasted a lot of time debugging it but i guess I am doing some small mistake which I am not getting how to solve.

what would really help you is proper indentation.

Jonathan Ong
– Jonathan Ong

2012-12-08 04:21:47 +00:00
Commented Dec 8, 2012 at 4:21 — Jonathan Ong
– Jonathan Ong, Commented Dec 8, 2012 at 4:21

Jonathan Lonowski · Accepted Answer · 2012-12-08 08:13:59Z

If you adjust your logs to include a unique message with each (such as the comments currently next to them), they probably appear in a particular, and perhaps unexpected, order:

log2
log2
log3
log1
log1

The issue you're experiencing isn't one of scope; it's one of timing. Functions can be called at any time -- not only immediately or synchronously as with _.each(), but also "later" or asynchronously as with SolrClient.select().

To ensure that all of the queries with SolrClient.select() finish before continuing, you can swap out _.each for async.forEach:

//...
function(category, waterfallCallback) {
    var categoryData = {};

    async.forEach(category, function (item, forEachCallback) {
        categoryData[item.id] = item;

        SolrClient.select(query, function (data) {
            categoryData[item.id]["data"] = data;
            forEachCallback(null);
        });
    }, function (err) {
        waterfallCallback(null, categoryData);
    });
}
//...

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