handling asynchronous call backs in nodejs

Question

I am newbie to nodejs.It's very hard to handle callbacks at nodejs level. I have code like this,

getItems(request,function(jsonObject){
     var itemData={};
     var itemDetails=new Array();
     for(var i=0;i < jsonObject.length;i++){
         getItemDetails(jsonObject[i].value.item_id,function(jsonObject){
            itemDetails.push(jsonObject);
         });
    }
    itemData["itemDetails"]=itemDetails;
    response.contentType("application/json");
    response.send({"data":itemData});
});

while executing the above code, the for loop is continuing with out getting callback from getItemDetails method and response sent to client. My requirement is the loop will wait until getting the call back from the getItemDetails then response should send. I have tried with process.nextTick(), but i am unable to find where i have to use that process.nextTick().. Please anybody provide suggestions.

Thanks in advance.

alessioalex · Accepted Answer · 2012-06-08 07:15:20Z

1

You need to send the response only after you get all the items, so modify your code like so:

getItems(request,function(jsonObject) {
    var itemData = {},
        itemDetails = [],
        itemsLeft = len = jsonObject.length,
        i;

    function sendResponse(itemDetails) {
        itemData["itemDetails"] = itemDetails;
        response.contentType("application/json");
        response.send({ "data": itemData });
    }

    for (i = 0; i < len; i++) {
        getItemDetails(jsonObject[i].value.item_id, function(jsonObject) {
            itemDetails.push(jsonObject);
            // send response after all callbacks have been executed
            if (!--itemsLeft) {
                sendResponse(itemDetails);
            }
        });
    }
});

Note: I've used itemLeft here since it's a more generic way to solve these kind of problems, but Ianzz approach is also ok since you can compare the length of the two arrays.

edited Jun 8, 2012 at 7:15

answered Jun 7, 2012 at 13:24

alessioalex

63.8k16 gold badges158 silver badges123 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Aruna Over a year ago

After changing the code also still i am facing the same problem. At least it is not checking the if condition, directly the i value changed and continuing further statements.

alessioalex Over a year ago

My bad, it was actually --itemsLeft, try now.

lanzz · Accepted Answer · 2012-06-07 13:20:28Z

0

You can't get the loop to wait, but you can modify your code to get the behavior you are expecting:

getItems(request,function(outerJsonObject){
    var itemData={};
    var itemDetails=new Array();
    for(var i=0;i < outerJsonObject.length;i++){
        getItemDetails(jsonObject[i].value.item_id,function(innerJsonObject){
            itemDetails.push(innerJsonObject);
            if (itemDetails.length == outerJsonObject.length) {
                // got all item details
                itemData["itemDetails"]=itemDetails;
                response.contentType("application/json");
                response.send({"data":itemData});
            }
        });
    }
});

answered Jun 7, 2012 at 13:20

lanzz

43.4k12 gold badges93 silver badges99 bronze badges

Collectives™ on Stack Overflow

handling asynchronous call backs in nodejs

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related