handling asynchronous call backs in nodejs

Question

handling asynchronous call backs in nodejs

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.

node.js
callback

Answer 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.

Answer 2

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});
            }
        });
    }
});