How to return value from a for loop which is querying the database in Javascript?

Use recursion:

function getUrls(data,done) {
    var links = new Array();

    function doGetUrl(i) {
        var user = data[i];
        Url.find({where:{data.id}}).
        success(function(url){
            links.push({
                "url": ur.text,
                "date": data.syncedTime
            });

            if (links.length == data.length){
                done(links);
            } else {
                doGetUrl(i + 1); // get next url
            }
        }).
        failure(function(err) {    
            doGetUrl(i); // on error, try to get current url again
            // other error handling code
        });
    }

    doGetUrl(0);
}

It will be easy for you, if you use async.js.

I used mapSeries here. It takes 3 parameters.

• collection/array
• iterator, which will be called for each item in the passed collection/array with 2 arguments. 1. item in collection, 2. callback. After completing the job in iterator, You should call the callback in node style(err first, results follows).
• Final callback, which will be called after all the items in the collection mapped.

---

function getUrls(data,done){

  var async = require('async');

  async.mapSeries(data, function(user, cb) {//If you want it to be async `async.map`

    Url.find({where:{user.id}}).success(function(url){
      cb(null, {
            "url": url.text,
            "date": user.syncedTime
      });
    });
  }, function(err, results) {
    //results is an array. Its the same as `links` in your old code.
    done(results);
  });

}

geturls(data,function(urls){
  var data = {
    "data": [
        { "userProfile": userP },
        { "urls": urls }
    ]
  };
  res.send(data);
}); 

I would probably make use of the complete callback, in jQuery terms. Have a counter that records how many records have been processed and update this in complete, as this executes on success or failure. Then when that counter is >= the length of the data array you can exit.

As an aside, I would always do a >= rather than an == for the comparison you are doing there, that way, if for any crazy reason, the count is upped more than it should you still exit.

If alll you want to do is avoif the problem of checking links.length to determine when you are done then I think its just a matter of adding a separate counter that gets incremented even if the urk database fails. If you do that you can continue using your current stype where the async requests are run in parallel.

var nreq = 0;
for (var i=0; i<data.length; i++){
   doTheAsyncOperation(function(){
     //Run this part in both the success and error cases
     nreq = nreq + 1;
     if(nreq >= data.length){ done(links) }
   })
}

On the other hand, if you want to run one query after the other you will need to rewrite the for to use recursion. This time, you don't need to worry about keeping a separate counter since you know when the final request runs:

function loop(i){
  if(i >= data.length){
    done(links);
  }else{
    doTheAsyncOperation(function(){
      loop(i+1);
    })
  }
}

loop(0);

Finally, its good to know how to code this sort of patterns yourself but in the long run I highly recommend using a control flow library to keep things cleaner.