How to write synchronous function in Node.js

this is a common problem when switching to callback framework from the "standard" one.

This is how it works, first it synchronously run this

for(var i=0; i<feedArr.length; i++){
    var feedId = feedArr[i].feedid;
    var listFeedImageQuery  = "select imageUrl from feedImages where feedId='"+feedId+"'";
    var deleteFeedImagesQuery = "delete from feedImages where feedId='"+feedId+"'";
    model.client.query(listFeedImageQuery,function(err,result){
        //to be executed later
    });
}

The method model.client.query is pushed with each cycle to the "stack of asynchronous calls", therefore in the moment when the all these queries are executed, the feedId is equal to feedArr[feedArr.length-1].feedid

One simple workaround is to wrap it around methods with parameter and then call them immediately.

for(var i=0; i<feedArr.length; i++){        
    (function(feedId) {
       var listFeedImageQuery  = "select imageUrl from feedImages where feedId='"+feedId+"'";
       var deleteFeedImagesQuery = "delete from feedImages where feedId='"+feedId+"'";
       model.client.query(listFeedImageQuery,function(err,result){
           //to be executed later
       });
    })(feedArr[i].feedid);
}

Now it should work.

for (var i = 0; i < 10; i++) {
  console.log('sync', i);   
  setTimeout(function () {
    console.log('wrong async', i)
  }, 1);
}


for (var i = 0; i < 10; i++) {  
  setTimeout((function (j) {
    console.log('right async', i)
  })(i), 1);
}

Figure it out, maybe you will know where you was wrong.

For your issue, you can extract the async code as a function, like this:

function deleteImages(feedId) {
    var listFeedImageQuery  = "select imageUrl from feedImages where feedId='"+feedId+"'";
    var deleteFeedImagesQuery = "delete from feedImages where feedId='"+feedId+"'";
    model.client.query(listFeedImageQuery,function(err,result){
        if(result.rows.length > 0){
            for(var j=0; j<result.rows.length; j++){
                fs.unlink(result.rows[j].imageurl, function(err) {
                    if(err)
                        console.log(err);
                });
            }
            model.client.query(deleteFeedImagesQuery,function(err,result){
                if(err)
                    console.log(err)
                else
                    console.log(feedId +' were removed'); // last feedId only gets removing from database
            });
        }   
    });
}

for(var i=0; i<feedArr.length; i++){
    var feedId = feedArr[i].feedid;
    deleteImages(feedId);
}

For the more complex issue, if you don't want the callback hell, try async or bluebird