How to make Asynchronous Route in nodejs

AsyncRoute.js

var calls = [];

// HomeStreamPost Part-2
// route to get the homeStream post of a particular user
server.get("/user/homeStreamPost/:email_address",function(req,res,next){
  var result=[];
  req.assert('email_address','Email Address is required').notEmpty().isEmail();
  var errors = req.validationErrors();
  if (errors) {
    helpers.failure(res,next,errors[0],400);
  }

  UserModel.findOne({email_address: req.params.email_address }, function (err, user) {
    if(err) {
      helpers.failure(res,next,'Something went wrong while fetching user from the database',500);
    }
    if(user === null) {
      helpers.failure(res,next,'This user does not exist',404);
    }

    var f_posts = user.following_post;

    var pushDoc = function(item, callback) {
              if(item) {
                PostModel.findOne({ _id: item}, function(err, post) {

                  if(post != null) {
                    result.push(post);
                    //Return to function which called this function
                    callback();
                  }
                  else callback();
                });
              }
            };

//This function will call callback for each user following list to pushDoc function
            async_query.forEach(f_posts, pushDoc , function(err) {
              //err will be generated when finished traversing the error
              if(err)
                console.log(err);
                helpers.success(res,next,result);
            });

  });
});