Async method in NodeJS

Question

I'd like to make an addition using Async() in NodeJS but it doesn't work...

My code :

    var id = request.params.id;
    var self = this;
    var total;
    var asyncTasks = [];

asyncTasks.push(function(callback){
  self.orderDao.getAllOfUser(id).success(function (orders) {
    orders.forEach( function(order){
      total = total + order.price; // here I'd like to make the addition
      console.log(total);
    });
  });
  callback();
});

    async.parallel(asyncTasks, function(){
      self.orderDao.getAllOfUser(id).success(function (orders) {
        response.render('order/index', {orders: orders, total: total});
      });
    });

Result of total : NaN

Unrelated to your question, but shouldn't callback(); be moved up one block? Then it'll be called when your query & the loop are done. Right now async.parallel's callback might be executed before your queries have time to finish. — RickN
– RickN, Commented Oct 10, 2014 at 9:13

Michael · Accepted Answer · 2014-10-10 09:13:14Z

this is how its done with parallel, in you attempt the first callback was been called right after the start of getAllOfUser(id) without waiting for the response. it was just luck that your finish callback run long enough for the total aggregation to finish:

var id = request.params.id;
var self = this;

async.parallel({
  total: function(callback){
    self.orderDao.getAllOfUser(id).success(function (orders) {
      var total=0;
      orders.forEach( function(order){
        total = total + order.price; // here I'd like to make the addition
        console.log(total);
      });
      callback(null, total);
    });
  },
  orders: function (callback) {
    self.orderDao.getAllOfUser(id).success(function (orders) {
      callback(null, orders);
    });
  }
}, function(err, res){
  response.render('order/index', {orders: res.orders, total: res.total});
});

But there is a better solution where you wouldn't need to do getAllOfUser twice. like:

var id = request.params.id;
var self = this;
var total=0;
self.orderDao.getAllOfUser(id).success(function (orders) {
  orders.forEach( function(order){
    total = total + order.price; // here I'd like to make the addition
    console.log(total);
  });
  response.render('order/index', {orders: orders, total: total});
});

RickN · Accepted Answer · 2014-10-10 09:09:37Z

0

At the top of your script, you do:

var total;

This initialises the variable with the value undefined: typeof total === "undefined"; //true

When you do this:

total = total + order.price;

You're actually doing this:

total = undefined + someNumber

Which is NaN, because undefined+5 is not a number.

To fix this, change the declaration at the top of your script to:

var total = 0;

Also you can (but don't have to) shorten the addition in the loop to:

total += order.price;

answered Oct 10, 2014 at 9:09

RickN

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