Issue while getting array value in NodeJS

Question

Issue while getting array value in NodeJS

I am using NodeJS to count the number of employees in different section. I am using Mongoose as ODM and MongoDB as database.That is my code (very simple for testing purposes).

   exports.list= function( req, res){
    var array = ['sectionA', 'sectionB'];
    var i;
    for(i = 0; i<array.length; i++){
        Issue.count({ 'section.name': array[i]}, function (err, count) {
          console.log('Section name is: ' + array[i] + ' number of employees: ' + count );
        )};
     }
    }

But the value of array[i] is undefined inside Issue.count({ 'section.name': array[i]}, function (err, count) {});. But the value of count is absolutely right. I want an output like:

Section name is: sectionA number of employees: 50
Section name is: sectionB number of employees: 100

But my current output is

 Section name is: undefined number of employees: 50
 Section name is: undefined number of employees: 100

This is because value of i inside Issue.count({ 'section.name': array[i]}, function (err, count) {}); is always 2.

javascript
node.js
express
mongoose

Answer 1

Is it possible that Issue.count function is asynchronous? So your loop is completing before the callback of:

function (err, count) {
  console.log('Section name is: ' + array[i] + ' number of employees: ' + count );
}

is executed. When the callbacks are executed the value of i is undefined as a result.

Answer 2

@eshortie is correct: Issue.count is asynchronous and that's causing the problem.

Here's a solution:

for (i = 0; i<array.length; i++) {
  Issue.count({ 'section.name': array[i]}, function(sectionName, err, count) {
    console.log('Section name is: ' + sectionName + ' number of employees: ' + count );
  }.bind(null, array[i]));
}

Answer 3

Don't try to execute asynchronous functions using a regular for loop. It is asking for problems. Use async.eachSeries or async.each instead https://github.com/caolan/async#eachseriesarr-iterator-callback

var async = require('async')
var Issue = {} // mongoose isue model here
var elements = ['foo', 'bar']
async.eachSeries(
  elements,
  function(name, cb) {
    var query = {
      'section.name': name
    } 
    Issue.count(query, function(err, count) {
      if (err) { return cb(err) }
      console.dir(name, count)
    })
  },
  function(err) {
    if (err) {
      console.dir(err)
    }
    console.log('done getting all counts')
  }
)