mongodb aggregate.js

aggregate.js

var mongo = require('mongodb').MongoClient;
var url = 'mongodb://localhost:27017/learnyoumongo';
var size = process.argv[2];
mongo.connect(url, function(err,db){
  if(err) throw err;
  var prices = db.collection('prices');
  prices.aggregate([{
    $match: {
      size: size
    }
  },{
    $group:{
      _id: 'avg',
      avg:{
        $avg: '$price'
      }
    }
  }]).toArray(function(err, results){
    if(err) throw err;
    var o = results[0];
    console.log(Number(o.avg).toFixed(2));
    db.close();
  });
});