aggregate.js

/*
output the average price
of the items that match
the size criteria input
the average is converted
to number and rounded to
2 decimals
*/

var mongoClient = require('mongodb').MongoClient;

var collectionName = 'prices';

var databaseName = 'learnyoumongo';

var size = process.argv[2];

var url = 'mongodb://localhost:27017/learnyoumongo';

mongoClient.connect(url, function(err, DB) {
	var dataBase = DB.db(databaseName);

	dataBase.collection(collectionName)
			.aggregate([
				{ $match: { size: size } },
				{ $group: {
					_id: 'average',
					total: { $avg: '$price' } 
				} }
			]).toArray(function(err, results) {
				if (err) {
					console.log(err);
				}
				if (!results.length) {
					throw new Error('No results found')
				}
				console.log(Number(results[0].total).toFixed(2));
			});

	DB.close();

});