Meteor.js local collection change hooks

test.js

var Layers = App.Collection('layers');

Layers.find().observeChanges({
	'added': function (id, fields) {
		var doc = this.results._map[id];
		
		// now we could access and modify the doc
		// ---------------------------------------------------------------------
		// here I wanna know a type of the layer and then fill some default 
		// properties for doc

		if(fields.type && fields.type == 'text')
			doc.addedText = "original has text";

		if(fields.type && fields.type == 'image')
			doc.addedImage = "original has image";

		if(fields.type && fields.type == 'svg')
			doc.addedSvg = "original has svg";
	},

	'changed': function (id, fields) {
		var doc = this.results._map[id];

		// now we could access and modify the doc
		// ---------------------------------------------------------------------
		// here I wanna know and store increment of "x".
		// Lets calculate "delta" as "fields.x" minus "doc.lastx" and write the value
		// in "delta" field

		if(fields.x) {
			var delta = fields.x - doc.lastx || fields.x;
			doc.delta = delta;
			doc.lastx = doc.lastx || fields.x;
		}
	}
});

// test added
// ---------------------------------------------------------------------

Layers._collection.insert({'_id': '00000000000', type: 'text'});
Layers._collection.insert({'_id': '00000000001', type: 'image'});
Layers._collection.insert({'_id': '00000000002', type: 'svg'});

console.log(Layers.find({}, {reactive: false}).fetch());

// test added
// ---------------------------------------------------------------------

Layers._collection.remove({});
Layers._collection.insert({'_id': '00000000000'});

Layers._collection.update('00000000000', {$set: {'x': 500}});

Layers._collection.update('00000000000', {$inc: {'x': Math.random() * 10}}, function () {
	console.log(Layers.findOne('00000000000'))

	Layers._collection.update('00000000000', {$set: {'x': Math.random() * 100}}, function () {
		console.log(Layers.findOne('00000000000'))

		Layers._collection.update('00000000000', {$inc: {'x': Math.random() * 1000}}, function () {
			console.log(Layers.findOne('00000000000'))
		});

	});
});