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can.batch.start( batchStopHandler ) and can.batch.stop( force, callStart ) are used to specify atomic operations. start prevents change events from being fired until stop is called.

This player has a favorite tvshow. We are listening for any changes to her preferences.

var player = new can.Map({
  tvshow: "The Simpsons"

player.bind("change", function(ev, attr, how, newVal, oldVal) {
  console.log("[change triggered] " + how + ": " + attr);

Normally, if a favorite tvshow were replaced with a favorite song, the "change" callback handler would immediately be called when tvshow is removed and when song is added.

By incorporating can.batch, the calls to the "change" callback handler will not occur until after tvshow is removed, song is added, and can.batch.stop is called.


player.attr("song", "What makes you beautiful");


Performance and correctness are the two most common reasons to use batch operations.


Sometimes, an object can temporarily be in an invalid state. For example, the previous player should have a tvshow or song property, but not both. Event listeners should never be called in an intermediate state. The can/map/define plugin uses can.batch.start and can.batch.stop to accomplish this when calling a setter.

import "can/map/define/"

var Player = can.Map.extend({
  define: {
    tvshow: {
      set: function(newValue) {

        return newValue;
    song: {
      set: function(newValue) {

        return newValue;

var player = new Player({ song: "Amish Paradise" });

player.bind("change", function(ev, attr, how, newVal, oldVal){
  var song = this.attr("song");
  var tvshow = this.attr("tvshow");

  if(song) {
    console.log("The greatest song is " + song + ". TV is overrated.");
  if(tvshow) {
    console.log("The greatest TV show is " + tvshow +
      ". Music is overrated.");

player.attr("tvshow", "Breaking Bad");

Use can.batch.start and can.batch.stop to ensure that events are only triggered when a subject is in a valid state.


CanJS synchronously dispatches events when a property changes. This makes certain patterns easier. For example, if you are utilizing live-binding and change a property, the DOM is immediately updated.

Occasionally, you may find yourself changing many properties at once. To prevent live-binding from performing unnecessary updates, update the properties within a pair of calls to can.batch.start and can.batch.stop.

Consider this list of items.

var items = new Items([
  {selected: false},
  {selected: true},
  {selected: false}

This template renders the number of selected items.

var renderer = can.stache("{{count}}");

  count: function() {
    var count = 0;
    items.each(function(item) {
      count += item.attr("selected") ? 1 : 0;
    return count;

Using can.batch will ensure that the DOM is only updated once the whole list of items has been updated instead of every time an individual item is flipped.

$("#selectAll").click(function() {
    item.attr('selected', true);


All events created within a set of start / stop calls share the same batchNum value. This can be used to respond only once for a given batchNum.

var batchNum;
obs.bind("change", function(ev, attr, how, newVal, oldVal) {
  if(!ev.batchNum || ev.batchNum !== batchNum) {
    batchNum = ev.batchNum;
    // your code here!

Automatic Batching

Libraries like Angular and Ember always batch operations. This behavior can be reproduced by batching everything that happens within the same thread of execution and/or within 10ms of each other.

setTimeout(function() {
  can.batch.stop(true, true);
  setTimeout(arguments.callee, 10);
}, 10);