May 19th, 2013 by pheuter


Knowing when an element is inserted into the DOM is an essential piece of information when it comes to developing web applications that depend on various UI states (such as height).

Backbone, especially Marionette, does an excellent job providing developers with intuitive constructs for encapsulating and working with DOM elements. From initializers to destructors you can track the state of your Views with ease. Unfortunately, the View does not have any notion of being inserted into the DOM.

Note, render does not imply a DOM insertion. What usually happens when a Backbone.View is rendered is its el gets populated with some data, but that el can be sitting somewhere in memory detached from the DOM.

It is possible to design your applications so that you can execute DOM-dependent logic after you know that the View you are working with should have been inserted into the DOM. Marionette's Region triggers an onShow event on a View that has just been rendered onto a Region that exists in the DOM, implying that the View should be in the DOM as well.

There are some drawbacks to this pattern. While sometimes it is good design, you may not always want to show a View onto a Region. Coupling your Views to Regions just for the onShow may be overhead you could be better off without. Also, you may start to find your views implementing logic in onShow that should go into an onRender instead. onShow implies your View will be part of some layout or parent view but you may prefer to de-couple your views from their implementation and not have to constrain yourself.

This is where Backbone-Domo comes in.

Domo arigato, Mr. Roboto

Backbone-Domo utilizes Mutation observers to efficiently react when your View's el gets inserted into the DOM and notify the View via an event.

Many browsers currently do not support Mutation observers, but luckily the Polymer team developed a polyfill that Domo utilizes as a fallback.


Using Backbone-Domo in your views couldn't get any simpler:

initialize: function() {
  Backbone.Domo.arigato(this, 'dom:insert');

  this.listenTo(this, 'dom:insert', function(event) {
    console.log("View is now in the DOM!");
    console.log("View width: " + this.$el.width())

When you're ready to close your view:

close: function() {

For more, check out the repo on GitHub.