Backbone Views in a Dynamic DOM

When you start writing applications with BackboneJS, that means that you're dealing with some dynamic UI. There are elements being pushed into and pulled out of the DOM. I was having problems trying to reference DOM elements that were dynamically inserted into the DOM by other Backbone views. The fix was simple but not immediately obvious without cracking open Backbone.

Backbone View Element

The el property of a Backbone.View is the dom element within which your view contents will be displayed. By default it's an empty div, otherwise it can be specified by a combination of tagName, className, and id properties. For these options, Backbone assumes that your DOM element isn't living within the document; rather, it will be created in memory and then inserted where you later specify.

If you want to specify a pre-existing DOM element within which to insert your Backbone view contents, you set the el property of your view directly. This is where I ran into problems.

Shifting Plates of the DOM

Backbone views assume the $ alias is either jQueryor Zepto. They do the same thing, just use css-like selectors to get references to DOM elements. So, I would have a view and set the el property to a DOM element selected by jQuery (note: examples use RequireJS to manage dependencies), such as MountainView.js:

define(function () {
  return Backbone.View.extend({
    el: $('#mountains'),
    render: function () {
      $(this.el).html(makePrettyMountains());
      return this;
    }
  });      
});

Problem is that earlier in the code, another view was being relied upon to insert the #mountains element into the DOM. Let's say SceneryStarterView.js looked something like this:

define(['MountainView'], function (MountainView) {
  return Backbone.View.extend({
    initialize: function () {
      $('body').html(makePlaceForMountains());
      var mountain = new MountainView();
      mountain.render().el;
    }
  });
});

It was importing (via RequireJS) MountainView.js. At import time, $('#mountains') was evaluated but nothing was found, because makePlaceForMountains() hadn't been called yet. It was looking for #mountains too early! How import the needed view, let it specify somewhere on the DOM that doesn't exist yet but will by the time it's instantiated and then rendered? Backbone has a mechanism for this...

Into the Asthenosphere

To find the answer, I had to peel back the Backbone crust and do a little dumpster diving. When my MountainView is new'ed up, the constructor for Backbone.View is hit. It calls a function called _ensureElement():

// Ensure that the View has a DOM element to render into.
// If `this.el` is a string, pass it through `$()`, take the first
// matching element, and re-assign it to `el`. Otherwise, create
// an element from the `id`, `className` and `tagName` proeprties.
_ensureElement : function() {
  if (!this.el) {
    var attrs = this.attributes || {};
    if (this.id) attrs.id = this.id;
    if (this.className) attrs['class'] = this.className;
    this.el = this.make(this.tagName, attrs);
  } else if (_.isString(this.el)) {
    this.el = $(this.el).get(0);
  }
}

Hmmmm... If there isn't an el field, grab id, className, and tagName and create an in-memory element with document.createElement(). And it's the else if that gives us the option we need. If the el property is a string, now try and select it with jQuery. Perfect! If we change our code for MountainView.js to reflect that one change:

define(function () {
  return Backbone.View.extend({
    el: '#mountains',
    // render, etc
  });      
});

It works like a charm. Why? Now the order of events is:

• SceneryStartView is instantiated somewhere
• It imports MountainView as a dependency
• SceneryStartView inserts the #mountians DOM element
• Then it news up MountainView
• In the constructor for Backbone.View, jQuery selects my string value for el
• One more round of hallelujah is sung!