Setting id and className dynamically in Backbone.js views

Question

I am in process of learning and using Backbone.js.

I have an Item model and a corresponding Item view. Each model instance has item_class and item_id attributes, that I want to be reflected in as the 'id' and 'class' attributes of the corresponding view. What's the correct way to achieve this ?

Example:

var ItemModel = Backbone.Model.extend({      
});

var item1 = new ItemModel({item_class: "nice", item_id: "id1"});
var item2 = new ItemModel({item_class: "sad", item_id: "id2"});

var ItemView = Backbone.View.extend({       
});

How should I implement the view so that the the views 'el's will translate to:

<div id="id1" class="nice"></div>
<div id="id2" class="sad"> </div>

In most examples I have seen, the view's el serves as a meaningless wrapper element inside which one has to manually write the 'semantic' code.

var ItemView = Backbone.View.extend({
   tagName:  "div",   // I know it's the default...

   render: function() {
     $(this.el).html("<div id="id1" class="nice"> Some stuff </div>");
   }       
});

So when rendered, one gets

<div> <!-- el wrapper -->
    <div id="id1" class="nice"> Some stuff </div>
</div>

But this seems like a waste - why have the external div ? I want the el to translate directly into the internal div!

JMM JMM · Accepted Answer · 2012-04-26T16:04:06

Summary: dynamically set view attributes with model data

http://jsfiddle.net/5wd0ma8b/

// View class with `attributes` method
var View = Backbone.View.extend( {
  attributes : function () {
    // Return model data
    return {
      class : this.model.get( 'item_class' ),
      id : this.model.get( 'item_id' )
    };
  }
  // attributes
} );

// Pass model to view constructor
var item = new View( {
  model : new Backbone.Model( {
    item_class : "nice",
    item_id : "id1"
  } )
} );

This example assumes that you're allowing Backbone to generate a DOM element for you.
The attributes method is called after the properties passed to the view constructor are set (in this case, model), allowing you to dynamically set the attributes with the model data before Backbone creates el.
In contrast to some of the other answers: doesn't hard-code attribute values in the view class, dynamically sets them from model data; doesn't wait until render() to set attr vals; doesn't repeatedly set attr vals in every call to render(); doesn't unnecessarily manually set attr vals on DOM element.
Note that if setting the class when calling Backbone.View.extend or a view constructor (e.g. new Backbone.View), you have to use the DOM property name, className, but if setting it via the attributes hash / method (as in this example) you have to use the attribute name, class.
As of Backbone 0.9.9:

When declaring a View...el, tagName, id and className may now be defined as functions, if you want their values to be determined at runtime.

I mention this in case there's a situation where that would be useful as an alternative to using an attributes method as illustrated.

Using an existing element

If you're using an existing element (e.g. passing el to the view constructor)...

var item = new View( { el : some_el } );

...then attributes won't be applied to the element. If the desired attributes aren't already set on the element, or you don't want to duplicate that data in your view class and another location, then you may want to add an initialize method to your view constructor that applies attributes to el. Something like this (using jQuery.attr):

View.prototype.initialize = function ( options ) {
  this.$el.attr( _.result( this, 'attributes' ) );
};

Usage of `el`, rendering, avoiding the wrapper

In most examples I have seen, the view's el serves as a meaningless wrapper element inside which one has to manually write the 'semantic' code.

There's no reason view.el needs to be "a meaningless wrapper element". In fact, that would often break the DOM structure. If a view class represents a <li> element for example, it needs to be rendered as an <li> -- rendering it as a <div> or any other element would break the content model. You'll likely want to focus on correctly setting up your view's element (using properties like tagName, className, and id) and then rendering its content thereafter.

The options for how to have your Backbone view objects interact with the DOM are wide open. There are 2 basic initial scenarios:

You can attach an existing DOM element to a Backbone view.
You can allow Backbone to create a new element that is disconnected from the document, then somehow insert it into the document.

There are various ways you can generate the content for the element (set a literal string, as in your example; use a templating library like Mustache, Handlebars, etc.). How you should use the el property of the view depends what you're doing.

Existing element

Your rendering example suggests that you have an existing element that you're assigning to the view, although you don't show instantiation of the views. If that's the case, and the element is already in the document, then you may want to do something like this (update the content of el, but don't alter el itself):

render : function () {
  this.$el.html( "Some stuff" );
}

http://jsfiddle.net/vQMa2/1/

Generated element

Let's say you don't have an existing element and you allow Backbone to generate one for you. You may want to do something like this (but it's likely better to architect things so that your view isn't responsible for knowing about anything outside itself):

render : function () {
  this.$el.html( "Some stuff" );
  $( "#some-container" ).append( this.el );
}

http://jsfiddle.net/vQMa2/

Templates

In my case, I'm using templates, e.g.:

<div class="player" id="{{id}}">
<input name="name" value="{{name}}" />
<input name="score" value="{{score}}" />
</div>
<!-- .player -->

The template represents the complete view. In other words, there will be no wrapper around the template -- div.player will be the root or outermost element of my view.

My player class will look something like this (with very simplified example of render()):

Backbone.View.extend( {
  tagName : 'div',
  className : 'player',

  attributes : function () {
    return {
      id : "player-" + this.model.cid
    };
  },
  // attributes

  render : function {
    var rendered_template = $( ... );

    // Note that since the top level element in my template (and therefore
    // in `rendered_template`) represents the same element as `this.el`, I'm
    // extracting the content of `rendered_template`'s top level element and
    // replacing the content of `this.el` with that.
    this.$el.empty().append( rendered_template.children() );
  }      
} );