Computed properties in Backbone

I have a scenario where the data being manipulated on the client is presented and interacted with in a different way than it is represented on the server.

Consider the following event resource returned from the server.

  "id": 123,
  "start_at": 1331336004906,
  "end_at": 1331337704906

And the following template for editing:

  <!-- Notice how date and time are separated in the interface -->
  <input type="text" name="start_date" value="{{start_date}}" />
  <input type="text" name="start_time" value="{{start_time}}" />

  <!-- Instead of asking for an end date/time, we ask for the duration -->
  <input type="text" name="duration" value="{{duration}}" />

  <input type="submit" />

How would I go about treating start_date, start_time, and duration as attributes in my Backbone model without sending them to the server? Am I supposed to modify .toJSON()?


Your model should correspond as closely as possible to what you have server side. So stick with start_at and end_at. That will greatly simplify your sync() operations.

On your edit form's View, you can:

  1. Compute start_date, start_time, duration through simple functions and call them in the template.
  2. Convert to start_at and end_at on submitting.

I am using a combination of the initialize() function together with change event listeners to update derived attributes. The idea is first to compute the attributes on model initialization, and second to let the model listen to its own changes and update the attributes accordingly.

My solution looks roughly like this:

MyModel: Backbone.Model.extend({
    initialize: function() {
        this.on('change:start_at', this.updateDerivedAttributes, this);
    updateDerivedAttributes: function() {
            start_date: Utils.dateFromDate( this.get( "start_at" ) ),
            start_time: Utils.timeFromDate( this.get( "start_at" ) ),
            duration: Utils.youGetTheIdea()
        }, {silent:true});

We are very used to send model.toJSON() to feed the template. And this method is very tricky to overwrite because is used by other components.

But we can feed the template with a custom model.toJSONDecorated() method that can look like this:

# in your Backbone.Model
toJSONDecorated: function(){
        start_date : Utils.dateFromDate( this.get( "start_at" ) ),
        start_time : Utils.timeFromDate( this.get( "start_at" ) ),
        duration   : Utils.youGetTheIdea( :) )

Of course this is breaking a few patterns, I can live with it, if you don't you can move this logic to a Decorator class as people have suggested in other answers.

You have some options:

  • Override toJSON to return the computed duration

  • Create a duration() method on the Backbone Model. Only downside that you have to call it differently (obj.duration() instead of obj.get('duration')). In your view that hands obj.toJSON() to your template, mix in the duration attribute

  • Use (or similar) to create a derived getter for duration

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