Controllers : Formats

Part of Locomotive's support for REST principles is the ability to represent a single resource with multiple data formats. In technical terms this is known as content negotiation, as it gives a client the ability to specify what format it prefers to receive in response to a request.


When rendering multiple data formats, it is advisable to use a template engine suited to the format. For example, Jade is an excellent choice for rendering HTML, but is not well-suited for rendering XML or JSON.

Template engines for formats are typically configured in config/initializers/02_views.js. For example, the following declares that xmlb be used when rendering responses in XML format.

this.format('xml', { engine: 'xmlb' });


Responding with the requested format is done by calling respond(), which takes an object containing format keys as an argument:

PostsController.index = function() {
    'xml': { template: 'feed' },
    'html': { template: 'index' }

The above code will render views/posts/feed.xml.xmlb for any client that requests content in XML format, while rendering views/posts/index.html.ejs for clients wanting HTML.

respond() works by automatically calling render(), passing along any options associated with the format key. If default options are sufficient, the format key can simply be set to true. For example, because the template is automatically derived from the action, the above could be reduced to:

  'xml': { template: 'feed' },
  'html': true
Default Format

By default, if a client requests a format that is not supported, Locomotive will respond with a 406 Not Acceptable error. If instead it is desired to respond using a default format, set a default format key.

  'xml': { template: 'feed' },
  default: { format: 'html' }

The above will continue to respond with XML to clients that request it, while rendering HTML for all other requests (for HTML or otherwise).

Fine-Grained Handling

If an application needs to perform specific handling when responding with a format, the format key can be set to a function. Instead of automatically calling render(), the function will be invoked allowing the application to handle the request as needed.

var self = this;
  'json': function() { self.res.json({ hello: 'world' }); },
  default: function() { self.render(); }