Controllers are responsible for handling a request and producing a response. After routing has determined what controller to use, an action function will be invoked. This action function does the work of loading data and rendering views.

Defining a Controller

In Locomotive, controllers are instances of Controller. Defining a new controller is as simple as creating an instance and exporting it via the module.

var PhotosController = new Controller();

module.exports = PhotosController;


Functions attached to the controller are known as action functions. When Locomotive receives a request, it will create a new instance of the controller and call the appropriate action function.

For example, if a request is received for /photos/123, Locomotive will call the show() action of PhotosController.

PhotosController.show = function() {

Controllers are responsible for sending a response to the request. This is typically accomplished by render()ing a view or issuing a redirect().


Requests often contain data that the controller needs to access when building a response. The param() function is used to get data contained in the route, query, or body parameters.

SearchController.find = function() {
  this.query = this.param('query');
  // execute search query...

The value returned by param() will be found by checking parameters in the following order:

  • Check route params (req.params), ex: /photos/:id
  • Check query string params (req.query), ex: ?id=12
  • Check urlencoded body params (req.body), ex: id=12

Rendering a View

Rendering a view is accomplished by calling render(). Any instance variables attached to the controller will be made available to the view. By convention, variables named with a leading underscore are considered private and will not be made available to the view.

PhotosController.show = function() {
  // this._photo is "private", and not available in the view
  this.title = this._photo.title;
  this.description = this._photo.description;

The view found in views/photos/show.html.ejs will be rendered.

<h2><%= title %></h2>
<p><%= description %></p>
Render another Action's View

By default, the view corresponding to the current action will be rendered, in this case show. A different action's view can be rendered by specifying it as an argument:

Render another Controller's Action's View

If you want to render a view belonging to an action in an entirely separate controller, that can be accomplished as follows:

Rendering Options

Options can be used to render a different format or use a different template engine.

this.render({ format: 'xml' });
// => renders `action.xml.ejs`

this.render({ format: 'xml', engine: 'xmlb' });
// => renders `action.xml.xmlb`

Refer to formats for settings used to associate a format with a template engines.

Capturing Render Output

By default, the output of rendering is automatically sent as a response to the request. By passing a callback as the final argument to render(), the output can instead be captured.

this.render('email', { name: 'Jack' }, function(err, html) {
  // ...

This is useful in cases where you want to use a template to generate an email message, rather than a response.

Redirecting a Request

Instead of rendering a view, a controller may want to redirect the request.


Request and Response

If an application needs access to the raw request and response, each is available as an instance variable within the controller.

PhotosController.show = function() {
  var req = this.req;  // also aliased as this.request
  var res = this.res;  // also aliased as this.response

In Locomotive, requests are dispatched through Express and any middleware in use before arriving at the controller. As such, the entirety of the Express API and the core Node.js HTTP API apply to these objects.

In most cases, the controller provides functions to access commonly needed properties on the request and response, such as param(). It's recommended to use these functions whenever possible.