rails respond_to format.js API

respond_to do |format|
  format.js # actually means: if the client ask for js -> return file.js
end

js here specifies a mime-type that the controller method would send back as a response;
Default Rails mime-types.
If you try also with format.yaml:

respond_to do |format|
  format.js
  format.yaml
end

that will mean that your controller will return yml or js depending on what the client-side is asking;

{} in terms of ruby is a block;
If you don’t specify any rails will try to render a default file from app/views/[contoller name]/[controller method name].[html/js/…]

# app/controllers/some_controller.rb
def hello
  respond_to do |format|
    format.js
  end
end

will look for /app/views/some/hello.js.erb; // at least in Rails v. 2.3.

If you do specify block:

respond_to do |format|
    # that will mean to send a javascript code to client-side;
    format.js { render             
        # raw javascript to be executed on client-side
        "alert('Hello Rails');", 
        # send HTTP response code on header
        :status => 404, # page not found
        # load /app/views/your-controller/different_action.js.erb
        :action => "different_action",
        # send json file with @line_item variable as json
        :json => @line_item,
        :file => filename,
        :text => "OK",
        # the :location option to set the HTTP Location header
        :location => path_to_controller_method_url(argument)
      }

  end