What's the use case of 'respond_to' in rails?

Question

respond_to do |format|
  format.html
  format.xml  { render :xml => @mah_blogz }
end

respond_to do |format|
      format.js
end

What's this respond_to, format.html, format.xml and format.js? What's their purpose and how do they work?

Answer 1

Here's the link to the documentation

http://api.rubyonrails.org/classes/ActionController/MimeResponds/ClassMethods.html#method-i-respond_to

Its a way of responding to the client based on what they are asking for, if the client asks for HTML, Rails will send back HTML to the client, if they ask for XML then XML.

Answer 2

Say you are doing this:

    class UsersController < ApplicationController

      def create
        #
        #your code
        #

        respond_to do |format|
          format.xml {render :xml => xxx}
          format.json {render :json => xxx}
          format.html {render xxx}
        end
      end

      def edit
        #
        #your code
        #

        respond_to do |format|
          format.xml {render :xml => xxx}
          format.json {render :json => xxx}
          format.html {render xxx}
        end
      end

    end

rather do:

    class UsersController < ApplicationController

      respond_to :xml, :json, :html

      def create
        #
        #your code
        #

        respond_with xxx

      end

      def edit
        #
        #your code
        #

        respond_with xxx

      end

    end

and thats how you keep the code DRY (Dont Repeat Yourself)