How does respond_to and respond_with work in rails?

respond_with

For a given controller action, respond_with generates an appropriate response based on the mime-type requested by the client.

This basically means your controller will send the appropriate data on a request basis - for example, if you did the following:

#app/controllers/articles_controller.rbClass ArticlesController < ApplicationController   def show       @article = Article.find params[:id]       respond_with @article   endend

This would basically respond with the data from @article each time you send a request. If the request is in the json mime-type, it will come back as a JSON object; if it's an HTML request, it will come back with the HTML object on the show view

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respond_to

Basically allows you to tailor the specific responses to different mime-types. If you send a JS request, you can manage the JS response etc

respond_to blocks inside the controller actions are very cumbersome, and only really meant for specific changes / edits to the response itself.

A much simpler way to handle the respond_to is to declare it at the top of the controller file, essentially telling Rails that every action will use the options defined in that method:

#app/controllers/your_controller.rbClass YourController < ApplicationController   respond_to :js, :json, :html #-> the same as using respond_to block for each actionend

note that in Rails 4 respond_with feature has been extracted to the gem 'responders' (https://github.com/plataformatec/responders).

The cases of when you need/don't need each format.*whatever* line.

Normally, you don't need any of it. Rails by default looks for an html file (aka template) matching the action name under the view folder matching the controller's name.

I'm not sure when/why the json and html are sometimes paired together(like in the scaffold-generated code). Perhaps the json line is for turbolinks (please confirm/correct this). But I do know you use a respond_to block with various types of format lines when you want each type to behave differently (e.g. serve up 10 results at a time through js, but more results through html).

The js format is needed when you use remote: true in a form or link. This is because Using this remote: true disables the html template from being served up, and instead looks for a js file matching the action's name, and executes/renders that file. You don't actually need the respond to json line if you're doing things in js only.

Bonus hint: if your js files have js.erb, you can access the instance variables (how about local variables? please confirm/correct this) that you set in your action. This kind of makes sense because your *.js.erb file is technically a view. Views can access its corresponding actions' variables (hmm what about when vies get rendered from another controller?). So if you already have access to your action's variables in your js file, this can eliminate the need to make ajax calls or json calls in many situations.

I'm actually not sure when you need the json line when also using remote: true / javascript. explicit jQuery.ajax() method calls that want json data might warrant the use of the respond to json line.