This extension is part of the Sinatra::Contrib project. Run gem install sinatra-contrib to have it available.


Sinatra::JSON adds a helper method, called json, for (obviously) json generation.


Classic Application

In a classic application simply require the helper, and start using it:

require "sinatra"
require "sinatra/json"

# define a route that uses the helper
get '/' do
  json :foo => 'bar'

# The rest of your classic application code goes here...

Modular Application

In a modular application you need to require the helper, and then tell the application you will use it:

require "sinatra/base"
require "sinatra/json"

class MyApp < Sinatra::Base

  # define a route that uses the helper
  get '/' do
    json :foo => 'bar'

  # The rest of your modular application code goes here...


By default it will try to call to_json on the object, but if it doesn't respond to that message, it will use its own rather simple encoder. You can easily change that anyways. To use JSON, simply require it:

require 'json'

The same goes for Yajl::Encoder:

require 'yajl'

For other encoders, besides requiring them, you need to define the :json_encoder setting. For instance, for the Whatever encoder:

require 'whatever'
set :json_encoder, Whatever

To force json to simply call to_json on the object:

set :json_encoder, :to_json

Actually, it can call any method:

set :json_encoder, :my_fancy_json_method


It will automatically set the content type to “application/json”. As usual, you can easily change that, with the :json_content_type setting:

set :json_content_type, :js

Overriding the Encoder and the Content-Type

The json helper will also take two options :encoder and :content_type. The values of this options are the same as the :json_encoder and :json_content_type settings, respectively. You can also pass those to the json method:

get '/'  do
  json({:foo => 'bar'}, :encoder => :to_json, :content_type => :js)
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