In Rails 3.1 the only difference I can spot is that the if formatter is js, then the answer is sent as plain text. If it is json then it is encoded as json.

if format.json

format.json {

  render :json => @accounts.map(&:attributes)

}

server send:

data:application/json;base64,

if format.js

format.js {

  render :json => @accounts.map(&:attributes)

}

server send:

[{id:33,email:[email protected]}]