It is modified version on Bob Ippolito's <bob@mochimedia.com> mochijson2 module, one can find in excellent mochiweb, erlang webserver.

Yet another JSON (RFC 4627) library for Erlang. jsonerl works with binaries as strings, arrays as lists (without an {array, _}) wrapper and it only knows how to decode UTF-8 (and ASCII).

It has the following mapping:

Erlang data type:        =>          JavaScript data type:        =>          Erlang data type:
-----------------------------------------------------------------------------------------------

1.    atom true                            boolean true                             atom true  

2.    atom false                           boolean false                            atom false

3.    atom undefined                       null                                     atom  undefined                         

4.    any other atom                       string                                   binary holding string

5.    list                                 array                                    list
 
6.    number                               number                                   number

7.    binary                               string                                   binary   

8.    proptuple                            object                                   proptuple
-----------------------------------------------------------------------------------------------

Note: proptuple is result of passing proplist to list_to_atom bif ;)
This is concept of mine to nicely model javascript object without a need of tagging structures on erlang side.

Eg. JavaScript object 

{
     name: "Luc Besson",
     year_of_birth: 1959,
     city: "Paris",
     photo: "http://is.gd/3pYJF",
     movies: ["Taken","Bandidas","Taxi"]
}

results in

{
  {<<"name">>, <<"Luc Besson">>},
  {<<"year_of_birth">>, 1959},
  {<<"city">>, <<"Paris">>},
  {<<"photo">>, <<"http://is.gd/3pYJF">>},
  {<<"movies">>, [<<"Taken">>,<<"Bandidas">>,<<"Taxi">>]}
}

-----------------------------------------------------------------------------------------------

Additionally there is utility macros delivered with jsonerl that helps turning erlang records to json and back. 

For example:

-include("jsonerl.hrl").
-record(artist, {name, year_of_birth, city, photo, movies}).

Artist = #artist{name = <<"Luc Besson">>, year_of_birth = 1959, city = <<"Paris">>, photo = <<"http://is.gd/3pYJF">>, movies = [<<"Taken">>,<<"Bandidas">>,<<"Taxi">>]},
Json = ?record_to_json(artist, Artist),
Artist = ?json_to_record(artist, Json).


the resulting json will be:

<pre lang="javascript" line="1">
{
     name: "Luc Besson",
     year_of_birth: 1959,
     city: "Paris",
     photo: "http://is.gd/3pYJF",
     movies: ["Taken","Bandidas","Taxi"]
}