A blazing fast JSON parser and generator in pure Elixir.

The parser and generator are at least twice as fast as other Elixir/Erlang libraries (most notably Poison). The performance is comparable to jiffy, which is implemented in C as a NIF. Jason is usually only twice as slow.

Both parser and generator fully conform to RFC 8259 and ECMA 404 standards. The parser is tested using JSONTestSuite.


The package can be installed by adding jason to your list of dependencies in mix.exs:

def deps do
  [{:jason, "~> 1.2"}]

Basic Usage

iex(1)> Jason.encode!(%{"age" => 44, "name" => "Steve Irwin", "nationality" => "Australian"})
"{\"age\":44,\"name\":\"Steve Irwin\",\"nationality\":\"Australian\"}"

iex(2)> Jason.decode!(~s({"age":44,"name":"Steve Irwin","nationality":"Australian"}))
%{"age" => 44, "name" => "Steve Irwin", "nationality" => "Australian"}

Full documentation can be found at https://hexdocs.pm/jason.

Use with other libraries


Versions starting at 0.14.0 use Jason by default. For earlier versions, please refer to previous versions of this document.


Versions starting at 3.0.0 use Jason by default. For earlier versions, please refer to previous versions of this document.

Plug (and Phoenix)

Phoenix starting at 1.4.0 uses Jason by default. For earlier versions, please refer to previous versions of this document.


You need to pass the :json_codec option to Absinthe.Plug

# When called directly:
plug Absinthe.Plug,
  schema: MyApp.Schema,
  json_codec: Jason

# When used in phoenix router:
forward "/api",
  to: Absinthe.Plug,
  init_opts: [schema: MyApp.Schema, json_codec: Jason]


Detailed benchmarks (including memory measurements): https://gist.github.com/michalmuskala/4d64a5a7696ca84ac7c169a0206640d5

HTML reports for the benchmark (only performance measurements): http://michal.muskala.eu/jason/decode.html and http://michal.muskala.eu/jason/encode.html


Benchmarks against most popular Elixir & Erlang json libraries can be executed after going into the bench/ folder and then executing mix bench.encode and mix bench.decode. A HTML report of the benchmarks (after their execution) can be found in bench/output/encode.html and bench/output/decode.html respectively.

Differences to Poison

Jason has a couple feature differences compared to Poison.

  • Jason follows the JSON spec more strictly, for example it does not allow unescaped newline characters in JSON strings - e.g. "\"\n\"" will produce a decoding error.
  • no support for decoding into data structures (the as: option).
  • no built-in encoders for MapSet, Range and Stream.
  • no support for encoding arbitrary structs - explicit implementation of the Jason.Encoder protocol is always required.
  • different pretty-printing customisation options (default pretty: true works the same)

If you require encoders for any of the unsupported collection types, I suggest adding the needed implementations directly to your project:

defimpl Jason.Encoder, for: [MapSet, Range, Stream] do
  def encode(struct, opts) do
    Jason.Encode.list(Enum.to_list(struct), opts)

If you need to encode some struct that does not implement the protocol, if you own the struct, you can derive the implementation specifying which fields should be encoded to JSON:

@derive {Jason.Encoder, only: [....]}
defstruct # ...

It is also possible to encode all fields, although this should be used carefully to avoid accidentally leaking private information when new fields are added:

@derive Jason.Encoder
defstruct # ...

Finally, if you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:

Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])
Protocol.derive(Jason.Encoder, NameOfTheStruct)


Jason is released under the Apache License 2.0 - see the LICENSE file.

Some elements of tests and benchmarks have their origins in the Poison library and were initially licensed under CC0-1.0.