A blazing fast JSON parser and generator in pure Elixir.

elixir, json



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, "~> 1.1"}]

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


You need to define a custom "types" module in an .ex file, somewhere in lib:

Postgrex.Types.define(MyApp.PostgresTypes, [], json: Jason)

## If using with ecto, you also need to pass ecto default extensions:

Postgrex.Types.define(MyApp.PostgresTypes, [] ++ Ecto.Adapters.Postgres.extensions(), json: Jason)

Then you can use the module, by passing it to Postgrex.start_link.


To replicate fully the current behaviour of Poison when used in Ecto applications, you need to configure Jason to be the default encoder in config/config.exs:

config :ecto, json_library: Jason

Additionally, when using PostgreSQL, you need to define a custom types module as described above, and configure your repo to use it (in either config/config.exs or config/<env>.exs):

config :my_app, MyApp.Repo, types: MyApp.PostgresTypes

Plug (and Phoenix)

First, you need to configure Plug.Parsers to use Jason for parsing JSON. You need to find, where you're plugging the Plug.Parsers plug (in case of Phoenix, it will be in the Endpoint module) and configure it in your endpoint module (lib/app_web/endpoint.ex), for example:

plug Plug.Parsers,
  parsers: [:urlencoded, :multipart, :json],
  pass: ["*/*"],
  json_decoder: Jason

Additionally, for Phoenix, you need to configure the "encoder" in config/config.exs:

config :phoenix, :format_encoders,
  json: Jason

A custom JSON encoder for Phoenix channels is unfortunately a bit more involved, you can find code for a custom serializer and how to use it in here.


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 with 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 2.0 License - 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.