Jason Documentation

To use Jason in your Elixir project, add it to your deps in mix.exs.

def deps do
  [{:jason, "~> 1.4"}]
end

When using Absinthe, you must explicitly pass json_codec: Jason to Absinthe.Plug via the :json_codec option.

# 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]

If you own a struct, you can use @derive to implement the Jason.Encoder protocol. You can specify which fields to include using the only: option, or encode all fields by omitting it.

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

# Encode all fields (use carefully)
@derive Jason.Encoder
defstruct # ...

Jason does not support decoding into specific data structures (no as: option) and does not have built-in encoders for MapSet, Range, or Stream. You can implement the Jason.Encoder protocol manually for these types.

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

To avoid unnecessary decoding/encoding roundtrips, use Jason.Fragment.new/1 to mark parts of a structure that are already valid JSON strings.

already_encoded_json = Jason.encode!(%{hello: "world"})
Jason.encode!(%{foo: Jason.Fragment.new(already_encoded_json)})