module JSON


The JSON module allows parsing and generating JSON documents.

NOTE To use JSON or its children, you must explicitly import it with require "json"

General type-safe interface

The general type-safe interface for parsing JSON is to invoke T.from_json on a target type T and pass either a String or IO as an argument.

require "json"

json_text = %([1, 2, 3])
Array(Int32).from_json(json_text) # => [1, 2, 3]

json_text = %({"x": 1, "y": 2})
Hash(String, Int32).from_json(json_text) # => {"x" => 1, "y" => 2}

Serializing is achieved by invoking to_json, which returns a String, or to_json(io : IO), which will stream the JSON to an IO.

require "json"

[1, 2, 3].to_json            # => "[1,2,3]"
{"x" => 1, "y" => 2}.to_json # => "{\"x\":1,\"y\":2}"

Most types in the standard library implement these methods. For user-defined types you can define a : JSON::PullParser) for parsing and to_json(builder : JSON::Builder) for serializing. The following sections show convenient ways to do this using JSON::Serializable.

NOTE JSON object keys are always strings but they can still be parsed and deserialized to other types. To deserialize, define a T.from_json_object_key?(key : String) : T? method, which can return nil if the string can't be parsed into that type. To serialize, define a to_json_object_key : String method can be serialized that way. All integer and float types in the standard library can be deserialized that way.

require "json"

json_text = %({"1": 2, "3": 4})
Hash(Int32, Int32).from_json(json_text) # => {1 => 2, 3 => 4}

{1.5 => 2}.to_json # => "{\"1.5\":2}"

Parsing with JSON.parse

JSON.parse will return an Any, which is a convenient wrapper around all possible JSON types, making it easy to traverse a complex JSON structure but requires some casts from time to time, mostly via some method invocations.

require "json"

value = JSON.parse("[1, 2, 3]") # : JSON::Any

value[0]               # => 1
typeof(value[0])       # => JSON::Any
value[0].as_i          # => 1
typeof(value[0].as_i)  # => Int32
value[0].as_i?         # => 1
typeof(value[0].as_i?) # => Int32 | Nil
value[0].as_s?         # => nil
typeof(value[0].as_s?) # => String | Nil

value[0] + 1       # Error, because value[0] is JSON::Any
value[0].as_i + 10 # => 11

JSON.parse can read from an IO directly (such as a file) which saves allocating a string:

require "json"

json ="path/to/file.json") do |file|

Parsing with JSON.parse is useful for dealing with a dynamic JSON structure.

Generating with

Use, which uses JSON::Builder, to generate JSON by emitting scalars, arrays and objects:

require "json"

string = do |json|
  json.object do
    json.field "name", "foo"
    json.field "values" do
      json.array do
        json.number 1
        json.number 2
        json.number 3
string # => %<{"name":"foo","values":[1,2,3]}>

Generating with to_json

to_json, to_json(IO) and to_json(JSON::Builder) methods are provided for primitive types, but you need to define to_json(JSON::Builder) for custom objects, either manually or using JSON::Serializable.

Defined in: