JavaScript Object Notation (JSON)

JSON is a lightweight data-interchange format.

JSON is easy for us humans to read and write, and equally simple for machines to read (parse) and write (generate).

JSON is language-independent, making it an ideal interchange format for applications in differing programming languages and on differing operating systems.

JSON Values

A JSON value is one of the following:

• Double-quoted text: "foo".

• Number: 1, 1.0, 2.0e2.

• Boolean: true, false.

• Null: null.

• Array: an ordered list of values, enclosed by square brackets:

  Example

  Run in Playground

  ["foo", 1, 1.0, 2.0e2, true, false, null]
  

• Object: a collection of name/value pairs, enclosed by curly braces; each name is double-quoted text; the values may be any JSON values:

  Example

  Run in Playground

  {"a": "foo", "b": 1, "c": 1.0, "d": 2.0e2, "e": true, "f": false, "g": null}
  

A JSON array or object may contain nested arrays, objects, and scalars to any depth:

{"foo": {"bar": 1, "baz": 2}, "bat": [0, 1, 2]}
[{"foo": 0, "bar": 1}, ["baz", 2]]

Using Module JSON

To make module JSON available in your code, begin with:

require 'json'

All examples here assume that this has been done.

Parsing JSON

You can parse a String containing JSON data using either of two methods:

• JSON.parse(source, **opts)

• JSON.parse!(source, **opts)

where

• source is a Ruby object.

• opts are keyword arguments that control both input allowed and output formatting.

The difference between the two methods is that JSON.parse! omits some checks and may not be safe for some source data; use it only for data from trusted sources. Use the safer method JSON.parse for less trusted sources.

Parsing JSON Arrays

When source is a JSON array, JSON.parse by default returns a Ruby Array:

json = '["foo", 1, 1.0, 2.0e2, true, false, null]'
ruby = JSON.parse(json)
ruby # => ["foo", 1, 1.0, 200.0, true, false, nil]
ruby.class # => Array

The JSON array may contain nested arrays, objects, and scalars to any depth:

json = '[{"foo": 0, "bar": 1}, ["baz", 2]]'
JSON.parse(json) # => [{"foo"=>0, "bar"=>1}, ["baz", 2]]

Parsing JSON Objects

When the source is a JSON object, JSON.parse by default returns a Ruby Hash:

json = '{"a": "foo", "b": 1, "c": 1.0, "d": 2.0e2, "e": true, "f": false, "g": null}'
ruby = JSON.parse(json)
ruby # => {"a"=>"foo", "b"=>1, "c"=>1.0, "d"=>200.0, "e"=>true, "f"=>false, "g"=>nil}
ruby.class # => Hash

The JSON object may contain nested arrays, objects, and scalars to any depth:

json = '{"foo": {"bar": 1, "baz": 2}, "bat": [0, 1, 2]}'
JSON.parse(json) # => {"foo"=>{"bar"=>1, "baz"=>2}, "bat"=>[0, 1, 2]}

Parsing JSON Scalars

When the source is a JSON scalar (not an array or object), JSON.parse returns a Ruby scalar.

String:

ruby = JSON.parse('"foo"')
ruby # => 'foo'
ruby.class # => String

Integer:

ruby = JSON.parse('1')
ruby # => 1
ruby.class # => Integer

Float:

ruby = JSON.parse('1.0')
ruby # => 1.0
ruby.class # => Float
ruby = JSON.parse('2.0e2')
ruby # => 200.0
ruby.class # => Float

Boolean:

ruby = JSON.parse('true')
ruby # => true
ruby.class # => TrueClass
ruby = JSON.parse('false')
ruby # => false
ruby.class # => FalseClass

Null:

ruby = JSON.parse('null')
ruby # => nil
ruby.class # => NilClass

Parsing Options

Input Options

Option max_nesting (Integer) specifies the maximum nesting depth allowed; defaults to 100; You can set it to false to disable depth checking entirely, but that is dangerous when parsing untrusted input.

With the default, 100:

source = '[0, [1, [2, [3]]]]'
ruby = JSON.parse(source)
ruby # => [0, [1, [2, [3]]]]

Too deep:

# Raises JSON::NestingError (nesting of 2 is too deep):
JSON.parse(source, max_nesting: 1)

Bad value:

# Raises TypeError (no implicit conversion of Symbol into Integer):
JSON.parse(source, max_nesting: :foo)

---

Option allow_duplicate_key specifies whether duplicate keys in objects should be ignored or cause an error to be raised:

When set to false, the default:

JSON.parse('{"a": 1, "a": 2}') # duplicate key "a" at line 1 column 1 (JSON::ParserError)

When set to true:

# The last value is used.
JSON.parse('{"a": 1, "a": 2}', allow_duplicate_key: true) # => {"a" => 2}

---

Option allow_nan (boolean) specifies whether to allow NaN, Infinity, and MinusInfinity in source; defaults to false.

With the default, false:

# Raises JSON::ParserError (unexpected token 'NaN]' at line 1 column 2):
JSON.parse('[NaN]')
# Raises JSON::ParserError (unexpected token 'Infinity]' at line 1 column 2):
JSON.parse('[Infinity]')
# Raises JSON::ParserError (invalid number: '-Infinity]' at line 1 column 2):
JSON.parse('[-Infinity]')

Allow:

source = '[NaN, Infinity, -Infinity]'
ruby = JSON.parse(source, allow_nan: true)
ruby # => [NaN, Infinity, -Infinity]

---

Option allow_trailing_comma (boolean) specifies whether to allow trailing commas in objects and arrays; defaults to false.

With the default, false:

JSON.parse('[1,]') # unexpected character: ']' at line 1 column 4 (JSON::ParserError)

When enabled:

JSON.parse('[1,]', allow_trailing_comma: true) # => [1]

---

Option allow_comments (boolean) specifies whether to allow JavaScript style comments (either // comment or /* comment */); defaults to false.

When set to false, the default:

JSON.parse('/* comment */ {"a": 1, "a": 2}') # unexpected token '/*' at line 1 column 1 (JSON::ParserError)

When set to true, comments are ignored:

JSON.parse('/* comment */ {"a": 1} // more comment', allow_comments: true) # => {"a" => 1}

---

Option allow_control_characters (boolean) specifies whether to allow unescaped ASCII control characters, such as newlines, in strings; defaults to false.

With the default, false:

JSON.parse(%{"Hello\nWorld"}) # invalid ASCII control character in string: \nWorld" at line 2 column 0 (JSON::ParserError)

When enabled:

JSON.parse(%{"Hello\nWorld"}, allow_control_characters: true) # => "Hello\nWorld"

---

Option allow_invalid_escape (boolean) specifies whether to ignore backslahes that are followed by an invalid escape character in strings; defaults to false.

With the default, false:

JSON.parse('"Hell\o"') # invalid escape character in string: '\o"' at line 1 column 6 (JSON::ParserError)

When enabled:

JSON.parse('"Hell\o"', allow_invalid_escape: true) # => "Hello"

Output Options

Option freeze (boolean) specifies whether the returned objects will be frozen; defaults to false.

Option symbolize_names (boolean) specifies whether returned Hash keys should be Symbols; defaults to false (use Strings).

With the default, false:

source = '{"a": "foo", "b": 1.0, "c": true, "d": false, "e": null}'
ruby = JSON.parse(source)
ruby # => {"a"=>"foo", "b"=>1.0, "c"=>true, "d"=>false, "e"=>nil}

Use Symbols:

ruby = JSON.parse(source, symbolize_names: true)
ruby # => {a: "foo", b: 1.0, c: true, d: false, e: nil}

---

Option object_class (Class) specifies the Ruby class to be used for each JSON object; defaults to Hash.

With the default, Hash:

source = '{"a": "foo", "b": 1.0, "c": true, "d": false, "e": null}'
ruby = JSON.parse(source)
ruby.class # => Hash

Use class OpenStruct:

ruby = JSON.parse(source, object_class: OpenStruct)
ruby # => #<OpenStruct a="foo", b=1.0, c=true, d=false, e=nil>

---

Option array_class (Class) specifies the Ruby class to be used for each JSON array; defaults to Array.

With the default, Array:

source = '["foo", 1.0, true, false, null]'
ruby = JSON.parse(source)
ruby.class # => Array

Use class Set:

ruby = JSON.parse(source, array_class: Set)
ruby # => Set["foo", 1.0, true, false, nil]

Generating JSON

To generate a Ruby String containing JSON data, use method JSON.generate(source, opts), where

• source is a Ruby object.

• opts is a Hash object containing options that control both input allowed and output formatting.

Generating JSON from Arrays

When the source is a Ruby Array, JSON.generate returns a String containing a JSON array:

ruby = [0, 's', :foo]
json = JSON.generate(ruby)
json # => '[0,"s","foo"]'

The Ruby Array array may contain nested arrays, hashes, and scalars to any depth:

ruby = [0, [1, 2], {foo: 3, bar: 4}]
json = JSON.generate(ruby)
json # => '[0,[1,2],{"foo":3,"bar":4}]'

Generating JSON from Hashes

When the source is a Ruby Hash, JSON.generate returns a String containing a JSON object:

ruby = {foo: 0, bar: 's', baz: :bat}
json = JSON.generate(ruby)
json # => '{"foo":0,"bar":"s","baz":"bat"}'

The Ruby Hash array may contain nested arrays, hashes, and scalars to any depth:

ruby = {foo: [0, 1], bar: {baz: 2, bat: 3}, bam: :bad}
json = JSON.generate(ruby)
json # => '{"foo":[0,1],"bar":{"baz":2,"bat":3},"bam":"bad"}'

Generating JSON from Other Objects

When the source is neither an Array nor a Hash, the generated JSON data depends on the class of the source.

When the source is a Ruby Integer or Float, JSON.generate returns a String containing a JSON number:

JSON.generate(42) # => '42'
JSON.generate(0.42) # => '0.42'

When the source is a Ruby String, JSON.generate returns a String containing a JSON string (with double-quotes):

JSON.generate('A string') # => '"A string"'

When the source is true, false or nil, JSON.generate returns a String containing the corresponding JSON token:

JSON.generate(true) # => 'true'
JSON.generate(false) # => 'false'
JSON.generate(nil) # => 'null'

When the source is none of the above, JSON.generate returns a String containing a JSON string representation of the source:

JSON.generate(:foo) # => '"foo"'
JSON.generate(Complex(0, 0)) # => '"0+0i"'
JSON.generate(Dir.new('.')) # => '"#<Dir:0x...>"'

Generating Options

Input Options

Option allow_nan (boolean) specifies whether NaN, Infinity, and -Infinity may be generated; defaults to false.

With the default, false:

# Raises JSON::GeneratorError (NaN not allowed in JSON):
JSON.generate(JSON::NaN)
# Raises JSON::GeneratorError (Infinity not allowed in JSON):
JSON.generate(JSON::Infinity)
# Raises JSON::GeneratorError (-Infinity not allowed in JSON):
JSON.generate(JSON::MinusInfinity)

Allow:

ruby = [Float::NAN, Float::INFINITY, JSON::NaN, JSON::Infinity, JSON::MinusInfinity]
JSON.generate(ruby, allow_nan: true) # => '[NaN,Infinity,NaN,Infinity,-Infinity]'

---

Option allow_duplicate_key (boolean) specifies whether hashes with duplicate keys should be allowed or produce an error. Defaults to false, which raises an error.

With the default, false:

JSON.generate({foo: 1, "foo" => 2})
# detected duplicate key "foo" in {foo: 1, "foo" => 2} (JSON::GeneratorError)

With true:

JSON.generate({foo: 1, "foo" => 2}, allow_duplicate_key: true)
# => '{"foo":1,"foo":2}'

---

Option max_nesting (Integer) specifies the maximum nesting depth in obj; defaults to 100.

With the default, 100:

obj = [[[[[[0]]]]]]
JSON.generate(obj) # => '[[[[[[0]]]]]]'

Too deep:

# Raises JSON::NestingError (nesting of 2 is too deep. Did you try to serialize objects with circular references?):
JSON.generate(obj, max_nesting: 2)

With false:

obj = []
obj[0] = obj
# Raises SystemStackError (stack level too deep):
JSON.generate(obj, max_nesting: false)

Setting max_nesting to false or a very large number can lead to a stack overflow which may leave the process in an unrecoverable state. It is highly discouraged.

Escaping Options

Options script_safe (boolean) specifies wether '\u2028', '\u2029' and '/' should be escaped as to make the JSON object safe to interpolate in script tags.

Options ascii_only (boolean) specifies wether all characters outside the ASCII range should be escaped.

Output Options

The default formatting options generate the most compact JSON data, all on one line and with no whitespace.

You can use these formatting options to generate JSON data in a more open format, using whitespace. See also JSON.pretty_generate.

• Option array_nl (String) specifies a string (usually a newline) to be inserted after each JSON array; defaults to the empty String, ''.

• Option object_nl (String) specifies a string (usually a newline) to be inserted after each JSON object; defaults to the empty String, ''.

• Option indent (String) specifies the string (usually spaces) to be used for indentation; defaults to the empty String, ''; has no effect unless options array_nl or object_nl specify newlines.

• Option space (String) specifies a string (usually a space) to be inserted after the colon in each JSON object’s pair; defaults to the empty String, ''.

• Option space_before (String) specifies a string (usually a space) to be inserted before the colon in each JSON object’s pair; defaults to the empty String, ''.

• Option sort_keys (boolean or Proc) controls whether and how the keys of a hash are sorted when generating the output; defaults to false. When true, keys are sorted lexicographically. When a Proc, it receives the entire Hash and must return a Hash with its pairs in the desired order, allowing for arbitrary sort orders.

• Option rfc8785 (boolean) controls whether the generated JSON will be canonicalized as defined in RFC8785. Integers are converted to Float before formatting and may be rounded. Integers outside the finite Float range raise JSON::GeneratorError. Nonempty indent, space, space_before, object_nl, or array_nl, and enabled ascii_only, script_safe, or allow_nan raise ArgumentError. JSON.dump defaults allow_nan to false when rfc8785 is enabled. JSON::Fragment contents are inserted as is; the caller must ensure they are canonical.

In this example, obj is used first to generate the shortest JSON data (no whitespace), then again with all formatting options specified:

obj = {foo: [:bar, :baz], bat: {bam: 0, bad: 1}}
json = JSON.generate(obj)
puts 'Compact:', json
opts = {
  array_nl: "\n",
  object_nl: "\n",
  indent: '  ',
  space_before: ' ',
  space: ' '
}
puts 'Open:', JSON.generate(obj, opts)

Output:

Compact:
{"foo":["bar","baz"],"bat":{"bam":0,"bad":1}}
Open:
{
  "foo" : [
    "bar",
    "baz"
  ],
  "bat" : {
    "bam" : 0,
    "bad" : 1
  }
}

fragment = JSON::Fragment.new("[1, 2, 3]")
JSON.generate({ count: 3, items: fragments })