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Programming language: Elixir
License: MIT License
Tags: JSON    

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exjsx (v4.0.0)

json for elixir

based on jsx

testing provided by travis-ci

Build Status

exjsx is released under the terms of the MIT license

copyright 2013, 2014, 2015, 2016, 2017 alisdair sullivan



build the library and run tests

$ mix compile
$ mix test

convert a json string into an elixir term

iex> JSX.decode "{\"library\": \"jsx\", \"awesome\": true}"
{:ok, %{"awesome" => true, "library" => "jsx"}}
iex> JSX.decode "[\"a\",\"list\",\"of\",\"words\"]"
{:ok, ["a", "list", "of", "words"]}

convert an elixir term into a json string

iex> JSX.encode %{"library" => "jsx", "awesome" => true}
{:ok, "{\"awesome\":true,\"library\":\"jsx\"}"}
iex> JSX.encode [library: "jsx", awesome: true]
{:ok, "{\"library\":\"jsx\",\"awesome\":true}"}
iex> JSX.encode ["a","list","of","words"]
{:ok, "[\"a\",\"list\",\"of\",\"words\"]"}

check if a binary or a term is valid json

iex> JSX.is_json? "[\"this is json\"]"
iex> JSX.is_json? ["this is not"]
iex> JSX.is_term? ["this is a term"]
iex> JSX.is_term? self()

minify some json

iex> JSX.minify "{
...>   \"a list\": [
...>     1,
...>     2,
...>     3
...>   ]
...> }"
{:ok,"{\"a list\":[1,2,3]}"}

prettify some json

iex> JSX.prettify "{\"a list\":[1,2,3]}"
{:ok, "{
  \"a list\": [


exjsx is an elixir application for consuming, producing and manipulating json

json has a spec but common usage deviates in a number of cases. exjsx attempts to address common usage while following the spirit of the spec

all json produced and consumed by exjsx should be utf8 encoded text or a reasonable approximation thereof. ascii works too, but anything beyond that i'm not going to make any promises. especially not latin1

json <-> elixir mapping

json elixir
number Float and Integer
string String
true and false true and false
null nil
array List and Enumerable
object Map


javascript and thus json represent all numeric values with floats. as this is woefully insufficient for many uses, exjsx, just like elixir, supports bigints. whenever possible, this library will interpret json numbers that look like integers as integers. other numbers will be converted to elixir's floating point type, which is nearly but not quite iee754. negative zero is not representable in elixir (zero is unsigned in elixir and 0 is equivalent to -0) and will be interpreted as regular zero. numbers not representable are beyond the concern of this implementation, and will result in parsing errors

when converting from elixir to json, numbers are represented with their shortest representation that will round trip without loss of precision. this means that some floats may be superficially dissimilar (although functionally equivalent). for example, 1.0000000000000001 will be represented by 1.0


the json spec is frustratingly vague on the exact details of json strings. json must be unicode, but no encoding is specified. javascript explicitly allows strings containing codepoints explicitly disallowed by unicode. json allows implementations to set limits on the content of strings. other implementations attempt to resolve this in various ways. this implementation, in default operation, only accepts strings that meet the constraints set out in the json spec (strings are sequences of unicode codepoints deliminated by " (u+0022) that may not contain control codes unless properly escaped with \ (u+005c)) and that are encoded in utf8

the utf8 restriction means improperly paired surrogates are explicitly disallowed. u+d800 to u+dfff are allowed, but only when they form valid surrogate pairs. surrogates encountered otherwise result in errors

json string escapes of the form \uXXXX will be converted to their equivalent codepoints during parsing. this means control characters and other codepoints disallowed by the json spec may be encountered in resulting strings, but codepoints disallowed by the unicode spec will not be. in the interest of pragmatism there is an option for looser parsing

all elixir strings are represented by BitStrings. the encoder will check strings for conformance. noncharacters (like u+ffff) are allowed in elixir utf8 encoded binaries, but not in strings passed to the encoder (although, again, see options)

when encoding, atoms are first converted to BitStrings

this implementation performs no normalization on strings beyond that detailed here. be careful when comparing strings as equivalent strings may have different utf8 encodings

true, false and null/nil

the json primitives true, false and null are represented by the elixir atoms true, false and nil


json arrays are represented with elixir lists of json values as described in this section. elixir enumerables like Stream, Range and MapSet are serialized to json arrays


json objects are represented by elixir maps. keys are atoms, bitstrings or integers and values are valid json values. structs, keylists and dicts are serialized to objects automagically but there is currently no way to perform the reverse. stay tuned tho

frequently made accusations

your lib sucks and encodes my structs wrong

so you have this struct:

defmodule Character do
  defstruct name: nil, rank: nil
iex> JSX.encode %Character{name: "Walder Frey", rank: "Lord"}
{:ok, "{\"name\":\"Walder Frey\",\"rank\":\"Lord\"}"}

but you don't like that encoding. ok. do this:

defimpl JSX.Encoder, for: Character do
  def json(record) do
    [:start_object, "name", record.rank <> " " <> record.name, :end_object]
iex> JSX.encode Character.new(name: "Walder Frey", rank: "Lord")
{:ok, "{\"name\":\"Lord Walder Frey\"}"}

apart from the jsx internal format you can also generate you own json and pass it to the encoder with [{:raw, "{\"name\": \"Lord Walder Frey\"}"}]

someone should write a macro that does this and make a pull request

you forgot to document incompletes

no i didn't. they are jsx only for now. stay tuned tho


exjsx functions all take a common set of options. not all flags have meaning in all contexts, but they are always valid options. functions may have additional options beyond these. see individual function documentation for details


json strings are escaped according to the json spec. this means forward slashes (solidus) are only escaped when this flag is present. otherwise they are left unescaped. you may want to use this if you are embedding json directly into a html or xml document


by default both the encoder and decoder return strings as utf8 binaries appropriate for use in elixir. escape sequences that were present in decoded terms are converted into the appropriate codepoint while encoded terms are unaltered. this flag escapes strings as if for output in json, removing control codes and problematic codepoints and replacing them with the appropriate escapes


escape all codepoints outside the ascii range for 7 bit clean output. note this escaping takes place even if no other string escaping is requested (via escaped_strings)


javascript interpreters treat the codepoints u+2028 and u+2029 as significant whitespace. json strings that contain either of these codepoints will be parsed incorrectly by some javascript interpreters. by default, these codepoints are escaped (to \u2028 and \u2029, respectively) to retain compatibility. this option simply removes that escaping


json escaping is lossy; it mutates the json string and repeated application can result in unwanted behaviour. if your strings are already escaped (or you'd like to force invalid strings into "json" you monster) use this flag to bypass escaping. this can also be used to read in really invalid json strings. everything between unescaped quotes are passed as is to the resulting string term. note that this takes precedence over any other options


as mentioned earlier, exjsx is pragmatic. if you're more of a json purist or you're really into bdsm stricter adherence to the spec is possible. the following restrictions are available

  • :comments

    comments are disabled and result in ArgumentError or {:error, :badarg}

  • :utf8

    invalid codepoints and malformed unicode result in ArgumentError or {:error, :badarg}

  • :single_quotes

    only keys and strings delimited by double quotes (u+0022) are allowed. the single quote (u+0027) results in ArgumentError or {:error, :badarg}

  • trailing_commas

    trailing commas in an object or list result in badarg errors

  • :escapes

    escape sequences not adhering to the json spec result in ArgumentError or {:error, :badarg}

any combination of these can be passed to exjsx by using {:strict, [strict_option()]}. :strict is equivalent to {:strict, [:comments, :bad_utf8, :single_quotes, :escapes]}


decode(json, opts)

decode parses a json text (a BitString) and produces {:ok, result} or {:error, reason}

opts has the default value [] and can be a list containing any of the standard exjsx options plus the following

  • {:labels, :binary} json object's keys will be decoded to BitStrings. the default

  • {:labels, :atom} json object's keys will be decoded to Atoms

  • {:labels, :existing_atom} json object's keys will be decoded to Atoms if they are already known to the runtime, otherwise the decoder will return an error

iex> JSX.decode "[true, false, null]"
iex> JSX.decode("{\"key\": true}", [{:labels, :binary}])
{:ok, %{"key" => true}}
iex> JSX.decode("{\"key\": true}", [{:labels, :atom}])
{:ok, %{key: true}}
iex> JSX.decode [:a, :b, :c]
{:error, :badarg}

decode!(json, opts)

decode! parses a json text (a BitString) and produces result or an ArgumentError exception

see decode for opts

iex> JSX.decode! "[true, false, null]"
[true, false, nil]
iex> JSX.decode! [:a, :b, :c]
** (ArgumentError) argument error

encode(term, opts)

encode produces takes an elixir term and produces {:ok, json} or {:error, :badarg}

opts has the default value [] and can be a list containing any of the standard exjsx options plus the following

  • {:space, n} inserts n spaces after every comma and colon in your json output. :space is an alias for {:space, 1}. the default is {:space, 0}

  • {:indent, n} inserts a newline and n spaces for each level of indentation in your json output after each comma. note that this overrides spaces inserted after a comma. :indent is an alias for {:indent, 1}. the default is {:indent, 0}

iex> JSX.encode [true, false, nil]
{:ok, "[true,false,null]"}
iex> JSX.encode(%{:a => 1, :b => 2, :c => 3}, [{:space, 2}, :indent])
 \"a\":  1,
 \"b\":  2,
 \"c\":  3
iex> JSX.encode(%{:a => 1, :b => 2, :c => 3}, [:space, {:indent, 4}])
    \"a\": 1,
    \"b\": 2,
    \"c\": 3

encode!(json, opts)

encode! produces takes an elixir term and produces json or an ArgumentError exception

see encode for opts

iex> JSX.encode! [true, false, null]
[true, false, nil]
iex> JSX.encode! [self()]
** (ArgumentError) argument error

format(json, opts)

format parses a json text and produces formatted {:ok, json} or {:error, :badarg}

see encode for opts

iex> JSX.format "[true, false, null]"
{:ok, "[true,false,null]"}
iex> JSX.format("[true, false, null]", [space: 2])
{:ok, "[true,  false,  null]"}
iex> JSX.format("[true, false, null]", [space: 4])
{:ok, "[true,    false,    null]"}
iex> JSX.format "{\"foo\":true,\"bar\":false}"
{:ok, "{\"foo\":true,\"bar\":false}"}
iex> JSX.format("{\"foo\":true,\"bar\":false}", [:space])
{:ok, "{\"foo\": true,\"bar\": false}"}
iex> JSX.format("{\"foo\":true,\"bar\":false}", [space: 2, indent: 4])
{:ok, "{
    \"foo\":  true,
    \"bar\":  false
iex> JSX.format [self()]

format!(json, opts)

format! parses a json text and produces formatted json or an ArgumentError exception

see encode for opts

iex> JSX.format! "[true, false, null]"
iex> JSX.format!("{\"foo\":true,\"bar\":false}", [space: 2, indent: 4])
    \"foo\":  true,
    \"bar\":  false
iex> JSX.format! [self()]
** (ArgumentError) argument error


minify is an alias for format(json, [space: 0, indent: 0])

iex> JSX.minify "[true, false, null]"
iex> JSX.minify [self()]


minify! is an alias for format!(json, [space: 0, indent: 0])

iex> JSX.minify! "[true, false, null]"
iex> JSX.minify [self()]
** (ArgumentError) argument error


prettify is an alias for format(json, [space: 1, indent: 2])

iex> JSX.prettify "[true, false, null]"
iex> JSX.prettify [self()]


prettify! is an alias for format!(json, [space: 1, indent: 2])

iex> JSX.prettify! "[true, false, null]"
iex> JSX.prettify! [self()]
** (ArgumentError) argument error

is_json?(json, opts)

returns true if input is a valid json text, false if not

opts has the default value [] and can be a list containing any of the standard exjsx options

what exactly constitutes valid json may be altered

iex> JSX.is_json? "[true, false, null]"
iex> JSX.is_json? [self()]

is_term?(term, opts)

returns true if input is an elixir term that can be safely converted to json, false if not

opts has the default value [] and can be a list containing any of the standard exjsx options

what exactly constitutes valid json may be altered

iex> JSX.is_term? [true, false, nil]
iex> JSX.is_term? [self()]


exjsx wouldn't be what it is without the guidance and code review of yurii rashkovskii, eduardo gurgel and devin torres

*Note that all licence references and agreements mentioned in the exjsx README section above are relevant to that project's source code only.