an evented, streaming json parser based in part on yajl. master is latest stable release, develop is the current stable codebase and prior releases are tags
Clone or download
Pull request Compare This branch is 6 commits ahead, 538 commits behind talentdeficit:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


jsx (v1.2.1)

a sane json implementation for erlang, inspired by yajl

copyright 2011, 2012 alisdair sullivan

jsx is released under the terms of the MIT license

jsx uses rebar for it's build chain

Build Status



to build the library: rebar compile

to convert a utf8 binary containing a json string into an erlang term: jsx:to_term(JSON)

to convert an erlang term into a utf8 binary containing a json string: jsx:to_json(Term)

to check if a binary is valid json: jsx:is_json(JSON)

to check if a term is valid json: jsx:is_term(Term)

to minify a json string: jsx:format(JSON)


json <-> erlang mapping

json erlang
number integer() and float()
string binary()
true, false and null true, false and null
array [] and [JSON]
object [{}] and [{binary() OR atom(), JSON}]


json must be a binary encoded in utf8. if it's invalid utf8 or invalid json, it probably won't parse without errors. there are a few non-standard extensions to the parser available that may change that, they are detailed in the options section below

jsx also supports json fragments; valid json values that are not complete json. that means jsx will parse things like <<"1">>, <<"true">> and <<"\"hello world\"">> without complaint


only the erlang terms in the table above are supported. non supported terms result in badarg errors. jsx is never going to support erlang lists instead of binaries, mostly because you can't discriminate between lists of integers and strings without hinting, and hinting is silly


javascript and thus json represent all numeric values with floats. as this is woefully insufficient for many uses, jsx, just like erlang, supports bigints. whenever possible, this library will interpret json numbers that look like integers as integers. other numbers will be converted to erlang's floating point type, which is nearly but not quite iee754. negative zero is not representable in erlang (zero is unsigned in erlang 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 erlang 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 and 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 that appear otherwise are an error

json string escapes of the form \uXXXX will be converted to their equivalent codepoint 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 (like the two cases above) will not be

in the interests of pragmatism, there is an option for looser parsing, see options below

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

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

the json primitives true, false and null are represented by the erlang atoms true, false and null. surprise


json arrays are represented with erlang lists of json values as described in this section


json objects are represented by erlang proplists. the empty object has the special representation [{}] to differentiate it from the empty list. ambiguities like [true, false] prevent using the shorthand form of property lists using atoms as properties so all properties must be tuples. all keys must be encoded as in string, above, or as atoms (which will be escaped and converted to binaries for presentation to handlers). values should be valid json values


jsx functions all take a common set of options. not all flags have meaning in all contexts, but they are always valid options. flags are always atoms or {atom, Term} tuples. functions may have additional options beyond these, see individual function documentation for details


json text input and json strings SHOULD be utf8 encoded binaries, appropriately escaped as per the json spec. if this option is present attempts are made to replace invalid codepoints with u+FFFD as per the unicode spec. this applies both to malformed unicode and disallowed codepoints


json strings are escaped according to the json spec. this means forward slashes (solidus) are optionally escaped. this option is only relevant for encoding, you may want to use this if you are embedding JSON directly into a HTML or XML document. See: html4-non-html-data


some parsers allow double quotes (u+0022) to be replaced by single quotes (u+0027) to deliminate keys and strings. this option allows json containing single quotes as structural (deliminator) characters to be parsed without errors. note that the parser expects strings to be terminated by the same quote type that opened it and that single quotes must, obviously, be escaped within strings deliminated by single quotes

double quotes must ALWAYS be escaped, regardless of what kind of quotes deliminate the string they are found in

the parser will never emit json with keys or strings deliminated by single quotes


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 if, for some reason, you object to this


json has no official comments but some parsers allow c style comments. this flag allows comments (both // ... and /* ... */ style) anywhere whitespace is allowed


by default, both the encoder and decoder return strings as utf8 binaries appropriate for use in erlang. escape sequences that were present in decoded terms are converted into the appropriate codepoint and 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


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") use this flag to bypass escaping


during decoding, ignore unrecognized escape sequences and leave them as is in the stream. note that if you combine this option with escaped_strings the escape character itself will be escaped


this option treats all exhausted inputs as incomplete, as explained below. the parser will not attempt to return a final state until the function is called with the value end_stream


relax is a synonym for [replaced_bad_utf8, single_quoted_strings, comments, ignored_bad_escapes] for when you don't care how janky and awful your json input is, you just want the parser to do the best it can

{pre_encode, F}

F is a function of arity 1 that pre-process input to the encoder. only input evaluated in a value context is pre-processed in this manner (so keys are not pre-processed, but objects and arrays are). if more than one pre encoder is declared, a badarg exception will occur

input can be any term, but output from the function must be a valid type for input

incomplete input

jsx handles incomplete json texts. if a partial json text is parsed, rather than returning a term from your callback handler, jsx returns {incomplete, F} where F is a function with an identical API to the anonymous fun returned from decoder/3. it retains the internal state of the parser at the point where input was exhausted. this allows you to parse as you stream json over a socket or file descriptor or to parse large json texts without needing to keep them entirely in memory

however, it is important to recognize that jsx is greedy by default. if input is exhausted and the json text is not unambiguously incomplete jsx will consider the parsing complete. this is mostly relevant when parsing bare numbers like <<"1234">>. this could be a complete json integer or just the beginning of a json integer that is being parsed incrementally. jsx will treat it as a whole integer. the option explicit_end can be used to modify this behaviour, see above

the encoder and decoder

jsx is built on top of two finite state automata, one that handles json texts and one that handles erlang terms. both take a callback module as an argument that acts similar to a fold over a list of json 'events'. these events and the handler module's callbacks are detailed in the next section

jsx:decoder/3 and jsx:encoder/3 are the entry points for the decoder and encoder, respectively

decoder(Handler, InitialState, Opts) -> Fun((JSON) -> Any)

encoder(Handler, InitialState, Opts) -> Fun((Term) -> Any)


  • Handler = atom(), should be the name of a callback module, see below
  • InitialState = term(), passed as is to Handler:init/1
  • Opts = see above
  • JSON = utf8 encoded json text
  • Term = an erlang term as specified above in the mapping section
  • Any = term()

decoder returns an anonymous function that handles binary json input and encoder returns an anonymous function that handles erlang term input. these are safe to reuse for multiple inputs

handler callbacks

Handler should export the following pair of functions

Handler:init(InitialState) -> State

Handler:handle_event(Event, State) -> NewState


  • InitialState, State, NewState = any erlang term
  • Event =
    • start_object
    • end_object
    • start_array
    • end_array
    • end_json
    • {key, binary()}
    • {string, binary()}
    • {integer, integer()}
    • {float, float()}
    • {literal, true}
    • {literal, false}
    • {literal, null}

init/1 is called with the initialState argument from decoder/3 or encoder/3 and should take care of any initialization your handler requires and return a new state

handle_event/2 is called for each Event emitted by the decoder/encoder with the output of the previous handle_event/2 call (or init/1 call, if handle_event/2 has not yet been called)

the event end_json will always be the last event emitted, you should take care of any cleanup in handle_event/2 when encountering end_json. the state returned from this call will be returned as the final result of the anonymous function

both key and string are utf8 encoded binaries with all escaped values converted into the appropriate codepoints

converting json to erlang and vice versa

converting json to erlang terms

to_term parses a JSON text (a utf8 encoded binary) and produces an erlang term (see json <-> erlang mapping details above)

to_term(JSON) -> Term

to_term(JSON, Opts) -> Term


  • JSON = as above in the mapping section
  • Term = as above in the mapping section
  • Opts = as above in the opts section, but see also additional opts below
  • Opt =
    • labels
    • {labels, Label}
    • Label =
      • binary
      • atom
      • existing_atom
    • {post_decode, F}

the option labels controls how keys are converted from json to erlang terms. binary does no conversion beyond normal escaping. atom converts keys to erlang atoms, and results in a badarg error if keys fall outside the range of erlang atoms. existing_atom is identical to atom, except it will not add new atoms to the atom table

{post_decode, F} is a user defined function of arity 1 that is called on each output value (objects, arrays, strings, numbers and literals). it may return any value to be substituted in the returned term. for example:

    1> F = fun(V) when is_list(V) -> V; (V) -> false end.
    2> jsx:to_term(<<"{\"a list\": [true, \"a string\", 1]}">>, [{post_decode, F}]).
      [{<<"a list">>, [false, false, false]}]

if more than one decoder is declared a badarg exception will result

converting erlang terms to json

to_json parses an erlang term and produces a JSON text (see json <-> erlang mapping details below)

to_json(Term) -> JSON

to_json(Term, Opts) -> JSON


  • JSON = as above in the mapping section
  • Term = as above in the mapping section
  • Opts = as above in the opts section, but see also additional opts below
  • Opt =
    • space
    • {space, N}
    • indent
    • {indent, N}

the option {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}

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

formatting and minifying json text

formatting json texts

produces a JSON text from JSON text, reformatted

format(JSON) -> JSON

format(JSON, Opts) -> JSON


  • JSON = as above in the mapping section
  • Opts = as above in the opts section, but see also additional opts below
  • Opt =
    • space
    • {space, N}
    • indent
    • {indent, N}

the option {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}

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

calling format with no options results in minified json text

verifying json and terms are valid input

verifying json texts

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

is_json(MaybeJSON) -> true | false | {incomplete, Fun}

is_json(MaybeJSON, Opts) -> true | false | {incomplete, Fun}


  • MaybeJSON = any()
  • Opts = as above

verifying terms

returns true if input is a valid erlang term that represents a JSON text, false if not

is_term(MaybeJSON) -> true | false

is_term(MaybeJSON, Opts) -> true | false


  • MaybeJSON = any()
  • Opts = as above


jsx wouldn't be what it is without the contributions of paul davis, lloyd hilaiel, john engelhart, bob ippolito, fernando benavides, alex kropivny, steve strong, michael truog and dmitry kolesnikov