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In computing, JavaScript Object Notation (JSON) (/ˈdʒeɪsən/ 'Jason',^[1]/dʒeɪˈsɒn/) is an open-standardfile format that uses human-readable text to transmit data objects consisting of attribute–value pairs and array data types (or any other serializable value). It is a very common data format used for asynchronous browser–server communication, including as a replacement for XML in some AJAX-style systems.^[2]

JSON is a language-independent data format. It was derived from JavaScript, but as of 2017 many programming languages include code to generate and parse JSON-format data. The official Internet media type for JSON is application/json. JSON filenames use the extension .json.

Douglas Crockford originally specified the JSON format in the early 2000s; two competing standards, RFC8259 and ECMA-404,^[3] defined it in 2017. The ECMA standard describes only the allowed syntax, whereas the RFC covers some security and interoperability considerations.^[4]

A restricted profile of JSON, known as I-JSON (short for 'Internet JSON'), seeks to overcome some of the interoperability problems with JSON. It is defined in RFC7493.^[5]

• 2Data types and syntax
• 3Schema and metadata
• 5Applications
• 9Comparison with other formats
  • 9.3Samples

History[edit]

JSON grew out of a need for stateless, real-time server-to-browser communication protocol without using browser plugins such as Flash or Java applets, the dominant methods used in the early 2000s.^{[citation needed]}

Douglas Crockford first specified^[6] and popularized the JSON format. The acronym originated at State Software, a company co-founded by Crockford and others in March 2001. The co-founders agreed to build a system that used standard browser capabilities and provided an abstraction layer for Web developers to create stateful Web applications that had a persistent duplex connection to a Web server by holding two HTTP connections open and recycling them before standard browser time-outs if no further data were exchanged. The co-founders had a round-table discussion and voted whether to call the data format JSML or JSON, as well as under what license type to make it available. Crockford, being inspired by the words of then President Bush, should also be credited with coming up with the 'evil-doers' JSON license ('The Software shall be used for Good, not Evil.') in order to open-source the JSON libraries, but force (troll) corporate lawyers, or those who are overly pedantic, to seek to pay for a license from State. Chip Morningstar developed the idea for the State Application Framework at State Software.^[7][8] On the other hand, this clause led to license compatibility problems of the JSON license with other open-source licenses.^[9]

A precursor to the JSON libraries was used in a children's digital asset trading game project named Cartoon Orbit at Communities.com (the State co-founders had all worked at this company previously) for Cartoon Network, which used a browser side plug-in with a proprietary messaging format to manipulate DHTML elements (this system is also owned by 3DO). Upon discovery of early Ajax capabilities, digiGroups, Noosh, and others used frames to pass information into the user browsers' visual field without refreshing a Web application's visual context, realizing real-time rich Web applications using only the standard HTTP, HTML and JavaScript capabilities of Netscape 4.0.5+ and IE 5+. Crockford then found that JavaScript could be used as an object-based messaging format for such a system. The system was sold to Sun Microsystems, Amazon.com and EDS. The JSON.org^[10] website was launched in 2002. In December 2005, Yahoo! began offering some of its Web services in JSON.^[11]

JSON was originally intended to be a subset of the JavaScript scripting language (specifically, Standard ECMA-262 3rd Edition—December 1999^{[12][need quotation to verify]}) and is commonly used with Javascript, but it is a language-independent data format. Code for parsing and generating JSON data is readily available in many programming languages. JSON's website lists JSON libraries by language.

Though JSON was originally advertised and believed to be a strict subset of JavaScript and ECMAScript,^{[13][not in citation given]} it inadvertently allows some unescaped characters in strings that are illegal in JavaScript and ECMAScript string literals. See Data portability issues below.

JSON itself became an ECMA international standard in 2013 as the ECMA-404 standard.^[14]In the same year RFC7158 used ECMA-404 as reference. In 2014 RFC7159 became the main reference for JSON's internet uses (ex. MIME application/json), and obsoletes RFC4627 and RFC7158 (but preserving ECMA-262 and ECMA-404 as main references). In December 2017, RFC7159 was made obsolete by RFC8259.

Data types and syntax[edit]

JSON's basic data types are:

• Number: a signed decimal number that may contain a fractional part and may use exponential E notation, but cannot include non-numbers such as NaN. The format makes no distinction between integer and floating-point. JavaScript uses a double-precision floating-point format for all its numeric values, but other languages implementing JSON may encode numbers differently.
• String: a sequence of zero or more Unicode characters. Strings are delimited with double-quotation marks and support a backslash escaping syntax.
• Boolean: either of the values true or false
• Array: an ordered list of zero or more values, each of which may be of any type. Arrays use square bracket notation and elements are comma-separated.
• Object: an unordered collection of name–value pairs where the names (also called keys) are strings. Since objects are intended to represent associative arrays,^[14] it is recommended, though not required,^[15] that each key is unique within an object. Objects are delimited with curly brackets and use commas to separate each pair, while within each pair the colon ':' character separates the key or name from its value.
• null: An empty value, using the word null

Limited whitespace is allowed and ignored around or between syntactic elements (values and punctuation, but not within a string value). Only four specific characters are considered whitespace for this purpose: space, horizontal tab, line feed, and carriage return. In particular, the byte order mark must not be generated by a conforming implementation (though it may be accepted when parsing JSON). JSON does not provide syntax for comments.

Early versions of JSON (such as specified by RFC4627) required that a valid JSON 'document' must consist of only an object or an array type, which could contain other types within them.

Example[edit]

The following example shows a possible JSON representation describing a person.

Data portability issues[edit]

Although Douglas Crockford originally asserted that JSON is a strict subset of JavaScript, his specification actually allows valid JSON documents that are invalid JavaScript. Specifically, JSON allows the Unicode line terminatorsU+2028LINE SEPARATOR and U+2029PARAGRAPH SEPARATOR to appear unescaped in quoted strings, while ECMAScript 2018 and older does not.^[16][17] This is a consequence of JSON disallowing only 'control characters'. For maximum portability, these characters should be backslash-escaped. This subtlety is important when generating JSONP.

JSON exchange in an open ecosystem must be encoded in UTF-8.^[18] The encoding supports the full Unicode character set, including those characters outside the Basic Multilingual Plane (U+10000 to U+10FFFF). However, if escaped, those characters must be written using UTF-16 surrogate pairs, a detail missed by some JSON parsers. For example, to include the Emoji character U+1F602😂FACE WITH TEARS OF JOY in JSON:

Numbers in JSON are agnostic with regard to their representation within programming languages. No differentiation is made between an integer and floating-point value: some implementations may treat 42, 42.0, and 4.2E+1 as the same number while others may not. Furthermore, no requirements are made regarding implementation issues such as overflow, underflow, loss of precision, or rounding. Additionally, JSON says nothing about the treatment of signed zeros: whether 0.0 is distinct from -0.0. Most implementations that use the IEEE 754 floating-point standard, including JavaScript, preserve signed zeros; but not all JSON implementations may do so.

Using JSON in JavaScript[edit]

As of 2018, all major browsers support at least the fifth edition ECMAScript which provides^[19] a safe and fast method of decoding JSON:

Unsupported native data types[edit]

JavaScript syntax defines several native data types that are not included in the JSON standard:^[15] Map, Set, Date, Error, Regular Expression, Function, Promise, and undefined.^{[note 1]} These JavaScript data types must be represented by some other data format, with the programs on both ends agreeing on how to convert between the types. As of 2011, there are some de facto standards, e.g., converting from Date to String, but none universally recognized.^[20][21] Other languages may have a different set of native types that must be serialized carefully to deal with this type of conversion.

Schema and metadata[edit]

JSON Schema[edit]

JSON Schema^[22] specifies a JSON-based format to define the structure of JSON data for validation, documentation, and interaction control. It provides a contract for the JSON data required by a given application, and how that data can be modified.

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JSON Schema is based on the concepts from XML Schema (XSD), but is JSON-based. As in XSD, the same serialization/deserialization tools can be used both for the schema and data; and is self-describing. It is specified in an Internet Draft at the IETF, currently in its 7th draft, which was released on March 18, 2018.^[23] There are several validators available for different programming languages,^[24] each with varying levels of conformance.

There is no standard file extension, but some have suggested .schema.json.^[25]

Example JSON Schema (draft 4):

The JSON Schema above can be used to test the validity of the JSON code below:

MIME type[edit]

The official MIME type for JSON text is 'application/json',^[26] and most modern implementations have adopted this.

The (unofficial) MIME type 'text/json' or the content-type 'text/javascript' also get legacy support by many service providers, browsers, servers, web applications, libraries, frameworks, and APIs. Notable examples include the Google Search API,^[27] Yahoo!,^[27][28] Flickr,^[27] Facebook API,^[29]Lift framework,^[30] Dojo Toolkit 0.4,^[31] etc.

Applications[edit]

JSON-RPC[edit]

JSON-RPC is a remote procedure call (RPC) protocol built on JSON, as a replacement for XML-RPC or SOAP. It is a simple protocol that defines only a handful of data types and commands. JSON-RPC lets a system send notifications (information to the server that does not require a response) and multiple calls to the server that can be answered out of order.Example of a JSON-RPC 2.0 request and response using positional parameters.

AJAJ[edit]

Asynchronous JavaScript and JSON (or AJAJ) refers to the same dynamic web page methodology as Ajax, but instead of XML, JSON is the data format. AJAJ is a web development technique that provides for the ability of a webpage to request new data after it has loaded into the web browser. Typically it renders new data from the server in response to user actions on that webpage. For example, what the user types into a search box, client-side code then sends to the server, which immediately responds with a drop-down list of matching database items.

The following JavaScript code is an example of a client using XMLHttpRequest to request data in JSON format from a server. (The server-side programming is omitted; it must be set up to service requests to the url containing a JSON-formatted string.)

Security considerations[edit]

JSON is intended as a data serialization format. However, its design as a non-strict subset of JavaScript can lead to the misconception that it is safe to pass JSON strings to the JavaScript eval() function. This is not safe, due to the fact that certain valid JSON strings are actually not valid JavaScript code.^[32]

To avoid the many pitfalls caused by executing arbitrary code from the internet, a new function, JSON.parse() was first added to the fifth edition of ECMAScript^[33], which as of 2017 is supported by all major browsers. For non-supported browsers, an API-compatible JavaScript library^[34] is provided by Douglas Crockford.

Vulnerabilities in specific JSON parsers[edit]

Various JSON parser implementations have suffered from denial-of-service attack and mass assignment vulnerability.^[35][36]

Object references[edit]

The JSON standard does not support object references, but an IETF draft standard for JSON-based object references exists.^[37]The Dojo Toolkit supports object references using standard JSON; specifically, the dojox.json.ref module provides support for several forms of referencing including circular, multiple, inter-message, and lazy referencing.^[38][39][40]Alternatively, non-standard solutions exist such as the use of Mozilla JavaScript Sharp Variables. However this functionality became obsolete with JavaScript 1.8.5 and was removed in Firefox version 12.^[41]

Comparison with other formats[edit]

JSON is promoted as a low-overhead alternative to XML as both of these formats have widespread support for creation, reading, and decoding in the real-world situations where they are commonly used.^[42] Apart from XML, examples could include OGDL, YAML and CSV. Also, Google Protocol Buffers can fill this role, although it is not a data interchange language.

YAML[edit]

YAML version 1.2 is a superset of JSON; prior versions were 'not strictly compatible'. For example, escaping a slash (/) with a backslash () is valid in JSON, but was not valid in YAML. (This is common practice when injecting JSON into HTML to protect against cross-site scripting attacks.) Nonetheless, many YAML parsers can natively parse the output from many JSON encoders.^[43]

XML[edit]

XML has been used to describe structured data and to serialize objects. Various XML-based protocols exist to represent the same kind of data structures as JSON for the same kind of data interchange purposes. Data can be encoded in XML in several ways. The most expansive form using tag pairs results in a much larger representation than JSON, but if data is stored in attributes and 'short tag' form where the closing tag is replaced with '/>', the representation is often about the same size as JSON or just a little larger. If the data is compressed using an algorithm like gzip, there is little difference because compression is good at saving space when a pattern is repeated.

While XML can contain 'mixed content', that is, additional content between element tags, JSON must adhere to a strict 'key' → 'value' mapping. This makes it difficult for JSON to communicate document types of data.

XML separates 'data' from 'metadata' (via the use of elements and attributes), while JSON does not have such a concept.

Another key difference lies within the addressability of values. Since JSON is being based on a simple 'key' → 'value' mapping, every key must be unique, that is, there can be no two keys of the same name, whereas in XML addressing happens on 'nodes', which all receive a unique ID via the XML processor. Additionally, the XML standard defines a common attribute 'xml:id', that can be used by the user, to set an ID explicitly.

XML also has the concept of schema. This permits strong typing, user-defined types, predefined tags, and formal structure, allowing for formal validation of an XML stream in a portable way. Similarly, there is an IETF draft proposal for a schema system for JSON.^[44]

XML supports comments, but JSON does not.^[45]

Samples[edit]

JSON sample[edit]

Both of the following examples carry the same kind of information as the JSON example above in different ways. More JSON Examples.

YAML sample[edit]

The JSON code above is also entirely valid YAML. YAML also offers an alternative syntax intended to be more human-accessible by replacing nested delimiters like {}, [], and ' marks with off-side indentation.^[43]

XML samples[edit]

The properties can also be serialized using attributes instead of tags:

The XML encoding may therefore be comparable in length to the equivalent JSON encoding. A wide range of XML processing technologies exist, from the Document Object Model to XPath and XSLT. XML can also be styled for immediate display using CSS. XHTML is a form of XML so that elements can be passed in this form ready for direct insertion into webpages using client-side scripting.

See also[edit]

• Other formats
  • HOCON—Human-Optimized Config Object Notation, a superset of JSON
  • YAML—Another datastorage format that is a superset of JSON^[46]
  • S-expression—the comparable LISP format for trees as text.
  • JSONP—JSON with Padding, a pattern of usage commonly employed when retrieving JSON across domains
  • GeoJSON—an open format for encoding a variety of geographic data structures
  • JSON-LD—JavaScript object notation for linked data, a W3C recommendation
  • SOAPjr—a hybrid of SOAP and JR (JSON-RPC)
• Binary encodings for JSON
  • EXI4JSON (EXI for JSON)—representation by means of the Efficient XML Interchange (EXI) standard
• Implementations:
  • Jayrock—an open source implementation of JSON for the .NET Framework.
  • Ember data server implementations for PHP, Node.js, Ruby, Python, Go, .NET and Java.
  • Jackson for Java.
• Other

Notes[edit]

1. ^The undefined type was left out of the JSON standard, and one finds suggestions that null be used instead.In fact, the current standard says that for a sparse array such as:

   which behaves in JavaScript as if it were:

   with the undefined entries being only implicit rather than explicit, should translate to JSON as if it were:

   with explicit null fillers for the undefined entries.

   Furthermore, in JavaScript {a: undefined} often behaves the same as {}. Both translate as '{}' in JSON. However undefined as an explicit property value does have use in JavaScript inheritance situations such as:

   where the inheritance of x's property a is overridden in xi and makes it pretty much behave as if nothing was inherited. JSON.stringify itself ignores inherited values - it only translates the enumerable own properties as given by Object.keys(y). The default stringification, while not encoding inheritance, can (except for undefined values) encode enough of an object to reconstruct it in an environment that knows what inheritance it should have.To encode JavaScript objects that contain explicit undefined values a convention for representing undefined must be established, such as mapping it to the string 'UNDEFINED'. One can then pass JSON.stringify the optional replacer argument to translate with this convention:

   Converting this JSON back into JavaScript is not as straightforward. While JSON.parse can take an optional reviver argument that is, essentially, the inverse of a replacer, it can't be used in this situation. If that function returns undefined, the JSON.parse logic interprets this to mean to not define a property rather than define one with a undefined value. Instead one has to explicitly post process the result from JSON.parse replacing each 'UNDEFINED' with undefined.

References[edit]

1. ^'Doug Crockford 'Google Tech Talks: JavaScript: The Good Parts''. 7 February 2009.
2. ^'A Modern Reintroduction To AJAX'. Retrieved 12 April 2017.
3. ^'Standard ECMA-404'. www.ecma-international.org.
4. ^Bray, Tim. 'JSON Redux AKA RFC8259'. Ongoing. Retrieved 16 March 2014.
5. ^Bray, Tim (ed.), The I-JSON Message Format, Internet Engineering Task Force (IETF), RFC7493
6. ^'Douglas Crockford — The JSON Saga'. YouTube. 28 August 2011. Retrieved 23 September 2016.
7. ^'Chip Morningstar Biography'. n.d.
8. ^'State Software Breaks Through Web App Development Barrier With State Application Framework: Software Lets Developers Create Truly Interactive Applications; Reduces Costs, Development Time and Improves User Experience'. PR Newswire. February 12, 2002. Archived from the original on June 5, 2013. Retrieved March 19, 2013.
9. ^Apache and the JSON license on LWN.net by Jake Edge (November 30, 2016)
10. ^'JSON'. json.org.
11. ^Yahoo!. 'Using JSON with Yahoo! Web services'. Archived from the original on October 11, 2007. Retrieved July 3, 2009.
12. ^Crockford, Douglas (May 28, 2009). 'Introducing JSON'. json.org. Retrieved July 3, 2009.
13. ^Douglas Crockford (2016-07-10). 'JSON in JavaScript'. Archived from the original on 2016-07-10. Retrieved 2016-08-13.CS1 maint: BOT: original-url status unknown (link)
14. ^ ^ab'The JSON Data Interchange Format'(PDF). ECMA International. October 2013. Retrieved 23 September 2016.
15. ^ ^ab'JSON Web Token (JWT)'. IETF. May 2015. Retrieved 23 September 2016.
16. ^Holm, Magnus (15 May 2011). 'JSON: The JavaScript subset that isn't'. The timeless repository. Retrieved 23 September 2016.
17. ^'TC39 Proposal: Subsume JSON'. ECMA TC39 committee. 22 May 2018.
18. ^'The JavaScript Object Notation (JSON) Data Interchange Format'. IETF. December 2017. Retrieved 16 February 2018.
19. ^'Standard ECMA-262'. ecma-international.org. Retrieved 13 September 2015.
20. ^'jquery - Format a Microsoft JSON date? - Stack Overflow'. stackoverflow.com. Retrieved 13 September 2015.
21. ^'Tales from the Evil Empire - Dates and JSON'. asp.net. Retrieved 13 September 2015.
22. ^'JSON Schema and Hyper-Schema'. json-schema.org. Retrieved 13 September 2015.
23. ^'draft-handrews-json-schema-01 - JSON Schema: A Media Type for Describing JSON Documents'. json-schema.org/. Retrieved 10 February 2019.
24. ^'JSON Schema Implementations'. json-schema.org. Retrieved 10 February 2019.
25. ^'Json Schema file extension'. Stack Overflow.
26. ^'Media Types'. iana.org. Retrieved 13 September 2015.
27. ^ ^abc'Handle application/json & text/json by benschwarz · Pull Request #2 · mislav/faraday-stack'. GitHub. Retrieved 13 September 2015.
28. ^'Yahoo!, JavaScript, and JSON'. ProgrammableWeb. Retrieved 13 September 2015.
29. ^'Make JSON requests allow text/javascript content by jakeboxer · Pull Request #148 · AFNetworking/AFNetworking'. GitHub. Retrieved 13 September 2015.
30. ^'lift/Req.scala at master · lift/lift · GitHub'. GitHub. Retrieved 13 September 2015.
31. ^'BrowserIO.js in legacy/branches/0.4/src/io – Dojo Toolkit'. dojotoolkit.org. Retrieved 13 September 2015.
32. ^'JSON: The JavaScript subset that isn't'. Magnus Holm. Retrieved 16 May 2011.
33. ^'ECMAScript Fifth Edition'(PDF). Archived from the original(PDF) on April 14, 2011. Retrieved March 18, 2011.
34. ^'douglascrockford/JSON-js'. GitHub.
35. ^'Denial of Service and Unsafe Object Creation Vulnerability in JSON (CVE-2013-0269)'. Retrieved January 5, 2016.
36. ^'Microsoft .NET Framework JSON Content Processing Denial of Service Vulnerability'. Retrieved January 5, 2016.
37. ^Zyp, Kris (September 16, 2012). Bryan, Paul C. (ed.). 'JSON Reference: draft-pbryan-zyp-json-ref-03'. Internet Engineering Task Force.
38. ^Zyp, Kris. 'dojox.json.ref'. Dojo.
39. ^Zyp, Kris (June 17, 2008). 'JSON referencing in Dojo'. SitePen. Retrieved July 3, 2009.
40. ^von Gaza, Tys (Dec 7, 2010). 'JSON referencing in jQuery'. NUBUNTU. Archived from the original on May 7, 2015. Retrieved Dec 7, 2010.
41. ^'Sharp variables in JavaScript'. Mozilla Developer Network. April 4, 2015. Retrieved 21 April 2012.
42. ^'JSON: The Fat-Free Alternative to XML'. json.org. Retrieved 14 March 2011.
43. ^ ^ab'YAML Ain't Markup Language (YAML™) Version 1.2'. yaml.org. Retrieved 13 September 2015.
44. ^'JSON Schema'. json-schema.org. Retrieved 2017-04-10.
45. ^Saternos, Casimir (2014). Client-server web apps with Javascript and Java. p. 45. ISBN9781449369316.
46. ^Oren Ben-Kiki; Clark Evans; Ingy döt Net. 'YAML Ain't Markup Language (YAML™) Version 1.2'. Retrieved 29 August 2015.

External links[edit]

• 'ECMA-404 JSON Data Interchange Format'(pdf). ECMA Int'l.
• RFC8259, JSON Data Interchange Format
• RFC7049, Concise Binary Object Representation (CBOR) for JSON

I am writing an http middleware using Node.js that responds to requests with a certain content-type set as header. Now I'd like to unit test that the middleware does not respond to requests with the 'wrong' content-type.

Of course, I can basically use any content-type for this that is not intended to be supported by my component, but I was wondering whether there is a content-type that is intended to be used in this cases (similar to the domain example.com which is for illustrative purposes only and can be used in documentation.

Is there something such as content/invalid or something like this?

1 Answer

In short: no. You can have a 'personal' or 'vanity' mime type by prepending with prs. Media Type Specification, Personal/Vanity type..but there is no mime that is, by definition, invalid. There are obsolete and deleted ones, but they are not by definition invalid.

So you can pretty much do whatever you want, such as content/prs.golo.impossibru or use an oxymoron, like content/swift-government-action - or whatever. It's kind of like how in some languages 'foo' and 'bar' are such common example variable names, where in other languages that just isn't done. In mime types there just appears to be no such convention.

This is at least partially because its almost never the mime type that is invalid, because if an error will occur it's usually that the mime is one thing but the content actually sent does not match it.

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