JSON-LD Syntax 1.0

3.1 Linking Data

The following definition for Linked Data is the one that will be used for this specification.

1. Linked Data is a set of documents, each containing a representation of a linked data graph.
2. A linked data graph is an unordered labeled directed graph, where nodes are subjects or objects, and edges are properties.
3. A subject is any node in a linked data graph with at least one outgoing edge.
4. A subject should be labeled with an IRI (an Internationalized Resource Identifier as described in [RFC3987]).
5. An object is a node in a linked data graph with at least one incoming edge.
6. An object may be labeled with an IRI.
7. A node may be a subject and object at the same time.
8. A property is an edge of the linked data graph.
9. A property should be labeled with an IRI.
10. An IRI that is a label in a linked data graph should be dereferencable to a Linked Data document describing the labeled subject, object or property.
11. A value is an object with a label that is not an IRI

An illustration of a linked data graph would probably help here.

Note that the definition for Linked Data above is silent on the topic of unlabeled nodes. Unlabeled nodes are not considered Linked Data. However, this specification allows for the expression of unlabled nodes, as most graph-based data sets on the Web contain a number of associated nodes that are not named and thus are not directly de-referenceable.

JSON-LD defines a mechanism to map JSON terms, i.e., keys and values, to IRIs. This does not mean that JSON-LD requires every key or value to be an IRI, but rather ensures that keys and values can be mapped to IRIs if the developer desires to transform their data into Linked Data. There are a few techniques that can ensure that developers will generate good Linked Data for the Web. JSON-LD formalizes those techniques.

We will be using the following JSON markup as the example for the rest of this section:

{
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

{
  "@context":
  {
    "name": "http://xmlns.com/foaf/0.1/name",
    "depiction":
    {
      "@id": "http://xmlns.com/foaf/0.1/depiction",
      "@type": "@id"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    },
  }
}

{
  "@context": "http://json-ld.org/contexts/person",
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

{
  "@context":
  {
    "name": "http://xmlns.com/foaf/0.1/name",
    "depiction":
    {
      "@id": "http://xmlns.com/foaf/0.1/depiction",
      "@type": "@id"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    },
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

{
  "@context": [
    "http://json-ld.org/contexts/person",
    {
      "pic": "http://xmlns.com/foaf/0.1/depiction"
    }
  ],
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "pic": "http://twitter.com/account/profile_image/manusporny"
}

{
  "http://xmlns.com/foaf/0.1/name": "Manu Sporny",
  "http://xmlns.com/foaf/0.1/homepage": "http://manu.sporny.org"
  "http://xmlns.com/foaf/0.1/depiction": "http://twitter.com/account/profile_image/manusporny"
}

3.2 IRIs

Expressing IRIs are fundamental to Linked Data as that is how most subjects, all properties and many objects are identified. IRIs can be expressed in a variety of different ways in JSON-LD.

1. Except within a context definition, terms in the key position in a JSON object that have a mapping to an absolute IRI or another term in the active context are expanded to an IRI by JSON-LD processors.
2. An IRI is generated for the string value specified using @id or @type.
3. An IRI is generated for the string value of any key for which there are coercion rules in effect that identify the value as an @id.

IRIs may be represented as an absolute IRI, a relative IRI, a term, or a compact IRI.

An absolute IRI is defined in [RFC3987] containing a scheme along with path and optional query and fragment segments. A relative IRI is an IRI that is relative some other absolute IRI. In JSON-LD all relative IRIs are resolved relative to the base IRI associated with the document (typically, the directory that contains the document or the document itself).

IRIs can be expressed directly in the key position like so:

{
...
  "http://xmlns.com/foaf/0.1/name": "Manu Sporny",
...
}

In the example above, the key http://xmlns.com/foaf/0.1/name is interpreted as an IRI, as opposed to being interpreted as a string because it contains a colon (:) delimiting a valid IRI scheme.

Term expansion occurs for IRIs if the value matches a term defined within the active context:

{
  "@context":
  {
    "name": "http://xmlns.com/foaf/0.1/name"
...
  },
  "name": "Manu Sporny",
  "status": "trollin'",
...
}

Terms are case sensitive, and must be matched using a case-sensitive comparison.

JSON keys that do not expand to an absolute IRI are ignored, or removed in some cases, by the [JSON-LD-API]. However, JSON keys that do not include a mapping in the context are still considered valid expressions in JSON-LD documents - the keys just don't have any machine-readable, semantic meaning.

Prefixes are expanded when the form of the value is a compact IRI represented as a prefix:suffix combination, and the prefix matches a term defined within the active context:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/"
...
  },
  "foaf:name": "Manu Sporny",
...
}

foaf:name above will automatically expand out to the IRI http://xmlns.com/foaf/0.1/name. See Compact IRIs for more details.

An IRI is generated when a JSON object is used in the value position that contains an @id keyword:

{
...
  "homepage": { "@id": "http://manu.sporny.org" }
...
}

Specifying a JSON object with an @id key is used to identify that object using an IRI. This facility may also be used to link to another JSON object using a mechanism called embedding, which is covered in the section titled Embedding.

If type coercion rules are specified in the @context for a particular term or property IRI, an IRI is generated:

{
  "@context":
  {
    ...
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
    ...
  }
...
  "homepage": "http://manu.sporny.org/",
...
}

In the example above, even though the value http://manu.sporny.org/ is expressed as a JSON string, the type coercion rules will transform the value into an IRI when processed by a JSON-LD Processor.

3.3 Identifying the Subject

To be able to externally reference nodes in a graph, it is important that each node has an unambiguous identifier. IRIs are a fundamental concept of Linked Data, and nodes should have a de-referencable identifier used to name and locate them. For nodes to be truly linked, de-referencing the identifier should result in a representation of that node (for example, using a URL to retrieve a web page). Associating an IRI with a node tells an application that the returned document contains a description of the node requested.

JSON-LD documents may also contain descriptions of other nodes, so it is necessary to be able to uniquely identify each node which may be externally referenced.

A subject of an object in JSON is declared using the @id key. The subject is the first piece of information needed by the JSON-LD processor in order to create the (subject, property, object) tuple, also known as a triple.

{
...
  "@id": "http://example.org/people#joebob",
...
}

The example above would set the subject to the IRI http://example.org/people#joebob.

A JSON object used to define property values is called a subject definition. Subject definitions do not require an @id. A subject definition that does not contain an @id property is called an unlabeled node.

To ensure the best possible performance, it is a best practice to put the @id keyword before other key-value pairs in an object. If it isn't listed first, processors have to save each key-value pair until @id is processed before they can start generating triples. Not specifying the @id keyword first creates a memory and complexity burden for one-pass processors.

3.4 Specifying the Type

The type of a particular subject can be specified using the @type keyword. Specifying the type in this way will generate a triple of the form (subject, type, type-IRI). To be considered Linked Data, types must be uniquely identified by an IRI.

{
...
  "@id": "http://example.org/people#joebob",
  "@type": "http://xmlns.com/foaf/0.1/Person",
...
}

3.5 String Internationalization

At times, it is important to annotate a string with its language. In JSON-LD this is possible in a variety of ways. Firstly, it is possible to define a default language for a JSON-LD document by setting the @language key in the @context or in a term definition:

{
  "@context":
  {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "occupation": "科学者"
}

The example above would associate the ja language code with the two strings 花澄 and 科学者. Languages must be expressed in [BCP47] format.

It is possible to override the default language by using the expanded form of a value:

{
  "@context": {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "occupation": {
    "@value": "Scientist",
    "@language": "en"
  }
}

It is also possible to override the default language or specify a plain value by omitting the @language tag or setting it to null when expressing the expanded value:

{
  "@context": {
    ...
    "@language": "ja"
  },
  "name": {
    "@value": "Frank"
  },
  "occupation":  {
    "@value": "Ninja",
    "@language": "en"
  },
  "speciality": "手裏剣"
}

Please note that language associations must only be applied to plain literal strings. That is, typed values or values that are subject to type coercion won't be language tagged.

To clear the default language for a subtree, @language can be set to null in a local context as follows:

{
  "@context": {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "details": {
    "@context": {
      "@language": null
    },
    "occupation": "Ninja"
  }
}

JSON-LD allows one to associate language information with terms. See Expanded Term Definition for more details.

3.6 Sets and Lists

A JSON-LD author can express multiple values in a compact way by using arrays. Since graphs do not describe ordering for links between nodes, arrays in JSON-LD do not provide an ordering of the listed objects by default. This is exactly the opposite from regular JSON arrays, which are ordered by default. For example, consider the following simple document:

{
...
  "@id": "http://example.org/people#joebob",
  "nick": [ "joe", "bob", "jaybee" ],
...
}

The markup shown above would result in three triples being generated, each relating the subject to an individual object, with no inherent order:

<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "joe" .
<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "bob" .
<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "jaybee" .

Multiple values may also be expressed using the expanded object form:

{
  "@id": "http://example.org/articles/8",
  "dc:title": 
  [
    {
      "@value": "Das Kapital",
      "@language": "de"
    },
    {
      "@value": "Capital",
      "@language": "en"
    }
  ]
}

The markup shown above would generate the following triples, again with no inherent order:

<http://example.org/articles/8>
   <http://purl.org/dc/terms/title>
      "Das Kapital"@de .
<http://example.org/articles/8>
   <http://purl.org/dc/terms/title>
      "Capital"@en .

As the notion of ordered collections is rather important in data modeling, it is useful to have specific language support. In JSON-LD, a list may be represented using the @list keyword as follows:

{
...
  "@id": "http://example.org/people#joebob",
  "foaf:nick":
  {
    "@list": [ "joe", "bob", "jaybee" ]
  },
...
}

This describes the use of this array as being ordered, and order is maintained when processing a document. If every use of a given multi-valued property is a list, this may be abbreviated by setting @container to @list in the context:

{
  "@context":
  {
    ...
    "nick":
    {
      "@id": "http://xmlns.com/foaf/0.1/nick",
      "@container": "@list"
    }
  },
...
  "@id": "http://example.org/people#joebob",
  "nick": [ "joe", "bob", "jaybee" ],
...
}

List of lists are not allowed in this version of JSON-LD. If a list of lists is detected, a JSON-LD processor will throw an exception. This decision was made due to the extreme amount of added complexity when processing lists of lists.

Similarly to @list, there exists the keyword @set to describe unordered sets. While its use in the body of a JSON-LD document represents just syntactic sugar that must be optimized away when processing the document, it is very helpful when used within the context of a document. Values of terms associated with a @set or @list container are always represented in the form of an array - even if there is just a single value that would otherwise be optimized to a non-array form in a compacted document. This makes post-processing of the data easier as the data is always in array form, even if the array only contains a single value.

The use of @container in the body of a JSON-LD document, i.e., outside @context must be ignored by JSON-LD processors.

4.1 Typed Values

A value with an associated type, also known as a typed value, is indicated by associating a value with an IRI which indicates the value's type. Typed values may be expressed in JSON-LD in three ways:

1. By utilizing the @type keyword when defining a term within a @context section.
2. By utilizing the expanded form for specifying objects.
3. By using a native JSON type.

The first example uses the @type keyword to associate a type with a particular term in the @context:

{
  "@context":
  {
    "modified":
    {
      "@id": "http://purl.org/dc/terms/modified",
      "@type": "http://www.w3.org/2001/XMLSchema#dateTime"
    }
  },
...
  "modified": "2010-05-29T14:17:39+02:00",
...
}

The modified key's value above is automatically type coerced to a datetime value because of the information specified in the @context.

The second example uses the expanded form of setting the type information in the body of a JSON-LD document:

{
  "@context":
  {
    "modified":
    {
      "@id": "http://purl.org/dc/terms/modified"
    }
  },
...
  "modified":
  {
    "@value": "2010-05-29T14:17:39+02:00",
    "@type": "http://www.w3.org/2001/XMLSchema#dateTime"
  }
...
}

Both examples above would generate an object with the value of 2010-05-29T14:17:39+02:00 and the type of http://www.w3.org/2001/XMLSchema#dateTime.

The third example uses a built-in native JSON type, a number, to express a type:

{
...
  "@id": "http://example.org/people#joebob",
  "age": 31
...
}

The example above is really just a shorthand for the following:

{
...
  "@id": "http://example.org/people#joebob",
  "age":
  {
    "@value": "31",
    "@type": "http://www.w3.org/2001/XMLSchema#integer"
  }
...
}

The @type keyword is also used to associate a type with a subject. Although the same keyword is used in both places, the concept of an object type and a value type are different. This is similar to object-oriented programming languages where both scalar and structured types use the same class inheritance mechanism, even though scalar types and structured types are inherently different.

4.2 Compact IRIs

Terms in Linked Data documents may draw from a number of different vocabularies. At times, declaring every single term that a document uses can require the developer to declare tens, if not hundreds of potential vocabulary terms that are used across an application. This is a concern for at least two reasons: the first is the cognitive load on the developer of remembering all of the terms, and the second is the serialized size of the context if it is specified inline. In order to address these issues, the concept of a compact IRI is introduced.

A compact IRI is a way of expressing an IRI using a prefix and suffix separated by a colon (:) which is similar to the CURIE Syntax in [RDFA-CORE]. The prefix is a term taken from the active context and is a short string identifying a particular IRI in a JSON-LD document. For example, the prefix foaf may be used as a short hand for the Friend-of-a-Friend vocabulary, which is identified using the IRI http://xmlns.com/foaf/0.1/. A developer may append any of the FOAF vocabulary terms to the end of the prefix to specify a short-hand version of the absolute IRI for the vocabulary term. For example, foaf:name would be expanded out to the IRI http://xmlns.com/foaf/0.1/name. Instead of having to remember and type out the entire IRI, the developer can instead use the prefix in their JSON-LD markup.

Terms are interpreted as compact IRIs if they contain at least one colon and the first colon is not followed by two slashes (//, as in http://example.com). To generate the full IRI, the value is first split into a prefix and suffix at the first occurrence of a colon (:). If the active context contains a term mapping for prefix, an IRI is generated by prepending the mapped prefix to the (possibly empty) suffix using textual concatenation. If no prefix mapping is defined, the value is interpreted as an absolute IRI. If the prefix is an underscore (_), the IRI remains unchanged. This effectively means that every term containing a colon will be interpreted by a JSON-LD processor as an IRI.

Consider the following example:

{
  "@context":
  {
    "dc": "http://purl.org/dc/elements/1.1/",
    "ex": "http://example.org/vocab#"
  },
  "@id": "http://example.org/library",
  "@type": "ex:Library",
  "ex:contains":
  {
    "@id": "http://example.org/library/the-republic",
    "@type": "ex:Book",
    "dc:creator": "Plato",
    "dc:title": "The Republic",
    "ex:contains":
    {
      "@id": "http://example.org/library/the-republic#introduction",
      "@type": "ex:Chapter",
      "dc:description": "An introductory chapter on The Republic.",
      "dc:title": "The Introduction"
    }
  }
}

In this example, two different vocabularies are referred to using prefixes. Those prefixes are then used as type and property values using the compact IRI prefix:suffix notation.

It's also possible to use compact IRIs within the context as shown in the following example:

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "foaf": "http://xmlns.com/foaf/0.1/",
    "foaf:homepage": { "@type": "@id" },
    "picture": { "@id": "foaf:depiction", "@type": "@id" }
  },
  "@subject": "http://me.markus-lanthaler.com/",
  "@type": "foaf:Person",
  "foaf:name": "Markus Lanthaler",
  "foaf:homepage": "http://www.markus-lanthaler.com/",
  "picture": "http://twitter.com/account/profile_image/markuslanthaler"
}

4.3 External Contexts

Authors may choose to declare JSON-LD contexts in external documents to promote re-use of contexts as well as reduce the size of JSON-LD documents.

In order to use an external context, an author must specify an IRI to a valid JSON-LD document. The referenced document must have a top-level JSON object. The value of any @context key within that object is substituted for the IRI within the referencing document to have the same effect as if the value were specified inline within the referencing document.

The following example demonstrates the use of an external context:

{
  "@context": "http://json-ld.org/contexts/person",
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

Authors may also import multiple contexts or a combination of external and local contexts by specifying a list of contexts:

{
  "@context":
  [
    "http://json-ld.org/contexts/person",
    {
      "foaf": "http://xmlns.com/foaf/0.1/"
    },
    "http://json-ld.org/contexts/event",
  ]
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
  "celebrates":
  {
    "@type": "Event",
    "description": "International Talk Like a Pirate Day",
    "date": "R/2011-09-19"
  }
}

Each context in a list will be evaluated in-order. Duplicate mappings among the contexts must be overwritten on a last-defined-overrides basis. The context list must contain either de-referenceable IRIs or JSON objects that conform to the context syntax as described in this document.

An author may nest contexts within JSON objects, with the more deeply nested contexts overriding the values in previously defined contexts:

{
  "@context":
  {
    "name": "http://example.com/person#name",
    "details": "http://example.com/person#details"
  },
  "name": "Markus Lanthaler",
  ...
  "details":
  {
    "@context": {
      "name": "http://example.com/organization#name"
    },
    "name": "Graz University of Technology"
  }
}

In the example above, the name prefix is overridden in the more deeply nested details structure. Note that this is rarely a good authoring practice and is typically used when the JSON object has legacy applications using the structure of the object.

External JSON-LD context documents may contain extra information located outside of the @context key, such as documentation about the prefixes declared in the document. When importing a @context value from an external JSON-LD context document, any extra information contained outside of the @context value must be discarded. It is also recommended that a human-readable document is served as well to explain the correct usage of the JSON-LD context document.

4.4 Referencing Contexts from JSON Documents

Ordinary JSON documents can be transformed into JSON-LD documents by referencing to an external JSON-LD context in an HTTP Link Header. Doing this allows JSON to be unambiguously machine-readable without requiring developers to drastically change their workflow and provides an upgrade path for existing infrastructure without breaking existing clients that rely on the application/json media type.

In order to use an external context with an ordinary JSON document, an author must specify an IRI to a valid JSON-LD document in an HTTP Link Header [RFC5988] using the describedby link relation. The referenced document must have a top-level JSON object. The @context subtree within that object is added to the top-level object of the referencing document. If an array is at the top-level of the referencing document and its items are objects, the @context subtree is added to all array items. All extra information located outside of the @context subtree in the referenced document must be discarded.

The following example demonstrates the use of an external context with an ordinary JSON document:

GET /ordinary-json-document.json HTTP/1.1
Host: example.com
Accept: application/ld+json,application/json,*/*;q=0.1

====================================

HTTP/1.0 200 OK
...
Content-Type: application/json
Link: <http://json-ld.org/contexts/person>; rel="describedby"; type="application/ld+json"

{
  "name": "Markus Lanthaler",
  "homepage": "http://www.markus-lanthaler.com/",
  "depiction": "http://twitter.com/account/profile_image/markuslanthaler"
}

JSON-LD documents served with the application/ld+json media type must have all context information, including references to external contexts, within the body of the document.

4.5 Expanded Term Definition

Within a context definition, terms may be defined using an expanded notation to allow for additional information associated with the term to be specified (see also Type Coercion and Sets and Lists).

Instead of using a string representation of an IRI, the IRI may be specified using an object having an @id key. The value of the @id key must be either a term, a compact IRI, or an absolute IRI.

{
  "@context":
  {
    "foaf": { "@id": "http://xmlns.com/foaf/0.1/" },
    "name": { "@id": "http://xmlns.com/foaf/0.1/name" },
    "homepage": { "@id": "foaf:homepage" },
    "depiction": { "@id": "foaf:depiction" }
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

This allows additional information to be associated with the term. This may be used for Type Coercion, Sets and Lists), or to associate language information with a term as shown in the following example:

{
  "@context": {
    ...
    "ex": "http://example.com/",
    "@language": "ja",
    "name": { "@id": "ex:name", "@language": null },
    "occupation": { "@id": "ex:occupation" },
    "occupation_en": { "@id": "ex:occupation", "@language": "en" },
    "occupation_cs": { "@id": "ex:occupation", "@language": "cs" }
  },
  "name": "Yagyū Muneyoshi",
  "occupation": "忍者",
  "occupation_en": "Ninja",
  "occupation_cs": "Nindža",
  ...
}

The example above would associate 忍者 with the specified default language code ja, Ninja with the language code en, and Nindža with the language code cs. The value of name, Yagyū Muneyoshi wouldn't be associated with any language code since @language was reset to null in the expanded term definition.

Expanded terms may also be defined using compact IRIs or absolute IRIs as keys. If the definition does not include an @id key, the expanded IRI is determined by performing expansion of the key within the current active context. This mechanism is mainly used to associate type or language information with a compact IRI or an absolute IRI.

While it is possible to define a compact IRI, or an absolute IRI to expand to some other unrelated IRI (for example, foaf:name expanding to http://example.org/unrelated#species), such usage is strongly discouraged.

4.6 Type Coercion

JSON-LD supports the coercion of values to particular data types. Type coercion allows someone deploying JSON-LD to coerce the incoming or outgoing values to the proper data type based on a mapping of data type IRIs to terms. Using type coercion, value representation is preserved without requiring the data type to be specified with each piece of data.

Type coercion is specified within an expanded term definition using the @type key. The value of this key represents a type IRI and must take the form of a term, compact IRI, absolute IRI, or the keyword @id. Specifying @id indicates that within the body of a JSON-LD document, a string value of a term coerced to @id is to be interpreted as an IRI.

Terms or compact IRIs used as the value of a @type key may be defined within the same context. This means that one may specify a term like xsd and then use xsd:integer within the same context definition - the JSON-LD processor will be able to determine the proper expansion for xsd:integer.

The example below demonstrates how a JSON-LD author can coerce values to typed values, IRIs and lists.

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "http://xmlns.com/foaf/0.1/name",
    "age":
    {
      "@id": "http://xmlns.com/foaf/0.1/age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id",
      "@container": "@list"
    }
  },
  "name": "John Smith",
  "age": "41",
  "homepage":
  [
    "http://personal.example.org/",
    "http://work.example.com/jsmith/"
  ]
}

The example above would generate the following Turtle:

@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

[ foaf:name "John Smith";
  foaf:age  "41"^^xsd:integer;
  foaf:homepage ( <http://personal.example.org/> <http://work.example.com/jsmith/> )
] .

Terms may also be defined using absolute IRIs or compact IRIs. This allows coercion rules to by applied to keys which are not represented as a simple term. For example:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "foaf:age":
    {
      "@type": "xsd:integer"
    },
    "foaf:homepage":
    {
      "@type": "@id"
    }
  },
  "foaf:name": "John Smith",
  "foaf:age": "41",
  "foaf:homepage":
  [
    "http://personal.example.org/",
    "http://work.example.com/jsmith/"
  ]
}

In this case the @id definition is optional, but if it does exist, the compact IRI or IRI is treated as a term (not a prefix:suffix construct) so that the actual definition of a prefix becomes unnecessary.

Keys in the context are treated as terms for the purpose of expansion and value coercion. At times, this may result in multiple representations for the same expanded IRI. For example, one could specify that dog and cat both expanded to http://example.com/vocab#animal. Doing this could be useful for establishing different type coercion or language specification rules. It also allows a compact IRI (or even an absolute IRI) to be defined as something else entirely. For example, one could specify that the term http://example.org/zoo should expand to http://example.org/river, but this usage is discouraged because it would lead to a great deal of confusion among developers attempting to understand the JSON-LD document.

Type coercion is performed using the unexpanded value of the key, which must have an exact match for an entry in the active context.

4.7 IRI Expansion Within a Context

In general, normal IRI expansion rules apply anywhere an IRI is expected (see IRIs). Within a context definition, this can mean that terms defined within the context may also be used within that context as long as there are no circular dependencies. For example, it is common to use the xsd namespace when defining typed values:

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "http://xmlns.com/foaf/0.1/name",
    "age":
    {
      "@id": "http://xmlns.com/foaf/0.1/age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
  },
  ...
}

In this example, the xsd term is defined and used as a prefix for the @type coercion of the age property.

Terms may also be used when defining the IRI of another term:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "age":
    {
      "@id": "foaf:age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "foaf:homepage",
      "@type": "@id"
    }
  },
  ...
}

Compact IRIs and IRIs may be used on the left-hand side of a term definition.

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "foaf:age":
    {
      "@type": "xsd:integer"
    },
    "foaf:homepage":
    {
      "@type": "@id"
    }
  },
  ...
}

In this example, the compact IRI form is used in two different ways. In the first approach, foaf:age declares both the IRI for the term (using short-form) as well as the @type associated with the term. In the second approach, only the @type associated with the term is specified. The JSON-LD processor will derive the full IRI for foaf:homepage by looking up the foaf prefix in the context.

Absolute IRIs may also be used in the key position in a context:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "foaf:age":
    {
      "@id": "foaf:age",
      "@type": "xsd:integer"
    },
    "http://xmlns.com/foaf/0.1/homepage":
    {
      "@type": "@id"
    }
  },
  ...
}

In order for the absolute IRI to match above, the absolute IRI must also be used in the JSON-LD document. Also note that foaf:homepage will not use the { "@type": "@id" } declaration because foaf:homepage is not the same as http://xmlns.com/foaf/0.1/homepage. That is, a JSON-LD processor will use direct string comparison when looking up terms in a context before it applies the prefix lookup mechanism.

The only exception for using terms in the context is that they must not be used in a circular manner. That is, a definition of term-1 must not depend on the definition of term-2 if term-2 also depends on term-1. For example, the following context definition is illegal:

{
  "@context":
  {
    "term1": "term2:foo",
    "term2": "term1:bar"
  },
  ...
}

4.8 Embedding

Object embedding is a JSON-LD feature that allows an author to use the definition of JSON-LD objects as property values. This is a commonly used mechanism for creating a parent-child relationship between two subjects.

The example shows two subjects related by a property from the first subject:

{
...
  "name": "Manu Sporny",
  "knows":
  {
    "@type": "Person",
    "name": "Gregg Kellogg",
  }
...
}

An object definition, like the one used above, may be used in any value position in the body of a JSON-LD document.

4.9 Named Graphs

The @graph keyword is used to express a set of JSON-LD subject definitions that may not be directly related to one another through a property. The mechanism may also be used where embedding is not desirable to the application. For example:

{
  "@context": ...,
  "@graph":
  [
    {
      "@id": "http://manu.sporny.org/i/public",
      "@type": "foaf:Person",
      "name": "Manu Sporny",
      "knows": "http://greggkellogg.net/foaf#me"
    },
    {
      "@id": "http://greggkellogg.net/foaf#me",
      "@type": "foaf:Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/i/public"
    }
  ]
}

In this case, embedding doesn't work as each JSON-LD object references the other. Using the @graph keyword allows multiple resources to be defined within an array, and allows the use of a shared context. This is equivalent to using multiple JSON object definitions in array and defining the @context within each object:

[
  {
    "@context": ...,
    "@id": "http://manu.sporny.org/i/public",
    "@type": "foaf:Person",
    "name": "Manu Sporny",
    "knows": "http://greggkellogg.net/foaf#me"
  },
  {
    "@context": ...,
    "@id": "http://greggkellogg.net/foaf#me",
    "@type": "foaf:Person",
    "name": "Gregg Kellogg",
    "knows": "http://manu.sporny.org/i/public"
  }
]

JSON-LD allows you to name things on the Web by assigning an @id to them, which is typically an IRI. This notion extends to the ability to identify graphs in the same manner. A developer may name data expressed using the @graph keyword by pairing it with an @id keyword. This enables the developer to make statements about the linked data graph itself, rather than just a single JSON-LD object.

{
  "@context": ...,
  "@id": "http://example.org/graphs/73",
  "asOf": { "@value": "2012-04-09", "@type": "xsd:date" },
  "@graph":
  [
    {
      "@id": "http://manu.sporny.org/i/public",
      "@type": "foaf:Person",
      "name": "Manu Sporny",
      "knows": "http://greggkellogg.net/foaf#me"
    },
    {
      "@id": "http://greggkellogg.net/foaf#me",
      "@type": "foaf:Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/i/public"
    },
    "http://www.markus-lanthaler.com/"
  ]
}

The example above expresses a named linked data graph that is identified by the IRI http://example.org/graphs/73. That graph is composed of the statements about Manu and Gregg and a reference to another IRI, which could make statements about Markus. Meta-data about the graph itself is also expressed via the asOf property, which specifies when the information was retrieved from the Web.

4.10 Identifying Unlabeled Nodes

At times, it becomes necessary to be able to express information without being able to specify the subject. Typically, this type of node is called an unlabeled node or a blank node. In JSON-LD, unlabeled node identifiers are automatically created if a subject is not specified using the @id keyword. However, authors may provide identifiers for unlabeled nodes by using the special _ (underscore) prefix. This allows one to reference the node locally within the document, but makes it impossible to reference the node from an external document. The unlabeled node identifier is scoped to the document in which it is used.

{
...
  "@id": "_:foo",
...
}

The example above would set the subject to _:foo, which can then be used elsewhere in the JSON-LD document to refer back to the unlabeled node. This practice is frowned upon when generating Linked Data. If a developer finds that they refer to the unlabeled node more than once, they should consider naming the node using a de-referenceable IRI.

4.11 Aliasing Keywords

Each of the JSON-LD keywords, except for @context, may be aliased to application-specific keywords. This feature allows legacy JSON content to be utilized by JSON-LD by re-using JSON keys that already exist in legacy documents. This feature also allows developers to design domain-specific implementations using only the JSON-LD context.

{
  "@context":
  {
     "url": "@id",
     "a": "@type",
     "name": "http://schema.org/name"
  },
  "url": "http://example.com/about#gregg",
  "a": "http://schema.org/Person",
  "name": "Gregg Kellogg"
}

In the example above, the @id and @type keywords have been given the aliases url and a, respectively.

4.12 Expanded Document Form

The JSON-LD API [JSON-LD-API] defines an method for expanding a JSON-LD document. Expansion is the process of taking a JSON-LD document and applying a @context such that all IRIs, datatypes, and values are expanded so that the @context is no longer necessary.

For example, assume the following JSON-LD input document:

{
   "@context":
   {
      "name": "http://xmlns.com/foaf/0.1/name",
      "homepage": {
        "@id": "http://xmlns.com/foaf/0.1/homepage",
        "@type", "@id"
      }
   },
   "name": "Manu Sporny",
   "homepage": "http://manu.sporny.org/"
}

Running the JSON-LD Expansion algorithm against the JSON-LD input document provided above would result in the following output:

[
  {
    "http://xmlns.com/foaf/0.1/name": [
      { "@value": "Manu Sporny" }
    ],
    "http://xmlns.com/foaf/0.1/homepage": [
      { "@id": "http://manu.sporny.org/" }
    ]
  }
]

Expanded document form is useful when an application has to process input data in a deterministic form. It has been optimized to ensure that the code that developers have to write is minimized compared to the code that would have to be written to operate on compact document form.

4.13 Compact Document Form

The JSON-LD API [JSON-LD-API] defines a method for compacting a JSON-LD document. Compaction is the process of taking a JSON-LD document and applying a context such that the most compact form of the document is generated. JSON is typically expressed in a very compact, key-value format. That is, full IRIs are rarely used as keys. At times, a JSON-LD document may be received that is not in its most compact form. JSON-LD, via the API, provides a way to compact a JSON-LD document.

For example, assume the following JSON-LD input document:

[
  {
    "http://xmlns.com/foaf/0.1/name": [ "Manu Sporny" ],
    "http://xmlns.com/foaf/0.1/homepage": [
      {
       "@id": "http://manu.sporny.org/"
      }
    ]
  }
]

Additionally, assume the following developer-supplied JSON-LD context:

{
  "@context": {
    "name": "http://xmlns.com/foaf/0.1/name",
    "homepage": {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
  }
}

Running the JSON-LD Compaction algorithm given the context supplied above against the JSON-LD input document provided above would result in the following output:

{
  "@context": {
    "name": "http://xmlns.com/foaf/0.1/name",
    "homepage": {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/"
}

The compaction algorithm enables a developer to map any document into an application-specific compacted form by first expanding the document. While the context provided above mapped http://xmlns.com/foaf/0.1/name to name, it could have also mapped it to any arbitrary string provided by the developer. This powerful mechanism, along with another JSON-LD API technique called framing, allows the developer to re-shape the incoming JSON data into a format that is optimized for their application.