JSON-LD Syntax 1.0

4.1 Compact IRIs

Term s 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 term s 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 term s, 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 IRI s 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" }
  },
  "@id": "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"
}

4.2 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 two 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. values.
3. By using a native JSON type such as number , true , or false .

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 with the type of http://www.w3.org/2001/XMLSchema#dateTime . Note that it is also possible to use a term or a compact IRI to express the value of a type.

The @type keyword is also used to associate a type with a subject node . Although the same keyword is used in both places, the The concept of an object node 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.

{
...
  "@id": "http://example.org/posts#TripToWestVirginia",
  "@type": "http://schema.org/BlogPosting",
  "modified":
  {
    "@value": "2010-05-29T14:17:39+02:00",
    "@type": "http://www.w3.org/2001/XMLSchema#dateTime"
  }
...
}

The first use of @type associates a node type ( http://schema.org/BlogPosting ) with the node , which is expressed using the @id keyword . The second use of @type associates a value type ( http://www.w3.org/2001/XMLSchema#dateTime ) with the value expressed using the @value keyword . As a general rule, when @value and @type are used in the same JSON object , the @type keyword is expressing a value type . Otherwise, the @type keyword is expressing a node type .

4.3 External Contexts Language-tagged Strings

Authors may choose to declare JSON-LD context s 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, A string with an author must specify associated language, also known as a language-tagged string , is indicated by associating a string with an IRI language code as defined in [ BCP47 to a valid ]. Language-tagged strings may be expressed in JSON-LD document. The referenced document must have in four ways:

1. By defining a top-level subject definition . The value of any global language using the @context @language key keyword within that object is substituted for a @context section.
2. By utilizing the IRI @language keyword when defining a term within a @context section.
3. By utilizing the referencing document to have the same effect as if expanded form for specifying values.
4. By utilizing the @container keyword with a value were specified inline of @language when defining a term within the referencing document. a @context section. This usage pattern is called a language map .

The following first example demonstrates uses the use of an external context: { , "name": "Manu Sporny", "homepage": "http://manu.sporny.org/", "depiction": "http://twitter.com/account/profile_image/manusporny" } Authors may also import multiple contexts or @language keyword to associate a combination of external and local contexts by specifying type with a list of contexts: particular term in the @context :

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

  {
    "@type": "Event",
    "description": "International Talk Like a Pirate Day",
    "date": "R/2011-09-19"
  }

    "title":
    {
      "@id": "http://purl.org/dc/terms/title",
      "@language": "en"
    }
  },
...
  "title": "JSON-LD Syntax",
...

}

Each context in a list will be evaluated in-order. Duplicate mappings among the context s must be overwritten on a last-defined-overrides basis. The context list must modified contain either de-referenceable IRI s or JSON object s that conform key's value above is automatically language coerced to a English value because of the context syntax as described information specified in this document. the @context .

An author may nest contexts within subject definitions , with The second example uses the more deeply nested contexts overriding expanded form of setting the values language information in previously defined contexts: the body of a JSON-LD document:

{
  "@context":

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

    "title":
    {
      "@id": "http://purl.org/dc/terms/title"
    }
  },
...
  "title":
  {
    "@value": "JSON-LD Syntax",
    "@language": "en"
  }
...

}

In the example above, Both examples above would generate the value name JSON-LD Syntax prefix is overridden in tagged with the more deeply nested language details en structure. Note that this is rarely a good authoring practice and ; which is typically used when the JSON object has legacy applications using the structure of [ BCP47 ] code for the object. English language.

External JSON-LD context documents may contain extra information located outside of the @context key, Systems that support multiple languages often need to express data values in each language. Typically, such as documentation about systems also try to ensure that developers have a programatically easy way to navigate the prefixes datastructures for the language-specific data. In this case, language map declared s may be utilized.

{
  "@context":
  {
    "title":
    {
      "@id": "http://purl.org/dc/terms/title"
      "@container": "@language"
    }
  },
...
  "title":
  {
    "en": "JSON-LD Syntax",
    "ru": "JSON-LD Синтаксис",
    "ja": "JSON-LDの構文"
  }
...
}

In the example above, the title is expressed in three languages; English, Russian, and Japanese. To access the document. When importing data above in a @context value from an external JSON-LD context document, any extra information contained outside of programming language supporting dot-notation accessors for object properties, a developer may use the @context property.language value must be discarded. It is also recommended that a human-readable document is served as well pattern. For example, to explain access the correct usage Japanese version of the JSON-LD context document. title, a developer would use the following code snippet: obj.title.ja .

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 subject node definition . The @context subtree within that object is added to the top-level subject node definition of the referencing document. If an array is at the top-level of the referencing document and its items are subject node definitions , 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.jsonld>; rel="describedby"; type="application/ld+json"
{
  "name": "Markus Lanthaler",
  "homepage": "http://www.markus-lanthaler.com/",
  "depiction": "http://twitter.com/account/profile_image/markuslanthaler"
}

4.5 Expanded Term Definition

Within a context definition, term s may be defined using an expanded notation to allow for additional information associated with the term to be specified (see also 4.6 Type Coercion and 4.9 Sets and Lists ).

Instead of using a string representation of an IRI , the IRI may be specified using a JSON object having an @id key. The value of the @id key must be either a term , a compact IRI , or an absolute IRI . Such an object is called a subject node reference .

{
  "@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 4.6 Type Coercion , 4.9 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 .

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 IRI s to term s. 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 4.5 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 value s, 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 be 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.

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 Property Generators

At times, an author may find that they need to express the same value for multiple properties. The simplest approach to accomplish this goal would be to do the following:

{
  "@context":
  {
    "title1": "http://purl.org/dc/terms/title",    "title2": "http://schema.org/name",    "title3": "http://www.w3.org/2000/01/rdf-schema#label"
  },
  "@id": "http://example.com/book",
  "title1": "The Count of Monte Cristo",  "title2": "The Count of Monte Cristo",  "title3": "The Count of Monte Cristo"
}

Unfortunately, the approach above produces redundant data and would become a publishing burden for large data sets. In these situations, the author may use a property generator to express a term once, but have the JSON-LD processor expand the single statement into multiple statements. This method can be accomplished by using the following markup pattern:

{
  "@context":
  {
    "title": { "@id": ["http://purl.org/dc/terms/title",
                       "http://schema.org/name",
                       "http://www.w3.org/2000/01/rdf-schema#label"] },
  },
  "@id": "http://example.com/book",
  "title": "The Count of Monte Cristo"
}

While the term above is only used once outside of the @context , a JSON-LD processor will internally transform the document above into the following set of statements:

<http://example.com/book>
  <http://purl.org/dc/terms/title>
    "The Count of Monte Cristo" .
<http://example.com/book>
  <http://schema.org/name>
    "The Count of Monte Cristo" .
<http://example.com/book>
  <http://www.w3.org/2000/01/rdf-schema#label>
"The
Count
of
Monte
Cristo"
.

4.8 IRI Expansion Within a Context

In general, normal IRI expansion rules apply anywhere an IRI is expected (see 3.5 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 value s:

{
  "@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.

Term s 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 term s 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 4.9 Sets and Lists

A JSON-LD author can express multiple values in a compact way by using array s. Since graphs do not describe ordering for links between nodes, arrays in JSON-LD do not provide an ordering of the listed objects contained elements 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 node to an individual object , value, 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>

<http://xmlns.com/foaf/0.1/nick>

"jaybee"
.

Multiple values may also be expressed using the expanded 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>

<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" ],
...
}

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 4.15 Compact Document Form . 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.

4.9 4.10 Embedding

Object embedding Embedding is a JSON-LD feature that allows an author to use subject node definitions as property values. This is a commonly used mechanism for creating a parent-child relationship between two subject node s.

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

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

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

4.10 4.11 Named Graphs

The @graph keyword is used to express a set of JSON-LD subject node definition s 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 subject node definition references the other. Using the @graph keyword allows multiple resources to be defined within an array , and allows the use of a shared context . When used in a JSON object that is not otherwise a subject node definition , this describes resources in the default graph . This is equivalent to using multiple subject node definitions in array and defining the @context within each subject node definition :

[
  {
    "@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 a linked data graph itself, rather than just a single subject node .

{
  "@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"
    }
  ]
}

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. Meta-data about the graph itself is also expressed via the asOf property, which specifies when the information was retrieved from the Web.

4.11 4.12 Identifying Unlabeled Nodes

At times, it becomes necessary to be able to express information without being able to specify the subject. node. Typically, this type of node is called an unlabeled node or a blank node (see [ RDF-CONCEPTS ] Section 3.4: Blank Nodes ). In JSON-LD, unlabeled node identifiers are automatically created if a subject node 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 node to _:foo , which can then be used elsewhere in the JSON-LD document to refer back to the unlabeled node . 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 so that it can be referenced also from other documents.

4.12 4.13 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.

4.13 4.14 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, types, 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"

        "@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 4.15 Compact Document Form .

4.14 4.15 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 4.14 Expanded Document Form . 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.