JSON-LD Syntax 1.0

4.1 6.1 Compact IRIs

Term Terms 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 terms 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, 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 IRI IRIs 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 6.2 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 three ways:

1. By utilizing the @type keyword when defining a term within a @context section.
2. By utilizing the an expanded form for specifying values. typed value .
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 the value 2010-05-29T14:17:39+02:00 with the type 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 node . The concept of an a 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 6.3 Language-tagged Strings

A string with an associated language, also known as a language-tagged string , , is indicated by associating a string with an a language code as defined in [ BCP47 ]. Language-tagged strings may be expressed in JSON-LD in four ways:

1. By defining a global language using the @language keyword within a @context section.
2. By utilizing the @language keyword when defining a term within a @context section.
3. By utilizing the an expanded form for specifying values. language-tagged string .
4. By utilizing a language map , provided the underlying term is defined with a @container keyword with a whose value of is @language when defining a term within a @context section. This usage pattern is called a language map .

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

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

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

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

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

Both examples above would generate the value JSON-LD Syntax tagged with the language en ; which is the [ BCP47 ] code for the English language.

Systems that support multiple languages often need to express data values in each language. Typically, such systems also try to ensure that developers have a programatically easy way to navigate the datastructures for the language-specific data. In this case, language map maps s may be utilized.

{
  "@context":
  {
    "title":
    {
      "@id": "http://purl.org/dc/terms/title"
      "@container": "@language"

      "@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 data above in a programming language supporting dot-notation accessors for object properties, a developer may use the property.language pattern. For example, to access the Japanese version of the title, a developer would use the following code snippet: obj.title.ja .

4.4 6.4 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 http://www.w3.org/ns/json-ld#context link relation. The referenced document must have a top-level node definition object . The @context subtree within that object is added to the top-level node definition object of the referencing document. If an array is at the top-level of the referencing document and its items are node definitions 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.jsonld>; rel="http://www.w3.org/ns/json-ld#context"; type="application/ld+json"
{
  "name": "Markus Lanthaler",
  "homepage": "http://www.markus-lanthaler.com/",
  "depiction": "http://twitter.com/account/profile_image/markuslanthaler"
}

Please note that 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. Contexts linked via a http://www.w3.org/ns/json-ld#context HTTP Link Header must be ignored for such documents.

4.5 6.5 Expanded Term Definition

Within a context definition, term terms s may be defined using an expanded notation term definition to allow for additional information associated with the term to be specified (see also 4.6 6.6 Type Coercion and 4.9 6.9 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 key, and a term , a compact IRI , or an absolute IRI . Such an object is called a node reference . as value.

{
  "@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 6.6 Type Coercion , 4.9 6.9 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. 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. context . This mechanism is mainly used to associate type or language information with a compact IRI or an absolute IRI .

4.6 6.6 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 IRIs s to term s. 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 4.5 6.5 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 expands to an , absolute IRI , or . Alternatively, the keyword @id . Specifying @id indicates may be used as value to indicate 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 . definition.

The example below demonstrates how a JSON-LD author can coerce values to typed value s, 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"
    }
  },
  "@id": "http://example.com/people#john",

  "name": "John Smith",
  "age": "41",
  "homepage":
  [
    "http://personal.example.org/",
    "http://work.example.com/jsmith/"
  ]
}

The example markup shown above would generate the following Turtle: data. The data has no inherent order except for the values the http://xmlns.com/foaf/0.1/homepage property which represent an ordered list.

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":
    {
      "@id": "http://xmlns.com/foaf/0.1/age",
      "@type": "xsd:integer"
    },
    "":
    {

    "http://xmlns.com/foaf/0.1/homepage":
    {

      "@type": "@id"
    }

    }

  },
  "foaf:name": "John Smith",
  "foaf:age": ,
  "foaf:homepage":
  [

  "foaf:age": "41",
  "http://xmlns.com/foaf/0.1/homepage":
  [

    "http://personal.example.org/",
    "http://work.example.com/jsmith/"
  ]

  ]

}

In this case the @id definition in the term 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 has to match exactly an entry in the active context .

4.7 6.7 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 that maps to multiple properties in the JSON-LD processor expand the single statement into multiple statements. graph . 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 is equivalent to the following set of statements: following:

4.8 6.8 IRI Expansion Within a Context

In general, normal IRI expansion rules apply anywhere an IRI is expected (see 3.7 5.3 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 Terms 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 is determined 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 needs to be used in the JSON-LD document. 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 terms s are looked up in a context using direct string comparison before it applies the prefix lookup mechanism. mechanism is applied.

The only exception for using terms in the context is that they must not be used in a circular manner. definitions are not allowed. That is, a definition of term-1 must not cannot 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"
  },
  ...
}

4.9 6.9 Sets and Lists

A JSON-LD author can express multiple values in a compact way by using array s. arrays . Since graphs do not describe ordering for links between nodes, arrays in JSON-LD do not provide an ordering of the 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 the following data being generated, each relating the node to an individual value, with no inherent order:

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 data, again with no inherent order:

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 4.15 6.17 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.

4.10 6.10 Embedding

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

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

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

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

4.11 6.11 Named Graphs

The @graph keyword At times, it is used necessary to express make statements about a set of JSON-LD node definition graph s that may not be directly related to one another through itself, rather than just a property. The mechanism may also be used where embedding is not desirable to the application. For example: { "@context": ..., "": [ { "@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 single 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 This can be done by grouping a JSON object set of nodes that is not otherwise a node definition , this describes resources in the default graph . This is equivalent to using multiple node definitions in array and defining the @context within each node definition : { "@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" } ] JSON-LD allows you to name things on the Web by assigning an @id @graph to them, which is typically an IRI keyword . This notion extends to the ability to identify graphs in the same manner. A developer may also 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 node . as shown in the following example:

{
  "@context": {
    "asOf": "http://purl.org/net/provenance/ns#accessedResource",

    "generatedAt": "http://www.w3.org/ns/prov#generatedAtTime",

    "Person": "http://xmlns.com/foaf/0.1/Person",
    "name": "http://xmlns.com/foaf/0.1/name",
    "knows": "http://xmlns.com/foaf/0.1/knows",
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "@id": "http://example.org/graphs/73",
  "asOf": { "@value": "2012-04-09", "@type": "xsd:date" },

  "generatedAt": { "@value": "2012-04-09", "@type": "xsd:date" },

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

The example above expresses a named linked data JSON-LD graph that is identified by the IRI http://example.org/graphs/73 . That graph is composed of the statements about Manu and Gregg. Metadata about the graph itself is also expressed via the asOf generatedAt property, which specifies when the information graph was retrieved from the Web. generated. An alternative view of the information above is represented in table form below:

When @graph is used in a document's top-level object which has no other properties that are mapped to an IRI or a keyword it is considered to express the otherwise implicit default graph. This mechanism can be useful when a number of nodes thay may not directly relate to one another through a property or 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 node object references the other. Using the @graph keyword allows multiple nodes to be defined within an array , and allows the use of a shared context . This is equivalent to using multiple node objects in array and defining the @context within each node 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"
  }
]

4.12 6.12 Identifying Unlabeled Blank Nodes

At times, it becomes necessary to be able to express information without being able to specify the node. Typically, this This type of node is called an unlabeled node or a blank node (see [ RDF-CONCEPTS ] (see Section 3.4: Blank Nodes ). of [ RDF-CONCEPTS ]). In JSON-LD, unlabeled blank node identifiers are automatically created if a node is not specified using the @id keyword . However, authors may provide identifiers for unlabeled blank 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 blank node identifier is scoped to the document in which it is used.

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

The example above would set the node to _:foo , which can then be used elsewhere in the JSON-LD document to refer back to the unlabeled blank node . If a developer finds that they refer to the unlabeled blank node more than once, they should consider naming the node using a de-referenceable dereferenceable IRI so that it can also be referenced also from other documents.

4.13 6.13 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.

6.14 Data Annotations

It is common for developers using JSON to organize their data in ways that makes working with the data more efficient. It is often that these methods of organizing data are not meant to express Linked Data, but should survive transformation by JSON-LD. For example, if a developer organizes employees in a JSON-LD document by a company-issued ID number, JSON-LD should not destroy that 'database index' when transforming the data. Data annotations allow content that would otherwise be removed from a JSON-LD graph to be preserved by instructing the JSON-LD processor to syntactically preserve the annotation information and continue processing deeper into the JSON data structure.

{
  "@context":
  {
     "schema": "http://schema.org/",
     "Article": "schema:Blog",
     "name": "schema:name",
     "articleBody": "schema:articleBody",
     "wordCount": "schema:wordCount",
     "commentCount": "http://example.com/schema/wordCount",
     "blogPost": {
       "@id": "schema:blogPost",
       "@container": "@annotation"
  },
  "@id": "http://example.com/",
  "@type": "Blog",
  "name": "World Financial News",
  "blogPost": {
     "en": {
       "@id": "http://example.com/posts/1/en",
       "articleBody": "World commodities were up today with heavy trading of crude oil...",
       "wordCount": 1539,
       "commentCount": 64
     },
     "de": {
       "@id": "http://example.com/posts/1/de",
       "articleBody": "Welt Rohstoffe waren bis heute mit schweren Handel mit Rohöl...",
       "wordCount": 1204,
       "commentCount": 23
     }
  }
}

In the example above, the blogPost term has been marked as a data annotation container . The en , de , and ja keys will effectively be ignored semantically, but preserved syntactically, by the JSON-LD Processor as annotations . The interpretation of the data above is expressed in the table below. Note how the annotations do not appear in the Linked Data below, but would continue to exist if the document were compacted or expanded using a JSON-LD processor:

6.15 Explicitly Ignoring Data

At times, it becomes necessary to explicitly ignore data expressed in JSON documents because it has no semantic meaning. For example, when the @vocab keyword is used, every key in a JSON-LD object is appended to the vocabulary IRI . The author may not want that behavior to apply to every key, and it may be easier to specify just the keys that they want the JSON-LD processor to ignore. For this purpose, an author may associate the null keyword with a term in the JSON-LD Context.

{
  "@context":
  {
     "@vocab": "http://schema.org/",
     "databaseId": null
  },
  "name": "Manu Sporny",
  "description": "That guy",
  "gender": "Male",
  "databaseId": "23987520"
}

In the example above, the author has used @vocab as the base IRI for all terms in the document, but has expressed that the databaseId value should not be processed by the JSON-LD processor by associating it with the null keyword in the JSON-LD Context.

4.14 6.16 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"
      }
   },
   "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 4.15 6.17 Compact Document Form .

4.15 6.17 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 4.14 6.16 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 , mechanism allows the developer to re-shape the incoming JSON data into a format that is optimized for their application.