Vocabulary

One of the key features of Fedify library is that it provides a collection of type-safe objects that represent the Activity Vocabulary and the vendor-specific extensions.

There are tons of objects in the Activity Vocabulary, and it's not practical to list all of them here. Instead, we'll show a few examples of the objects that are available in the library: Create, Note, and Person. For the full list of the objects, please refer to the API reference.

Instantiation

You can instantiate an object by calling the constructor function with an object that contains the properties of the object. The following shows an example of instantiating a Create object:

typescript

import { Create
, Note
 } from "@fedify/fedify";

const create
 = new Create
({
  id
: new URL
("https://example.com/activities/123"),
  actor
: new URL
("https://example.com/users/alice"),
  object
: new Note
({
    id
: new URL
("https://example.com/notes/456"),
    content
: "Hello, world!",
    published
: Temporal.Instant
.from
("2024-01-01T00:00:00Z"),
  }),
});

Note that every URI is represented as a URL object. This is for distinguishing the URIs from the other strings.

TIP

You can instantiate an object from a JSON-LD document by calling the fromJsonLd() method of the object. See the JSON-LD section for details.

Properties

Every object in the Activity Vocabulary has a set of properties. The properties are categorized into the following types:

Functional or non-functional
Scalar or non-scalar

Functional properties are the properties that contain zero or a single value, while non-functional properties are the properties that contain zero or multiple values.

Scalar properties can contain only scalar values (e.g., string, number, boolean, URI), while non-scalar properties can contain both scalar and non-scalar values. Objects like Create, Note, and Person are non-scalar values. Non-scalar properties can contain either objects or URIs (object ID) of the objects.

Depending on the category of the property, the accessors of the property are different. The following table shows examples of the accessors:

	Functional	Non-functional
Scalar	`Object.published`	`Object.name`/`~Object.names`
Non-scalar	`Person.inboxId`/`~Person.getInbox()`	`Activity.actorId`/`~Activity.actorIds`/`~Activity.getActor()`/`~Activity.getActors()`

Some non-functional properties have both singular and plural accessors for the sake of convenience. In such cases, the singular accessors return the first value of the property, while the plural accessors return all values of the property.

NOTE

Some of the properties in Activity Vocabulary have been renamed in Fedify:

Original name	Accessor in Fedify
`alsoKnownAs`	`Application.getAliases()`/`Group.getAliases()`/`Organization.getAliases()`/`Person.getAliases()`/`Service.getAliases()`
`anyOf`	`Question.getInclusiveOptions()`
`attributedTo`	`Object.getAttributions()`
`hreflang`	`Link.language`
`inReplyTo`	`Object.getReplyTargets()`
`isCat`	`Application.cat`/`Group.cat`/`Organization.cat`/`Person.cat`/`Service.cat`
`movedTo`	`Application.successor`/`Group.successor`/`Organization.successor`/`Person.successor`/`Service.successor`
`oneOf`	`Question.getExclusiveOptions()`
`orderedItems`	`OrderedCollection.getItems()`
`publicKeyMultibase`	`Multikey.publicKey`
`publicKeyPem`	`CryptographicKey.publicKey`
`quoteUri`	`Article.quoteUrl`/`ChatMessage.quoteUrl`/`Note.quoteUrl`/`Question.quoteUrl`
`votersCount`	`Question.voters`
`_misskey_quote`	`Article.quoteUrl`/`ChatMessage.quoteUrl`/`Note.quoteUrl`/`Question.quoteUrl`

Object IDs and remote objects

Every object in the Activity Vocabulary has an id property, which is the URI of the object. It is used to identify and dereference the object.

For example, the following two objects are equivalent (where dereferencing URI https://example.com/notes/456 returns the Note object):

typescript

const a
 = new Create
({
  id
: new URL
("https://example.com/activities/123"),
  actor
: new URL
("https://example.com/users/alice"),
  object
: new Note
({
    id
: new URL
("https://example.com/notes/456"),
    content
: "Hello, world!",
    published
: Temporal.Instant
.from
("2024-01-01T00:00:00Z"),
  }),
});
const b
 = new Create
({
  actor
: new URL
("https://example.com/users/alice"),
  object
: new URL
("https://example.com/notes/456"),
});

How are the two objects equivalent? Because for the both objects, ~Activity.getObject() returns the equivalent Note object. Such get*() methods for non-scalar properties are called dereferencing accessors. Under the hood, the get*() methods fetch the remote object from the URI and return the object if no cache hit. In the above example, the await a.getObject() immediately returns the Note object because it's already instantiated, while the await b.getObject() fetches the remote object from the URI and returns the Note object.

If you only need the object ID without fetching the remote object, you can use the *Id/*Ids accessors instead of dereferencing accessors. In the same manner, both a.objectId and b.objectId return the equivalent URI.

TIP

Dereferencing accessors take option documentLoader to specify the method to fetch the remote object. By default, it uses the default document loader which utilizes th fetch() API.

If you want to implement your own document loader, see the DocumentLoader interface in the API reference.

See the Getting a DocumentLoader section for details.

Property hydration

The get*() accessor methods for non-scalar properties automatically populate the property with the remote object when the methods are called even if the property internally contains only the URI of the object. This is called property hydration.

For example, the following code hydrates the object property of the Create object:

typescript

const create
 = new Create
({
  object
: new URL
(
    "https://hollo.social/@fedify/0191e4f3-6b08-7003-9d33-f07d1e33d7b4",
  ),
});

// Hydrates the `object` property:
const note
 = await create
.getObject
();

// Returns the hydrated `object` property; therefore, the following code does
// not make any HTTP request:
const note2
 = await create
.getObject
();

Hydrating the property also affects the JSON-LD representation of the object.

For example, since the following code does not hydrate the object property, the JSON-LD representation of the Create object has the object property with the URI of the object:

typescript

const create
 = new Create
({
  object
: new URL
(
    "https://hollo.social/@fedify/0191e4f3-6b08-7003-9d33-f07d1e33d7b4",
  ),
});

const jsonLd
 = await create
.toJsonLd
();
console
.log
(JSON
.stringify
(jsonLd
));

The above code outputs the following JSON-LD document ("@context" is simplified for readability):

json

{
  "@context": ["https://www.w3.org/ns/activitystreams"],
  "type": "Create",
  "object": "https://hollo.social/@fedify/0191e4f3-6b08-7003-9d33-f07d1e33d7b4"
}

However, if the property is once hydrated, the JSON-LD representation of the object has the object property with the full object:

typescript

// Hydrates the `object` property:
await create
.getObject
();

const jsonLd
 = await create
.toJsonLd
();
console
.log
(JSON
.stringify
(jsonLd
));

The above code outputs the following JSON-LD document ("@context" and some attributes are simplified or omitted for readability):

json

{
  "type": "Create",
  "@context": ["https://www.w3.org/ns/activitystreams"],
  "object": {
    "id": "https://hollo.social/@fedify/0191e4f3-6b08-7003-9d33-f07d1e33d7b4",
    "type": "Note",
    "attributedTo": "https://hollo.social/@fedify",
    "content": "<p>...</p>\n",
    "published": "2024-09-12T06:37:23.593Z",
    "sensitive": false,
    "tag": [],
    "to": "as:Public",
    "url": "https://hollo.social/@fedify/0191e4f3-6b08-7003-9d33-f07d1e33d7b4"
  }
}

Immutability

Every object in the Activity Vocabulary is represented as an immutable object. This means that you cannot change the properties of the object after the object is instantiated. This is for ensuring the consistency of the objects and the safety of the objects in the concurrent environment.

In order to change the properties of the object, you need to clone the object with the new properties. Fortunately, the objects have a clone() method that takes an object with the new properties and returns a new object with the new properties. The following shows an example of changing the ~Object.content property of a Note object:

typescript

import { LanguageString
, Note
 } from "@fedify/fedify";

const noteInEnglish
 = new Note
({
  id
: new URL
("https://example.com/notes/123"),
  content
: new LanguageString
("Hello, world!", "en"),
  published
: Temporal.Now
.instant
(),
});
const noteInChinese
 = noteInEnglish
.clone
({
  content
: new LanguageString
("你好，世界！", "zh"),
});

Parameters of the clone() method share the same type with parameters of the constructor.

Looking up remote objects

See the Looking up remote objects section in the Context docs.

Traversing remote collections

See the Traversing remote collections section in the Context docs.

JSON-LD

Under the hood, every object in the Activity Vocabulary is represented as a JSON-LD document. The JSON-LD document is a JSON object that contains the properties of the object. The following shows an example of the JSON-LD representation of the Create object:

json

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/123",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "id": "https://example.com/notes/456",
    "content": "Hello, world!",
    "published": "2024-01-01T00:00:00Z"
  }
}

If you want to instantiate an object from a JSON-LD document, you can use the fromJsonLd() method of the object. The following shows an example of instantiating a Create object from the JSON-LD document:

typescript

import { Create
 } from "@fedify/fedify";

const create
 = await Create
.fromJsonLd
({
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/123",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "id": "https://example.com/notes/456",
    "content": "Hello, world!",
    "published": "2024-01-01T00:00:00Z"
  }
});

Note that the fromJsonLd() method can parse a subtype as well. For example, since Create is a subtype of Activity, the Activity.fromJsonLd() method can parse a Create object as well:

typescript

import { Activity
 } from "@fedify/fedify";

const create
 = await Activity
.fromJsonLd
({
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id": "https://example.com/activities/123",
  "actor": "https://example.com/users/alice",
  "object": {
    "type": "Note",
    "id": "https://example.com/notes/456",
    "content": "Hello, world!",
    "published": "2024-01-01T00:00:00Z"
  }
});

On the other way around, you can use the toJsonLd() method to get the JSON-LD representation of the object:

typescript

const jsonLd
 = await create
.toJsonLd
();

By default, the toJsonLd() method returns the JSON-LD document which is neither compacted nor expanded. Instead, it processes the JSON-LD document without the proper JSON-LD processor for efficiency.

The toJsonLd() method takes some options to customize the JSON-LD document. For example, you can compact the JSON-LD document with a custom context. In this case, the toJsonLd() method returns the compacted JSON-LD document which is processed by the proper JSON-LD processor:

typescript

const jsonLd
 = await create
.toJsonLd
({
  format
: "compact",
  context
: "https://example.com/context",
});

TIP

Why are the fromJsonLd() and toJsonLd() methods asynchronous? Because both methods may fetch remote documents under the hood in order to compact/expand a JSON-LD document. In fact, like the dereferencing accessors, both fromJsonLd() and toJsonLd() methods take option documentLoader to specify the method to fetch the remote document.

See the Getting a DocumentLoader section for details.

Scalar types

The Activity Vocabulary has a few scalar types that are used as the values of the properties. The following table shows the scalar types and their corresponding TypeScript types:

Scalar type	TypeScript type
`xsd:boolean`	`boolean`
`xsd:integer`	`number`
`xsd:nonNegativeInteger`	`number`
`xsd:float`	`number`
`xsd:string`	`string`
`xsd:anyURI`	`URL`
`xsd:dateTime`	`Temporal.Instant`
`xsd:duration`	`Temporal.Duration`
`rdf:langString`	`LanguageString`
`w3id:cryptosuiteString`	`"eddsa-jcs-2022"`
`w3id:multibase`	`Uint8Array`
Language tag (BCP 47)	`LanguageTag`
Public key PEM	`CryptoKey`
Public key Multibase	`CryptoKey`
Proof purpose	`"assertionMethod" \| "authentication" \| "capabilityInvocation" \| "capabilityDelegation" \| "keyAgreement"`
Units	`"cm" \| "feet" \| "inches" \| "km" \| "m" \| "miles" \| URL`

Vocabulary ​

Instantiation ​

Properties ​

Object IDs and remote objects ​

Property hydration ​

Immutability ​

Looking up remote objects ​

Traversing remote collections ​

JSON-LD ​

Scalar types ​

Vocabulary

Instantiation

Properties

Object IDs and remote objects

Property hydration

Immutability

Looking up remote objects

Traversing remote collections

JSON-LD

Scalar types