Event

An event is a fact the application records, such as “this post was published.” You name it and give it the schema of its payload. new stores a payload you already typed. parse checks an unknown value and then constructs the event.

class PostPublished extends Event('post.published', PostSchema) {}

Register the class on an event catalog when a publisher and a subscriber need to agree on the set. An event handler performs the work that follows from one event.

Why it exists

Publishing a post saves the post. Emailing subscribers is a consequence, and it is a different story: it can fail, retry, and run in another process without changing whether the post is published. The event is the fact the use case records. The handler is where the email happens. The payload schema keeps that fact the same shape on both sides.

How it’s used

The use case constructs the event from the post it just saved and hands it to a publisher port. The handler later reads event.payload.

class PostPublished extends Event('post.published', PostSchema) {}

await events.publish(
  new PostPublished({
    id: post.props.id,
    title: post.props.title,
    body: post.props.body,
    published: true,
  }),
)

new PostPublished(...) trusts that object. When the payload arrives as unknown JSON, the boundary uses PostPublished.parse(value), or the catalog’s parse for a { key, payload } message.

The string 'post.published' is the event’s token. A catalog map key, such as postPublished, is a separate name. The catalog page covers that split.

API

Event(token, schema) returns the class. token is a string literal. schema is a Standard Schema or a Schema class. A Schema class is stored as its definition.

Member	Role
PostPublished.token	The name you passed in.
PostPublished.schema	The payload schema.
new PostPublished(payload)	Stores payload. The constructor expects the schema’s output type.
PostPublished.parse(value)	Checks value and returns an instance of the class you called it on.
event.token	The same literal as the static token.
event.payload	The payload.

A rejected parse throws a coded error with code VALIDATION and the message hexok: post.published validation failed.

Override parse when the boundary needs to preprocess the value. Callers can tell two event classes apart with instanceof or by comparing event.token.