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Simple abstraction for first-class event emitters


A subscription is an object that emits events. Rather than overloading objects with other roles to emit events, and having a single object emit several types, as in .addEventListener style events, a subscription is a dedicated object representing a single event source.

// Instead of adding listeners by string type
object.addEventListener("message", m => { ... })

// You create a subscription object when you construct the base
// object...
class MyObject {
  constructor() {
    this.message = new Subscription

// ...and add handlers directly to that:
myObject.message.add(m => { ... })

The interface of a subscription is roughly based on js-signals, but whereas js-signals have a fixed dispatch algorithm, subscriptions come in several forms.

The perceived benefits of subscriptions are that they are easier to check statically (instead of a single method dispatching on a bunch of strings, you have actual objects representing your event types, each of which expects a single type of handler function), and lead to more explicit code.


This module is licensed under an MIT license. The recommended way to install it is with NPM.


class Subscription

A subscription is an object that you can add subscribers (functions) to, which will be called every time the subscription is dispatched.

add(f: Function, priority: ?number)

Add a function of the appropriate type for this subscription to be called whenever the subscription is dispatched. When priority is provided (default is zero), it determines when the function is called relative to other handlers—those with a high priority come first.

addOnce(f: Function, priority: ?number)

Add a function to be called once, the next time this subscription is dispatched.

remove(f: Function)

Remove the given subscriber from the subscription.

hasHandler() → bool

Returns true if there are any functions registered with this subscription.

dispatch(...args: [any])

Call all handlers for this subscription with the given arguments.

class PipelineSubscription extends Subscription

A type of subscription that passes the value it is dispatched with through all its subscribers, feeding the return value from each subcriber to the next one, and finally returning the result.

dispatch(value: any) → any

Run all handlers on the given value, returning the result.

class StoppableSubscription extends Subscription

A stoppable subscription is a subscription that stops calling further subscribers as soon as a subscriber returns a truthy value.

dispatch(...args: [any]) → any

Call handlers with the given arguments. When one of them returns a truthy value, immediately return that value.

class DOMSubscription extends Subscription

A DOM subscription can be used to register intermediate handlers for DOM events. It will call subscribers until one of them returns a truthy value or calls preventDefault on the DOM event.

dispatch(event: Event) → bool

Run handlers on the given DOM event until one of them returns a truty value or prevents the event's default behavior. Returns true if the event was handled.