awayback v7.0.0

A custom event system with type-safe events.

import awayback from 'awayback'
const events = awayback()

events.on('event', (data) => {
  console.log(data) // OUTPUT: data
})

events.emit('event', 'data')

Installation

$ npm i awayback

Afterwards:

// Import and create a new 'awayback' event system instance.
import awayback from 'awayback'
const events = awayback()

API

awayback(replayable?)

Create a new awayback event system instance.

• replayable <Array> Optional array of event names to enable replay functionality for specific events. When undefined (default), no events support replay.
• returns: Awayback<D, R> where D is your event definitions and R is the replayable array type

Event replay allows listeners to receive and process previous event emissions when added with the isReplaying option set to true. By default, no events are replayed. Specify an array of event names to enable replay functionality for only those events.

type Events = {
  login: (userId: string) => void
  logout: (userId: string) => void
}

// No events support replay by default
const events = awayback<Events>()

// isReplaying cannot be 'true' without a replay array
events.on(
  'login',
  (userId) => {
    console.log(userId)
  },
  { isReplaying: true }
) // TypeScript error: isReplaying must be false

// To enable replay, specify which events to track
const events = awayback<Events, ['login']>(['login'])

// Now isReplaying can be 'true' for 'login' events
events.on(
  'login',
  (userId) => {
    console.log(userId)
  },
  { isReplaying: true }
)

The TypeScript compiler helps prevent accidental usage of replay features on events that aren't configured for replay. This type safety ensures you can't mistakenly attempt to replay events that haven't been stored.

type Events = {
  login: () => void
  logout: () => void
}

// Only the 'login' event will support replay
const events = awayback<Events>(['login'])

// isReplaying can be 'true' for 'login' events
events.on('login', () => {}, { isReplaying: true })

// TypeScript error: isReplaying cannot be 'true' for 'logout' since it's not in the replay array
events.on('logout', () => {}, { isReplaying: true })

// Regular event listening works normally, just no replay of previous events
events.on('logout', () => {}, { isReplaying: false })

For optimal performance in applications with many events, use the replay parameter to restrict replay functionality to only the events that need it. Events not specified in the replay array will not store their historical data, reducing memory usage and improving performance when handling high-frequency events.

Type-Safe Definitions (PropertyKey)

The event definition type D extends Definition, which supports strings, numbers, and symbols as keys. This allows you to use unique symbols to avoid naming collisions in complex applications.

const symbol = Symbol('event')

type Events = {
  [symbol]: (status: string) => void // Symbol key
  100: (message: string) => void // Number key
  other: () => void // String key
}

const events = awayback<Events>()

events.on(symbol, (status) => {
  console.log(status)
})

events.emit(symbol, 'active')

Invalid Keys

The event name "*" is reserved for internal use (such as wildcard defaults in .bind()) and cannot be used as a key in your definitions.

.emit(event, ...data)

• event <String>
• ...data <Any>

Events can be fired using the function emit. The first required argument is the name of the event (key). The second argument (optional) is data that should be transmitted.

events.emit('name', 'data')

It is possible to use strings, symbols or numbers as event keys.

const symbol = Symbol('event')

events.emit('name', 'data')
events.emit(symbol, 'data')
events.emit(404, 'not found')

Multiple arguments

More than two arguments can be passed on.

// Listen to the event
events.on('event', (...data) => {
  console.log(data)
  // OUTPUT: ['some', 'data']
})

events.emit('event', 'some', 'data')

.on(event, callback, options)

• event <String>
• callback <Function>
  • ...data <Any>
• options <Object>
  • predicate <Function>
    • ...data <Any>
    • returns: <Boolean>
  • signal <AbortSignal>
  • isReplaying <Boolean>
  • isDistinct <Boolean>
• returns: <Function> A cleanup function that, when called, removes the listener.

The callback is executed each time the event is fired. The arguments from the emit function call are also exposed to the listener.

events.on('name', (data) => {
  // Event is fired, callback executed
  // Use transmitted data
  console.log(data)
})

If the predicate option is provided, the callback is only executed if the predicate function returns true.

events.on(
  'event',
  (data) => {
    // Event is fired, callback executed
    // Use transmitted data
    console.log(data)
  },
  {
    predicate: (data) => data[0] === 'some',
  }
)

events.emit('event', ['some', 'data'])
// OUTPUT: ['some', 'data']

If the signal option is provided, the listener can be aborted using an AbortController.

const controller = new AbortController()
const { signal } = controller

events.on(
  'event',
  (data) => {
    console.log(data)
    // OUTPUT: data
  },
  { signal }
)

events.emit('event', 'data')

// Abort the listener
controller.abort()

events.emit('event', 'more data')
// No output, listener has been aborted

The signal option allows for fine-grained control over event listeners, enabling you to cancel them when they are no longer needed.

If the isDistinct option is set to true, the listener will only be registered if the same callback function has not already been registered for this event. Duplicate registrations are silently ignored.

const handler = (data) => {
  console.log(data)
}

events.on('event', handler, { isDistinct: true })
events.on('event', handler, { isDistinct: true }) // Ignored — handler already registered

events.emit('event', 'data')
// OUTPUT: data  (printed once, not twice)

If the argument isReplaying in options is set to true, the listener executes event calls from before the initialization of the listener. Note that this only works for events specified in the replay array when creating the awayback instance.

// Create instance with replay enabled for 'event'
const events = awayback(['event'])

// Fire event before the initialization of .on()
// This event call will also be executed
events.emit('event', 'before')

setTimeout(() => {
  events.on(
    'event',
    (data) => {
      console.log(data)
      // OUTPUT: before
      // OUTPUT: after
      // OUTPUT: another call
    },
    {
      isReplaying: true,
    }
  )

  events.emit('event', 'after')
  events.emit('event', 'another call')
}, 1000)

The function returns an unsubscribe function, providing a convenient way to stop listening without manually calling .remove().

const unsubscribe = events.on('event', (data) => {
  console.log(data)
})

events.emit('event', 'hello') // Logs: hello

unsubscribe() // Stop listening

.once(event, callback, options)

• event <String>
• callback <Function>
  • ...data <Any>
• options <Object>
  • predicate <Function>
    • ...data <Any>
    • returns: <Boolean>
  • signal <AbortSignal>
  • isReplaying <Boolean>
  • isDistinct <Boolean>
• returns: <Function> A cleanup function that, when called, removes the listener.

The callback is only executed once when the event is called first. The arguments from the emit function call are also exposed to the listener.

events.once('event', (data) => {
  // Event is fired, callback executed
  // Use transmitted data
  console.log(data)
  // OUTPUT: one
})

events.emit('event', 'one')
events.emit('event', 'two')

If the predicate option is provided, the callback is only executed if the predicate function returns true.

events.once(
  'event',
  (data) => {
    // Event is fired, callback executed
    // Use transmitted data
    console.log(data)
  },
  {
    predicate: (data) => data[0] === 'some',
  }
)

events.emit('event', ['some', 'data'])
// OUTPUT: ['some', 'data']

If the signal option is provided, the listener can be aborted using an AbortController.

const controller = new AbortController()
const { signal } = controller

events.once(
  'event',
  (data) => {
    console.log(data)
    // OUTPUT: data
  },
  { signal }
)

events.emit('event', 'data')

// Abort the listener
controller.abort()

events.emit('event', 'more data')
// No output, listener has been aborted

The signal option allows for fine-grained control over event listeners, enabling you to cancel them when they are no longer needed.

If the argument isReplaying in options is set to true, the listener executes event calls from before the initialization of the listener. Note that this only works for events specified in the replay array when creating the awayback instance.

// Create instance with replay enabled for 'event'
const events = awayback(['event'])

// Fire event before the initialization of .once()
// This event call will also be executed
events.emit('event', 'before')

setTimeout(() => {
  events.once(
    'event',
    (data) => {
      console.log(data)
      // OUTPUT: before
    },
    {
      isReplaying: true,
    }
  )
}, 1000)

The function returns an unsubscribe function, providing a convenient way to stop listening without manually calling .remove().

const unsubscribe = events.once('event', (data) => {
  console.log(data)
})

events.emit('event', 'hello') // Logs: hello

unsubscribe() // Stop listening

.only(event, callback, options)

• event <String>
• callback <Function>
  • ...data <Any>
• options <Object>
  • predicate <Function>
    • ...data <Any>
    • returns: <Boolean>
  • signal <AbortSignal>
  • isReplaying <Boolean>
  • isDistinct <Boolean>
• returns: <Function> A cleanup function that, when called, removes the listener.

The callback is only executed if this callback is the first listener registered and no other listener has been executed yet. The arguments from the emit function call are also exposed to the listener.

events.only('name', (data) => {
  // Event is fired, callback executed
  // Use transmitted data
  console.log(data)
})

events.on('event', () => {})

events.only('event', (data) => {
  // Event is fired, callback is not executed
  // because it's not the first event listener
  console.log(data)
})

events.emit('event', 'data')

If the predicate option is provided, the callback is only executed if the predicate function returns true.

events.only(
  'event',
  (data) => {
    // Event is fired, callback executed
    // Use transmitted data
    console.log(data)
  },
  {
    predicate: (data) => data[0] === 'some',
  }
)

events.emit('event', ['some', 'data'])
// OUTPUT: ['some', 'data']

If the signal option is provided, the listener can be aborted using an AbortController.

const controller = new AbortController()
const { signal } = controller

events.only(
  'event',
  (data) => {
    console.log(data)
    // OUTPUT: data
  },
  { signal }
)

events.emit('event', 'data')

// Abort the listener
controller.abort()

events.emit('event', 'more data')
// No output, listener has been aborted

The signal option allows for fine-grained control over event listeners, enabling you to cancel them when they are no longer needed.

If the argument isReplaying in options is set to true, the listener executes event calls from before the initialization of the listener. Note that this only works for events specified in the replay array when creating the awayback instance.

// Create instance with replay enabled for 'event'
const events = awayback(['event'])

// Fire event before the initialization of .only()
// This event call will also be executed
events.emit('event', 'before')

setTimeout(() => {
  events.only(
    'event',
    (data) => {
      console.log(data)
      // OUTPUT: before
    },
    {
      isReplaying: true,
    }
  )
}, 1000)

The function returns an unsubscribe function, providing a convenient way to stop listening without manually calling .remove().

const unsubscribe = events.only('event', (data) => {
  console.log(data)
})

events.emit('event', 'hello') // Logs: hello

unsubscribe() // Stop listening

.bind(events, types, options)

• events <Object>
  • Key-value pairs of event names and their corresponding callback functions.
• types <Object> (Optional)
  • Key-value pairs of event names and their corresponding ListenerType (on, once, only).
  • Use the wildcard key '*' to define a default type for all bound events.
• options <Object> (Optional)
  • Key-value pairs of event names and their corresponding ListenerOptions.
  • Use the wildcard key '*' to define default options for all bound events.

Binds multiple event listeners simultaneously. This is a highly efficient way to register a group of related listeners, configure their listener types, and pass shared or specific options all at once.

import { ListenerType } from 'awayback'

// Basic binding
events.bind({
  login: (userId) => console.log('Logged in:', userId),
  logout: (userId) => console.log('Logged out:', userId),
})

// Binding with specific types and wildcard defaults
events.bind(
  {
    ready: () => console.log('App ready!'),
    error: (err) => console.error('App error:', err),
    update: (data) => console.log('App updated:', data),
  },
  {
    ready: ListenerType.once, // 'ready' will only execute once
    '*': ListenerType.on, // All others will execute normally (default)
  },
  {
    '*': { isDistinct: true }, // Applies `isDistinct: true` to all bound events
  }
)

Calling the returned function will unbind all events associated with that specific group.

const unsubscribe = events.bind({
  login: (id) => console.log('In:', id),
  logout: (id) => console.log('Out:', id),
})

unsubscribe() // Removes both login and logout listeners

.promise(event, options): Promise

• event <String>
• options <Object>
  • timeout <Number>
  • reject <Array>
  • predicate <Function>
    • ...data <Any>
    • returns: <Boolean>
  • signal <AbortSignal>
  • isReplaying <Boolean>
• returns: <Promise>

Returns a promise that resolves with the value of the current instance. This function is useful for converting callback-based code to promise-based code.

setTimeout(() => {
  // Fire event after 300ms
  events.emit('ready', ['some', 'data'])
}, 300)

const result = await events.promise('ready')

console.log(result)
// OUTPUT: ['some', 'data']

If the predicate option is provided, the promise will only be resolved if the predicate function returns true.

const result = await events
  .promise('ready', {
    predicate: (data) => data[0] === 'some',
  })
  .then((data) => {
    console.log(data)
    // OUTPUT: ['some', 'data']
  })

events.emit('ready', ['some', 'data'])

If the timeout option is provided, the promise will be rejected after the specified time.

const result = await events
  .promise('ready', {
    timeout: 100,
  })
  .catch(() => {
    console.log('timeout')
    // OUTPUT: timeout
  })

If the reject option is provided, the promise will be rejected as soon as one of the provided events is fired.

const result = await events
  .promise('ready', {
    reject: ['error'],
  })
  .catch(() => {
    console.log('error')
    // OUTPUT: error
  })

events.emit('error')

.listeners(event)

• event <String>
• returns: Readonly<Readonly<Listener<D, E, R>>[]>

Returns a read-only array of all listeners currently registered for the given event. Each listener entry is itself read-only and exposes its type, callback, emission count, execution count, and options. Returns an empty array if no listeners have been registered for the event.

const handler = (data) => {
  console.log(data)
}

events.on('event', handler)

console.log(events.listeners('event').length)
// OUTPUT: 1

This is useful for inspecting the current state of an event's listeners, for example to check whether a specific callback is already registered before adding it:

const handler = (data) => {
  console.log(data)
}

events.on('event', handler)

const isRegistered = events.listeners('event').some((listener) => listener[ListenerProperty.callback] === handler)

console.log(isRegistered)
// OUTPUT: true

.remove(event, callback)

• event <String>
• callback <Function>
  • ...data <Any>

Removes a listener from the event so that this is no longer executed when firing.

const handler = () => {}

events.on('event', handler)
events.remove('event', handler)

.destroy()

Iterates through all registered event listeners and removes them, effectively disabling any further event handling for the object.

events.destroy()

License

MIT License

Copyright (c) 2022-2026 Luca Raúl Joos

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.