typescript-event-emitter

Versatile and feature-rich TypeScript library for event management, providing a solid foundation for building event-driven applications in TypeScript.

*	Version	Supported
npm	>= 7.24.0	✅

main features

1. Throttling and Debouncing:
   • flexibility for handling events in scenarios where rapid or frequent triggering needs to be controlled.
2. Wildcard Listeners:
   • supporting wildcard listeners with the '*' namespace, allowing global event handling.
3. Namespace Support:
   • namespace support is a great way to organize and manage different types of events within your system.
4. Priority Queue:
   • prioritizing listeners based on a priority value ensures that critical listeners can be given precedence, offering more control over event execution order.
5. Event Filtering:
   • the ability to filter events based on registered filters provides a mechanism for selectively emitting events.
6. Async/Await Pattern:
   • leveraging async/await for asynchronous operations ensures that it can handle asynchronous listeners gracefully.
7. Global Event Bus Integration:
   • the integration with a global event bus enhances the modularity and usability of the event system.
8. Error Handling:
   • logging errors to the console.
9. Custom separator per listener and Global configs:
   • ability to set custom separator per listener which would override global separator dedicated for listeners.
   • ability to change global separator which is used for listeners where separator is not provided

installation

$ npm install --save typescript-event-emitter

usage

After installation, the only thing you need to do is require the module:

import { EventEmitter } from 'typescript-event-emitter';

or

const { EventEmitter } = require('typescript-event-emitter');

And you're ready to create your own EventEmitter instances.

Base usage

  const emitter = new EventEmitter();
  let context = { test: 'Some metada' }

  const onEventNameEmitted = (eventname:string, data:any) => {
      console.log(data === context) // true
      console.log(eventname === 'event-name') // true
  };

  emitter.on('event-name', onEventNameEmitted);  // adds listener
  emitter.emit('event-name', context); // emits listener

Remove listener

  const emitter = new EventEmitter();

  const onEventNameEmitted = (eventname:string, data:any) => {
      console.log(eventname,data )
  };

  emitter.on('event-name', onEventNameEmitted);// adds listener
  emitter.off('event-name', onEventNameEmitted); // removes listener

Throttling

1. First emit:

   • The 'throttleEvent' is emitted.
   • The throttled listener is executed immediately, and callCount becomes 1.

2. Second emit (within the 100-millisecond throttle delay):

   • The 'throttleEvent' is emitted again.
   • This emit is ignored because throttling prevents the listener from being executed within the 100-millisecond throttle period.

3. Third emit (within the 100-millisecond throttle delay):

   • The 'throttleEvent' is emitted once more.
   • This emit is also ignored due to throttling.

  const emitter = new EventEmitter();
  let callCount = 0;
  emitter.on(
    'throttleEvent',
    () => {
      callCount++;
    },
    { throttle: 100 }
  );
  emitter.emit('throttleEvent');
  emitter.emit('throttleEvent');
  emitter.emit('throttleEvent');

Debouncing

1. First emit:

   • The 'debounceEvent' is emitted.
   • The debounced listener is called but not immediately executed due to the debounce delay.
   • The debounced function is scheduled to be executed after 100 milliseconds.

2. Second emit (within the debounce delay):

   • The 'debounceEvent' is emitted.
   • The debounced listener is called again, but the previous scheduled execution is canceled, and a new one is scheduled for 100 milliseconds from the latest emit.

3. Third emit (within the debounce delay):

   • The 'debounceEvent' is emitted.
   • The debounced listener is called once more, canceling the previous scheduled execution again and scheduling a new one for 100 milliseconds from this emit.

So basically for the given example, the listener will be executed after 300 millisecond delay

  const emitter = new EventEmitter();
  let callCount = 0;
  emitter.on(
    'debounceEvent',
    () => {
      callCount++;
    },
    { debounce: 100 }
  );
  emitter.emit('debounceEvent');
  emitter.emit('debounceEvent');
  emitter.emit('debounceEvent');

Wildcard

  const emitter = new EventEmitter();
  emitter.on('*', () => { // listener will be executed for both emits, wildcard listens to anything
    console.log("Executed")
  });
  emitter.emit('someEvent');
  emitter.emit('namespace.someEvent');

  const emitter = new EventEmitter();
  emitter.on('namespace1.*', () => { // listener will be executed 2 times, wildcard for namespace listens to anything within that namespace
    console.log("Executed 1")
  });

  emitter.on('namespace2.*', () => { // listener will not be executed
    console.log("Executed 2")
  });

  emitter.emit('other.event1');
  emitter.emit('namespace1.event1');
  emitter.emit('namespace1.event2');

  const emitter = new EventEmitter();
  emitter.on('*.someEvent', () => { // wildcard listeners as namespace for event
     console.log("Executed")
  });

  emitter.emit('other.event1'); // No match, no listener executed
  emitter.emit('other.someEvent'); // Matches the pattern, listener executed
  emitter.emit('namespace1.event1'); // No match, no listener executed
  emitter.emit('namespace1.someEvent'); // Matches the pattern, listener executed

Namespace

  const emitter = new EventEmitter();

  emitter.on('someEvent', () => {
    console.log('Listener');
  });

  emitter.on('namespace.someEvent', () => {
    console.log('Listener');
  });

  emitter.emit('other.event'); // No match, no listener executed
  emitter.emit('namespace.someEvent'); // Matches the pattern, listener executed
  emitter.emit('namespace.event'); // No match, no listener executed

Priority Queue

  const emitter = new EventEmitter();
  //last to be executed
  emitter.on('priorityEvent', () => {
      console.log('Low Priority Listener');
  });
  //first to be executed
  emitter.on(
    'priorityEvent',
    () => {
      console.log('High Priority Listener');
    },
    { priority: 2 }
  );
  //second to be executed
  emitter.on(
    'priorityEvent',
    () => {
      console.log('Medium Priority Listener');
    },
    { priority: 1 }
  );

  emitter.emit('priorityEvent');

Event Filtering

  interface Message {
    id: number;
    content: string;
    messageType: string;
    sender: string;
  }

  const emitter = new EventEmitter();
  const currentUser = { username: 'example_user' };
  const notificationFilter: EventFilter = (eventName, namespace) => {
    if (namespace === 'dm') {
      return true;
    }
    if (eventName === 'notification') {
      return true;
    }
    if (namespace === 'mention' && currentUser.username === eventName) {
      return true;
    }

    return false;
  };

  const receivedNotifications: Message[] = [];
    emitter.on(
    '*',
    (_event, message: Message) => {
      receivedNotifications.push(message);  //array will have: directMessage, generalNotification, mentionNotification objects
    },
    { filter: notificationFilter }
  );

  const directMessage: Message = { id: 1, content: 'Hello!', messageType: 'dm', sender: 'user123' };
  const generalNotification: Message = {
    id: 2,
    content: 'General update',
    messageType: 'announcement',
    sender: 'system'
  };
  const mentionNotification: Message = {
      id: 3,
      content: 'You were mentioned!',
      messageType: 'mention',
      sender: 'other_user'
  };
  const unrelatedEvent: Message = { id: 4, content: 'Irrelevant event', messageType: 'other', sender: 'unknown' };

  emitter.emit('other.event', unrelatedEvent),
  emitter.emit('notification', generalNotification),
  emitter.emit('dm.newMessage', directMessage),
  emitter.emit('mention.example_user', mentionNotification)

Async/Await Pattern

  const emitter = new EventEmitter();
  let flag = false;
  emitter.on('asyncEvent', async () => {
    return new Promise<void>(resolve => {
      setTimeout(() => {
        flag = true;
        resolve();
      }, 100);
    });
  });
  await emitter.emit('asyncEvent');
  console.log(flag); // will be true

Error Handling

  const emitter = new EventEmitter();
  emitter.on('errorEvent', () => {
    throw new Error('Listener Error');
  });
  try {
    await emitter.emit('errorEvent');
  } catch (error) {
    console.log(error) // will be 'Listener Error'
  }

  const emitter = new EventEmitter();
  let firstListenerInvoked = false;
  let secondListenerInvoked = false;
  emitter.on('errorEvent', () => {
    throw new Error('Listener Error');
  });

  emitter.on('errorEvent', () => {
    firstListenerInvoked = true;
  });

  emitter.on('errorEvent', () => {
    secondListenerInvoked = true;
  });

  emitter.emit('errorEvent'); //all 3 will be fired and event flow won't be disrupted

Global Event Bus

The global event bus is a singleton that contains an instance of an event emitter. Functionality/features, etc is just a centralized mechanism for communication across different parts of an application.

import { globalEventBus } from 'typescript-event-emitter';

or

const { globalEventBus } = require('typescript-event-emitter');

  let context = { test: 'Some metada' }

  const onEventNameEmitted = (eventname:string, data:any) => {
      console.log(data === context) // true
      console.log(eventname === 'event-name') // true
  };

  globalEventBus.on('event-name', onEventNameEmitted);  // adds listener
  globalEventBus.emit('event-name', context); // emits listener

Custom separator per listener and Global configs

  const eventEmitter: EventEmitter = new EventEmitter({ separator: ':' }); // setting global separator if not provided it will revert to default "."

  eventEmitter.on("namespace:someEvent", () => {});

  const eventEmitter: EventEmitter = new EventEmitter(); // default separator '.'
  eventEmitter.setGlobalOptions({ separator: "-" }); // sets global separator which can be provided via constructor aswell

  eventEmitter.on("namespace-someEvent", () => {});
  eventEmitter.off("namespace-someEvent");

  eventEmitter.on("namespace:someEvent1", () => {}, { separator: ":" }); // listener separator will be ':'
  eventEmitter.off("namespace:someEvent1");

  eventEmitter.on("namespace:someEvent2", () => {}, { separator: ":" }); // listener separator will be ':'
  eventEmitter.on("namespace-someEvent3", () => {}); // listener separator will be '-' as it was set via setGlobalOptions

  eventEmitter.off("namespace:someEvent2");
  eventEmitter.off("namespace-someEvent3");

  const eventEmitter: EventEmitter = new EventEmitter({ separator: ':' }); // setting global separator if not provided it will revert to default "."

  globalEventBus.setGlobalOptions({ separator: "-" }); // sets global separator
  globalEventBus.on("namespace:someEvent", () => {}, { separator: ":" });  // listener separator will be ':'

Tests

This module is well-tested. You can run:

npm run test to run the tests under Node.js.
npm run test:nyc to run the tests under Node.js and get the coverage

Tests are not included in the npm package. If you want to play with them, you must clone the GitHub repository.

contributing

Please read our Contribution Guidelines before contributing to this project.

security

Please read our SECURITY REPORTS

license

MIT