Type-safe Event Emitter

Simple C#-style events (i.e. an emitter per event type).

interface ResizeEvent { oldSize: number, newSize: number }

const resized = new Event<ResizeEvent>();
resized.on(ev => console.log(`Resized from ${ev.oldSize} to ${ev.newSize}`));
resized.emit({ oldSize: 3, newSize: 5 });

Highlights

• 🔥 Zero dependencies
• 🗜 Tidy and compact
• 💻 Works in browser
• 🔬 Strongly typed

Usage

import { Event } from '@flexent/event';

type Weather = 'sunny' | 'cloudy' | 'snowy';

interface WeatherChangedEvent {
    oldWeather: Weather;
    newWeather: Weather;
}

// The event object is created for each event type
const weatherChanged = new Event<WeatherChangedEvent>();

// Basic subscription: ev has type WeatherChangedEvent
weatherChanged.on(ev => {
    console.log(`Weather has changed from ${ev.oldWeather} to ${ev.newWeather}`);
});

// Basic emitting: the payload should have type WeatherChangedEvent
weatherChanged.emit({
    oldWeather: { /* ... */ },
    newWeather: { /* ... */ },
});

// Only-once subscription
weatherChanged.once(ev => console.log(`Weather has changed, but I won't bother you again`));

// .on and .once return an unsubscribe function
const unsubscribe = weatherChanged.on(ev => {
    if (isGoodWeather(ev.newWeather)) {
        console.log(`Finally, a good weather!`);
        unsubscribe();
    }
});

Subscriber groups

You can use subscriber groups to quickly delete listeners.

This is especially useful if you use classes and do not want to store references to unsubscribe function returned by on and once.

// Subscriber groups can be used to quickly unsubscribe (super useful in classes!)
class SomeSubscriber {

    desiredWeather: Weather = 'sunny';

    subscribe() {
        // 1. Notice the second argument, `this` is our group identifier (can be anything except null)
        weatherChanged.on(ev => this.onWeatherChanged(ev), this);
    }

    onWeatherChanged(ev: WeatherChangedEvent) {
        if (ev.newWeather === this.desiredWeather) {
            console.log('Finally what we were waiting for!');
            // 2. Unsubscribe all listeners of group `this`
            weatherChanged.removeAll(this);
        }
    }
}

Trivia

It is advisable to adopt a naming conventions for the events and payload types.

Also, it's often convenient to group all related events into a single "event bus":

import { Event } from '@flexent/event';

class ChatEvents {
    roomCreated = new Event<Room>();
    roomDestroyed = new Event<Room>();
    message = new Event<Message>();
    personJoined = new Event<Person>();
    personLeft = new Event<Person>();
    personEntered = new Event<{ door: Door, person: Person }>();
}