30-seconds-of-code/snippets/createEventHub.md

createEventHub

Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.

Create an empty hub property using Object.create(null) to create a truly empty object that does not inherit properties from Object.prototype (which would be resolved if the event name matched one of the properties).

For emit, resolve the array of handlers based on the event argument and then run each one with Array.forEach() by passing in the data as an argument.

For on, create an array for the event if it does not yet exist, then use Array.push() to add the handler to the array.

For off, use Array.findIndex() to find the index of the handler in the event array and remove it using Array.splice().

const createEventHub = () => ({
  hub: Object.create(null),
  emit(event, data) {
    (this.hub[event] || []).forEach(handler => handler(data));
  },
  on(event, handler) {
    if (!this.hub[event]) this.hub[event] = [];
    this.hub[event].push(handler);
  },
  off(event, handler) {
    const i = (this.hub[event] || []).findIndex(h => h === handler);
    if (i > -1) this.hub[event].splice(i, 1);
  }
});

const handler = data => console.log(data);
const hub = createEventHub();

// Listen for different types of events
hub.on('message', handler); // subscribe the handler to listen for a 'message' event
hub.on('message', () => console.log('Message event fired')); // subscribe a different handler to listen for a 'message' event
hub.on('double', number => console.log(number * 2));  // subscribe a handler to listen for a 'double' event

// Emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('double', 5); // logs 10

hub.off('message', handler); // unsubscribe a single handler from the 'message' event