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

/**
 * createEventHub
 *
 * Creates a pub/sub (publish-subscribe) event hub with emit, on, and off
 * methods.
 *
 * Use `Object.create(null)` to create an empty `hub` object that does not
 * inherit properties from `Object.prototype`.
 *
 * For `emit`, resolve the array of handlers based on the `event` argument and
 * then run each one with `Array.prototype.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.prototype.push()` to add the handler to the array.
 *
 * For `off`, use `Array.prototype.findIndex()` to find the index of the handler
 * in the event array and remove it using `Array.prototype.splice()`.
 *
 * @see https://www.30secondsofcode.org/snippet/createEventHub
 */
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);
        if (this.hub[event].length === 0) delete this.hub[event];
    }
});

//
// Examples:

const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;

// Subscribe: listen for different types of events
hub.on("message", handler);
hub.on("message", () => console.log("Message event fired"));
hub.on("increment", () => increment++);

// Publish: 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("increment"); // `increment` variable is now 1

// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off("message", handler);