JavaScript Publish/Subscribe Pattern Example

In this article, I show you how to create and use public/subscribe pattern in Javascript.
The Publisher/Subscriber pattern, or “PubSub” for short, is a pattern that allows us to create modules that can communicate with each other without depending directly on each other. It’s a great pattern for decoupling our application and is quite common in JavaScript.
For the best learning experience, I highly recommended that you open a console (which, in Chrome and Firefox, can be done by pressing Ctrl+Shift+I), navigate to the "console" tab, copy-and-paste each JavaScript code example from this article, and run it by pressing the Enter/Return key.
Let's first write a code to design the Publish/Subscribe Pattern.

Publish/Subscribe Pattern Implementation

var events = {
    events: {},
    subscribe: function (eventName, fn) {
        this.events[eventName] = this.events[eventName] || [];
        this.events[eventName].push(fn);
    },
    unsubscribe: function (eventName, fn) {
        if (this.events[eventName]) {
            for (var i = 0; i < this.events[eventName].length; i++) {
                if (this.events[eventName][i] === fn) {
                    this.events[eventName].splice(i, 1);
                    break;
                }
            };
        }
    },
    publish: function (eventName, data) {
        if (this.events[eventName]) {
            this.events[eventName].forEach(function (fn) {
                fn(data);
            });
        }
    }
}

Usage

Here, multiple subscribers are subscribing to a single event named as "event1".
function test(){
    events.subscribe("event1", function(data){
        console.log("subscriber1 subscribe to event1 -> " + data);
    });

    events.subscribe("event1", function(data){
        console.log("subscriber2 subscribe to event1 -> " + data);
    });

    events.subscribe("event1", function(data){
        console.log("subscriber3 subscribe to event1 -> " + data);
    });

    events.subscribe("event1", function(data){
        console.log("subscriber4 subscribe to event1 -> " + data);
    });

    // publish some data to all subscribers whose were registered to event1
    events.publish("event1", "some data");
}

test();

Output

subscriber1 subscribe to event1 -> some data
subscriber2 subscribe to event1 -> some data
subscriber3 subscribe to event1 -> some data
subscriber4 subscribe to event1 -> some data

Using JavaScript class Keyword (after es2015)

Let's demonstrate the Pub/Sub pattern using JavaScript class keyword:
class Event{

    constructor(){
        this.events = {};
    }

    subscribe(eventName, fn) {

        this.events[eventName] = this.events[eventName] || [];
        this.events[eventName].push(fn);
    }

    unsubscribe(eventName, fn) {

        if (this.events[eventName]) {
            for (var i = 0; i < this.events[eventName].length; i++) {
                if (this.events[eventName][i] === fn) {
                    this.events[eventName].splice(i, 1);
                    break;
                }
            };
        }
    }

    publish(eventName, data) {

        if (this.events[eventName]) {
            this.events[eventName].forEach(function(fn) {
                fn(data);
            });
        }
    }

}

function test1(){
    var event = new Event();

    event.subscribe("event1", function(data){
        console.log("subscriber1 subscribe to event1 -> " + data);
    });

    event.subscribe("event1", function(data){
        console.log("subscriber2 subscribe to event1 -> " + data);
    });

    event.subscribe("event1", function(data){
        console.log("subscriber3 subscribe to event1 -> " + data);
    });

    event.subscribe("event1", function(data){
        console.log("subscriber4 subscribe to event1 -> " + data);
    });

    // publish some data to all subscribers whose were registered to event1
    event.publish("event1", "some data");
}

test1();

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