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Sourabh Parmar
Sourabh Parmar

Posted on • Updated on

How can you do your custom implementation for events.

I have been working on Node for quite some time and have extensively used Events. But never knew how it works internally until very recently.

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What are Events?

So basically, when something happens in your app like a user gets logged in, you fire an event to notify everybody listening to that event, that a user has logged in.

function login(data) {
    // fires an event
    CustomEvent.emit('user_logged_in', data);

function someOtherFunc() {
    // event listeners
    CustomEvent.subscribe('user_logged_in', function(data) {
        // do some operation

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Events Implementation

The Event class that we are designing will have the following methods:

  • emit - It fires an event, with an event name and data.
  • subscribe - It subscribes to the event and gets executed every time the given event is fired.
  • unsubscribe - It will unsubscribe the function from the event.
  • unsubscribeAll - It will unsubscribe all the functions listening to that event.
 * Disclaimer: This is not a production-quality code, lots of checks are not in place.
function CustomEvent() {
    // listers is an object
    // it will be a map of event_name
    // and all the functions subscribing to it
    this.listeners = {};

CustomEvent.prototype.subscribe = function(eventName, someFunction) {
    // checks if eventName exists
    // if doesn't initialise a new array
    // else push the function to the existing array
    if(!this.listeners[eventName]) {
        this.listeners[eventName] = [someFunction];

CustomEvent.prototype.emit = function(eventName, data) {
    // gets the event name
    // and loops over all the functions that it has to call
    const listeners = this.listeners[eventName];
    listeners && Array.isArray(listeners) && => {, data);


CustomEvent.prototype.unsubscribeAll = function(eventName) {
    this.listeners[eventName] = undefined;

CustomEvent.prototype.unsubscribe = function(eventName, someFunction) {

    const funcs = this.listeners[eventName];

    const filteredFuncs = funcs.filter(func => func.toString() !== someFunction.toString());

    this.listeners[eventName] = filteredFuncs;

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If you guys have a better implementation for unsubscribing in mind, please suggest.
Keep coding. Keep learning.

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