EventEmitter: newListener event

This event is emitted before a listener is added to its internal array of listeners.

Below table summarizes the arguments of callback function to newListener event.

Argument	Type	Description
eventName	String	The name of the event being listened for
listener	Function	Event handler function

HelloWorld.js

var EventEmitter = require('events').EventEmitter;

var em = new EventEmitter();


em.on('newListener', (event, listener) =>{
 console.log(`New listener is added for the event '${event}'`);
});


em.on('demoEvent', () => {
  console.log('Listener1 is added for demoEvent');
});

em.on('demoEvent', () => {
  console.log('Listener2 is added for demoEvent');
});

em.emit('demoEvent');

Output

New listener is added for the event 'demoEvent'
New listener is added for the event 'demoEvent'
Listener1 is added for demoEvent
Listener2 is added for demoEvent

There is one side effect with ‘newListener’ event, since this event is called before adding the actual listener, any listener added in this 'newListener' callback will be inserted before the listener that is in the process of being added.

HelloWorld.js

var EventEmitter = require('events').EventEmitter;

var em = new EventEmitter();

em.once('newListener', (event, listener) => {
    em.on('demoEvent', () => {
        console.log("B");
    })
});


em.on('demoEvent', () => {
    console.log('A');
});

em.emit('demoEvent');

Output

B

A

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