setTimeout():<\/strong> It executes after a threshold amount of time has passed.<\/p>\n The order of execution of the above timer depends where we are using them, if both are written in the main module then it can be impacted by other applications.<\/p>\n Example of setimmediate and setTimeout in Nodejs:<\/strong><\/p>\n When you run the below code you will observe that sometimes setTimeout() executes first and sometimes setImmediate() executes first.<\/p>\n Code for comparing setimmediate and setTimeout:<\/strong><\/p>\n Output:<\/strong><\/p>\n When we will be using these timers inside an I\/O cycle then always the setImmediate will execute first.<\/p>\n Code for setimmediate and setTimeout inside filesystem<\/strong><\/p>\n Output:<\/strong><\/p>\n It is a part of asynchronous api. Whenever we pass callbacks to process.nextTick() what happens is that all these callbacks are resolved before the continuation of the event loop. Due to this, starvation may occur.<\/p>\n It allows users to easily handle errors, cleanups and also if we want to run a callback after the call stack was unwound.<\/p>\n The callbacks in process.nextTick() are called immediately whereas in setImmediate() callbacks execute in the next iterations of the event loop.<\/p>\n Event Emitter is a class that is capable of emitting an event, attaching an event and also detaching it. Https and other node js core modules use the event emitter class.<\/p>\n Event emitter class calls all the events in the same order in which they were registered.<\/p>\n The on() method is used to attach the event handler, this method takes a callback function which gets attached to the event. More than one function can also be attached to a single event.<\/p>\n The emit() method is used to emit the attached event, basically it invokes the attached function.<\/p>\n Code for event emitter:<\/strong><\/p>\n Output:<\/strong><\/p>\n In the above image, you can see that the function event1 and event 2 has been registered by the name \u201csomeevent\u201d using the on() method.<\/p>\n Next we used the emit method to fire the event and we can see the output of both the functions which means that both the functions were invoked as soon as the \u201csomeevent\u201d event was fired.<\/p>\n We can also pass data to the call back function. In order to do so we have to pass the data in the second argument of emit() method.<\/p>\n Code for event emitter with parameter:<\/strong><\/p>\n Output:<\/strong><\/p>\n You can see that we have passed a name with value \u201cDataFlair\u201d in our second argument.This value is then used by the event1 function.<\/p>\n Below are the methods present in the event emitter with explanation and code.<\/p>\n We can detach the previously attached event by using the off() method.<\/p>\n Code for detaching event:<\/strong><\/p>\n Output<\/strong><\/p>\n You can see that we have used emit method twice to emit the same event but the output came only for that emit which was called before detaching the event using the off() method.<\/p>\n So once you detach an event then the emit method does not have any effect for that event.<\/p>\n listenerCount() method returns the number of listeners for that event. You can see in the below image that listenerCount() return 1 at first than as we called the off() method the listener for the event is detached and hence the count of listener is now zero.<\/p>\n Code for count listener:<\/strong><\/p>\n Output:<\/strong><\/p>\n It returns an array of listeners for that particular event.<\/p>\n Code for raw listener:<\/strong><\/p>\n Output:<\/strong><\/p>\n Before calling the off() method there was one listener so it returned its type and name.<\/p>\n After the off() method was called nothing was attached to the event so it returned an empty array.<\/p>\n This is similar to the on method. It takes the event name and a callback function and then attaches the event. It checks the event array to see if the event has already been attached or not. If the event is attached it returns the array, else return empty array.<\/p>\n Code for adding listener:<\/strong><\/p>\n Output:<\/strong><\/p>\n You can see that the result is same as the on() method<\/p>\n It is similar to the off() method. It checks in the event array for the event and it removes it from the event array.<\/p>\n Code for removing listener:<\/strong><\/p>\nsetTimeout(() => {\r\n console.log('setTimeout executed');\r\n}, 0);\r\n \r\nsetImmediate(() => {\r\n console.log('setImmediate executed');\r\n});\r\n<\/pre>\n![\"nodejs-setimmediate](\"https:\/\/data-flair.training\/blogs\/wp-content\/uploads\/sites\/2\/2021\/11\/nodejs-setimmediate-and-set-timeout-in-main.webp\")
<\/a><\/p>\nconst fs = require('fs');\r\n \r\nfs.readFile(\"DataFlair.txt\", () => {\r\n setTimeout(() => {\r\n console.log('timeout');\r\n }, 0);\r\n setImmediate(() => {\r\n console.log('immediate');\r\n });\r\n});<\/pre>\n<\/a><\/p>\nprocess.nextTick():<\/h3>\n
Why to use process.nextTick()?<\/h3>\n
process.nextTick() vs setImmediate() in Nodejs:<\/h3>\n
Nodejs Event Emitter and its working<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1() {\r\n console.log('First event occurred!')\r\n}\r\n \r\nfunction event2() {\r\n console.log('First event occurred!')\r\n}\r\n \r\nemitter.on('someevent', event1)\r\nemitter.on('someevent', event2)\r\nemitter.emit('someevent')<\/pre>\n<\/a><\/p>\nEmitting event with parameter.<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.on('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })<\/pre>\n<\/a><\/p>\nEvent Emitter Methods:<\/h3>\n
Detach Event:<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.on('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nemitter.off('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })<\/pre>\n<\/a><\/p>\nNodejs Count Listener:<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.on('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nconsole.log(emitter.listenerCount('someevent'))\r\nemitter.off('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nconsole.log(emitter.listenerCount('someevent'))<\/pre>\n<\/a><\/p>\nNodejs Raw Listener:<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.on('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nconsole.log(emitter.rawListeners('someevent'))\r\nemitter.off('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nconsole.log(emitter.rawListeners('someevent'))\r\n<\/pre>\n<\/a><\/p>\nNodejs Add Listener:<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.addListener('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })<\/pre>\n<\/a><\/p>\nRemove Listener:<\/h3>\n
const EventEmitter = require('events');\r\nconst emitter = new EventEmitter();\r\n \r\nfunction event1(arg) {\r\n console.log('Welcome to', arg.name);\r\n}\r\n \r\nemitter.addListener('someevent', event1)\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\nemitter.removeListener('someevent', event1);\r\nemitter.emit('someevent', { name: \"DataFlair\" })\r\n<\/pre>\n