Node.js Event Emitters: Practical Examples & Argument Handling Explained

Hello what's up guys today in this video we will see event module in our nodejs application and believe me today's video is very important like one of the most important video because today we are going to see event module and here like events i hope you understand in javascript everything is like event only you are clicking on a button so what is onclick it is an event whenever you submit a form then on submit what is that it is an event like event is very important in javascript world so in node event also works right so here is the same event but in a little different texture in a little different way here we don't directly use the event as we could do in javascript because here we are dealing with the server but still events are very important and you have to understand and i will try my best to explain you in the simplest way so event emitter is a core module in nodejs used to create and handle custom events so it's a part of the events module and it's often used for building events driven system in nodejs event driven system is also a very important term which is used in many places if you let me know in the comment section if you want to know more about this particular event driven system see this event emitter is basically a class which we will get from the event module and we will make an instance of it which will be called anything you can name it but it is widely used in the name of emitter so when we use the emitter object or basically the instance when we use that we get many methods in it in those many methods there are two methods which are widely used one of them is .emit and the other is .on you can also call it as add listener like both are same kind of so this .emit suppose you are calling an event or a function suppose you are calling a function and you will define a function on here you will call and here you will define I have tried to explain you this in the simplest way but in reality there is a difference there is a difference between the two that the way javascript works there basically the events of javascript are different here but kind of you understood emit means you pass the name of the event here and call it and here you will define it that whenever someone is emitting this particular event then I have to listen to that event and what operation you have to perform you have to write it here ok so see purpose emits or trigger an event with a specified event name you can also pass arguments that will be consumed by the listeners it's like calling a function but instead it triggers all the listeners attached to the specified event this is the beauty so guys let me know in the comment section let's say I have called a function in javascript by doing greet and I have defined that same greet function 5 times first time, second time, third time, fourth time, fifth time then when you will call that greet function will those 5 functions get called and if there will be only one call then the one at the top will get called or the one at the bottom which you have defined as function greet will get called let me know in the comment section but in event emitter but in node modules those 5 will get 5 calls this is the use difference here those 5 will get 5 calls ok and second is your .on method where attach a listener or a function to a specific event name this listener will execute when the event is emitted got it? remember it in a simple way here you are calling an event and here you are defining it ok just like this now let me show you then you will understand so guys let me do one thing I come inside this I create a new folder by doing events ok I create a new file inside this whose name will be app.js ok perfect ok const just a second first of all write require this is an event module so we have events no problem equal to every time we were getting fs from fs module we were getting os from os module but when you are playing with events then you are getting class that's why we will follow this Pascal case convention and that is event emitter ok this is it alright so I have import and event emitter then we need to create an instance of it and for some reason it was not like this sorry it is like this ok alright guys you can see that I have created an instance then we need to define an event listener and then we will trigger it by using this emit ok see everything now now I have to create an instance if you have ever dealt with classes or oops or objects oriented programming if you have ever done then you know that whenever you have to create any instance then whatever is the name of this class ok we have this class name ok before that we need to write the new and basically many people call it object so it's a const emitter you have to write so this is a convention like this and yes here they have added automatic which is a very good thing so now guys these two lines you have to do it always first of all import your class name event emitter and then you have to create a new instance ok now in this you have all the methods of our events module out of which only two are useful that is your emit and then your dot on or add listener both are kind of same now guys now I have to fire an event and that is greet will be an event ok greet is an event and whenever we will do it emit so I need hello Vinod or hello Thapa technical or hello whatever your name is that's all I need I you kind of understand it that I have to create a function and whenever I call this function so this particular whatever name will be I will get there ok so first of all why not I define this functionality I define it ok so I will go here define an event listener so see how this is your emitter inside this when you dot I mean when you press dot automatically gives you a suggestion BS code out of which we have this on either you can write add listener or you can write on both are same ok after this now here first of all you have to pass the name of the event that which event you want to listen I am assuming that my event name will be greet my event name will be greet ok so whenever someone will do this greet function emit or trigger or call so what do you want to show now what do you want to show them what do you want to produce then here we need to pass a callback function right simple whenever someone will do this particular event will do emit or trigger so whatever I will write here they will see it so I write simple console.log vinod thapa simple right I wrote simple now here I have to console it I have to greet now I have to trigger this particular event or call ok how so again we have this emitter now you have to play with this emitter and count that it is only two either you write on or you emit it emit means you produce right like give a signal this word is something like this you can say but I am trying to make you understand like a normal javascript world where you will understand that you are calling a function and that function's name is greet this is it so emitter what is this this is an instance of yours object dot emit method in which you are calling an event that is greet right and what are you doing here you with the help of on event or on method you write the name of that particular event here so that whenever you emit this particular event or trigger or call then this callback function will run and I will see this simple function call function define and you understand this simple but this although it is not clear but cd I have to go out first cd dot dot ok ls it's a cd events clear ls it's an ok perfect node app.js hello binod thapa how did you like it this is awesome that how you are emitting the event here right so now I would like to tell you one more thing here and that is here you are hardcoding but what we can do is we can also pass an argument we can also pass an argument here see again it's simple as it was in normal javascript it's the same here I will not do anything I will copy this I will paste now what I am doing is comment see from here now what I have to do here I have to pass an argument so that this hardcoded binod thapa is not there I will go here something like this simple now you tell me yourself that if I pass an argument and I want it here then how can I get it we have seen it thousands of times if you have done javascript then it's very easy when you will read the function then there is argument parameter we have read all this right so let's see I will write the user's name here we have already used the template literals which is a part of ECMAScript come here dollar callibrasis and pass the user name our work is done right either do this or do this but here somewhere you have the power that when you are doing the function emit then only you are telling that my name is this and the same name should be seen here understand and understand that now it is set in one place now anyone can come and emit it here as many times as they want right so see hello binod thapa how did you like it this is very awesome and let's say you want to add more extra data now add that too let's say your profession I will write deb ok full stack developer here I will write profession like this and hello binod now I will not say hello here I will do one thing wait here guys I will write profession hello hello binod right ok I mean write anything I am just telling that if two come then how can you use ok see hello binod thapa you are a full stack developer right yes right ok whatever come now here one more thing I will tell you now further you could have done this in a better way but it's best idea to take a single argument as an object now what does this mean see carefully further how could you do this better see here two user names came right now here suppose I will write more data so I will have to add more extra parameters here I have to add here I mean are you understanding so now what can we do these two data now here you can pass the argument as a single object these arguments you can pass them as a single object in this way where you give key value pair my name is binod my profession is this so how easy it is like it's a single object in that you are passing your bio data in the form of key value pair now here you don't have to work so hard you write a single data let's say just argument or say kind of variable or if you want say object no problem because it's an object only although it's a parameter but what you can do argument dot username argument dot your profession in this way okay either you can do this or you can do this but it's kind of like a sim I mean it feels a little connected in this because it's a single object in that it's full value has come data now if I go here then you will get the same and by the way hello undefined you are a full stack one second why undefined came here and double double data is coming what is the reason behind it right nothing is above okay that's why undefined because it's a name but I am making it username so it's a username and it's a profession now there should be no such problem so now if I go and boom guys see yeah this this is it hello Vinod you are a full stack right yeah perfect see so guys this was your emitter this was your event module in your node yes and believe me it's very important now you understand it emitter.emit you have to write the name of the event and you have to pass the argument and emitter.on you have to write the event first so yes this listener for this particular event I am writing you have to give a comma in the callback function then you have to define the function this way it is working for you it's a little I mean funda is different but it works like this so guys finally now I am in front of you regarding this particular event emitter basically event module I am going to bring a challenge tomorrow now be ready and I would like you to perform it yourself okay but I hope you liked the video a lot and you must have got an idea that how you can deal with this event module so guys how did you like the video do tell me by commenting and please like the video if you have any doubt I am telling you again do comment and we our next target is 8 lakh subscribers so if you haven't subscribed yet please do it see you in tomorrow's video till then guys thank you so much take care