事件和函数的区别？

Question

I am new to Node, and I am struggling to understand the main difference between Events and Functions. Both need to be triggered, so why do we need an Event at all if we have to trigger it anyway?

How is it different than having a Function triggered?

Example code:

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

eventEmitter.on('event1', function () {
    console.log('Event 1 executed.');
    eventEmitter.emit('event2');
});

eventEmitter.on('event2', function() {
    console.log('Event 2 executed.');
});

eventEmitter.emit('event1');
console.log('Program Ended.');

We can achieve the same result by functions, right?

I am sure this has some serious importance in Node (otherwise it would not exist, lol), but I am struggling to understand it.

Help appreciated: :)

Answer 1

Events deal with asynchronous operations. They aren't really related to functions in the sense that they are interchangeable.

eventEmitter.on is itself a function, it takes two arguments the event name, then a function (callback) to be executed when the event happens.

eventEmitter.on(evt, callback)

There is no way to tell WHEN the event will be emitted, so you provide a callback to be executed when the event occurs.

In your examples, you are controlling when the events are triggered, which is different than real world use where you may have a server listening for connections that could connect at anytime.

server.listen('9000', function(){
    console.log('Server started');
});

server.on('connection', function(client){
    console.log('New client connected');
    doSomethingWithClient(client);
});

//series of synchronous events
function doSomethingWithClient(client){
    //something with client
}

For server.listen the server doesn't start immediately, once its ready the callback is called

server.on('connection') listens for client connections, they can come at any time. The event is then triggered when a connection occurs, causing the callback to be run.

Then there is doSomethingWithClient this is just a function with a set of synchronous operations to be done when a client connection occurs.

Answer 2

events useful in webserver code (that is active on port) not in normal scripts, in normal scripts events will behave same as functions because events will be continually active & listening on port for requests as long as port is up so if we use function instead, function will run only once when.js file is ran thus functions cannot capture incoming request and respond.

Example: in below code if you see output of below function dummy_func() triggered immediately when js file is ran & printed statement ConditionReport: A client Connected: 0 only once as output, but the EventReport: A client Connected: printed only when opened http://localhost:58080 in browser and again in another tab if i open same http://localhost:58080 it printed again EventReport: A client Connected: 3

 const express = require('express'); const app = express(); const path = require('path'); const PORT = process.env.PORT || 58080; // Load all static html files in the directory, here index.html file open as default at http://localhost:58080/ but to load html like Resume.html we should call complete http://localhost:58080/Resume.html app.use(express.static(path.join(__dirname))); // Configure Port var server_object=app.listen(PORT, () => console.log("Server listening on port " + PORT)); //Possible incomming Events var incomming_events=server_object.eventNames() console.log("\nserver_object:",incomming_events); //On Event var i=0 server_object.on('connection',()=>{ i=i+1 console.log("\nEventReport: A client Connected:",i); }); //Using if condition instead of Event function dummy_func(j){ console.log("\nConditionReport: A client Connected:",j,"\n"); } var j=0 if (incomming_events.includes('connection')){ dummy_func(j) j=j+1 }

OUTPUT:

stataraju@statara-ltm7wjr Example2 % node index.js

server_object: [ 'request', 'connection', 'listening' ]

ConditionReport: A client Connected: 0 

Server listening on port 58080

EventReport: A client Connected: 1

EventReport: A client Connected: 2

EventReport: A client Connected: 3

Answer 3

An event is an identifier used within tools ( .on() , .emit() etc) to set and execute callbacks. Functions are reusable code.

Answer 4

I suppose I see the biggest difference I see is that an event emitter could trigger multiple events that are listening, whereas just calling a function only triggers one thing.

So for example, you could have numerous objects in a game that are all waiting for a step event that increments their animation.

They are such a pain to debug though that I'd much rather just use functions.