Node.js' efficiency is partly due to its event-based architecture. Unlike reading all necessary files (like PHP) every request, Node.js simply starts the server, initializes most variables, declares the function, and then waits for the event to happen. Although Node.js has some useful events built in, such as request
events, wouldn't it be more useful to be able to create and trigger custom events? This article will explore how to achieve this. First, we will demonstrate how to issue a normal event. For example, when someone enters a store, the bell rings to indicate its existence, which is similar to the observer pattern where events act as subjects and all functions attached to events are like observers. The store example is as follows:
const events = require('events'); const eventEmitter = new events.EventEmitter(); const ringBell = () => { console.log('ring ring ring'); }; eventEmitter.on('doorOpen', ringBell); eventEmitter.emit('doorOpen');
First, we load the Node.js core module events
. Then, create an instance of the EventEmitter
class (we will extend it later). After that, we put the ringBell
function into a variable so that it can be called this way. It just prints "ring ring ring" in the console. Next is the key point. We use the eventEmitter.on()
method to add the ringBell
function to the function list of doorOpen
events. The first parameter is the event name and the second parameter is the function to be added. This actually did nothing, just registered our function. The real magic happens in the next line, when we issue an event. Calling the emit()
method will execute all functions registered with the on
method. This is not that interesting, if we just want the bell to ring, we can call the function directly. But that's what's interesting about events: you can register as many functions as you like. For example, we can also do this:
eventEmitter.on('doorOpen', ringBell); eventEmitter.on('doorOpen', doSomething); eventEmitter.on('doorOpen', doSomethingElse); eventEmitter.emit('doorOpen');
This works just as well and makes full use of the features provided by EventEmitter
. We can also use a function with parameters as a listener:
eventEmitter.on('doorOpen', (ring) => { console.log(ring); }); eventEmitter.emit('doorOpen', 'ringeling');
We just need to pass the parameters in the emit()
method. Although this is very powerful, in the Node.js community, a common practice is to inherit from the EventEmitter
class. We can do this by creating a Door
class that has a open()
method that will emit doorOpen
events. Please see this code:
const events = require('events'); function Door(colour) { this.colour = colour; events.EventEmitter.call(this); this.open = () => { this.emit('open'); }; } Door.prototype.__proto__ = events.EventEmitter.prototype; const frontDoor = new Door('brown'); frontDoor.on('open', () => { console.log('ring ring ring'); }); frontDoor.open();
In our constructor of the Door
object, we set the door color and execute the constructor method of the EventEmitter
using the call()
object's EventEmitter
method. Then, we declare the open
method, which emits the "open" event. This line of code:
const events = require('events'); const eventEmitter = new events.EventEmitter(); const ringBell = () => { console.log('ring ring ring'); }; eventEmitter.on('doorOpen', ringBell); eventEmitter.emit('doorOpen');
Copy all EventEmitter
properties to Door
objects. Then we create frontDoor
, which is an instance of Door
with a brown color. Then we add an event listener, finally open the door and print a message in the console. I hope everyone can see this events
module is very powerful and useful! Finally, the events
module provides us with a way to list all event listeners attached to events and delete event listeners.
eventEmitter.on('doorOpen', ringBell); eventEmitter.on('doorOpen', doSomething); eventEmitter.on('doorOpen', doSomethingElse); eventEmitter.emit('doorOpen');
You can use the listeners
attribute to achieve it. Of course, this only works if you are not using anonymous functions as event listeners. If we want, we can remove the bell from the door:
eventEmitter.on('doorOpen', (ring) => { console.log(ring); }); eventEmitter.emit('doorOpen', 'ringeling');
Or, we can even remove all listeners:
const events = require('events'); function Door(colour) { this.colour = colour; events.EventEmitter.call(this); this.open = () => { this.emit('open'); }; } Door.prototype.__proto__ = events.EventEmitter.prototype; const frontDoor = new Door('brown'); frontDoor.on('open', () => { console.log('ring ring ring'); }); frontDoor.open();
Thank you for reading this guide and hope you gain something! See you next time!
Node.js Events and EventEmitter FAQs (FAQs)
What is the EventEmitter class in Node.js? How does it work?
TheEventEmitter
class is a core module in Node.js that facilitates communication between objects. It is part of the events
module for issuing and handling custom events. The EventEmitter
class works by registering a function or event handler to a named event. When an event is emitted by an EventEmitter
object, all functions attached to the event are called synchronously.
How to create an instance of EventEmitter?
Creating an instance of EventEmitter
is simple. First, you need to import the events
module. You can then create a new instance using the new
keyword. Here is a simple example:
Door.prototype.__proto__ = events.EventEmitter.prototype;
How to emit events using EventEmitter?
To issue an event, you can use the EventEmitter
method of the emit
instance. This method allows you to specify the event name and pass any number of parameters to the event listener. Here is an example:
const ring = () => { console.log('ring'); }; frontDoor.on('open', ring); console.log(require('util').inspect(frontDoor.listeners('open'))); // 輸出 [ ring ]
How to listen for events using EventEmitter?
To listen for events, you can use the EventEmitter
method of the on
instance. This method allows you to specify the event name and a callback function that will be called when the event is emitted. Here is an example:
frontDoor.removeListener('open', ring);
What is the difference between 'on' and 'once' methods in EventEmitter?
Theon
method allows you to add a callback function that will be called every time the event is emitted. On the other hand, the once
method allows you to add a callback function that will only be called when the event is emitted the first time.
How to remove event listener from EventEmitter?
To remove the event listener, you can use the EventEmitter
or removeListener
method of the off
instance. This method allows you to specify the event name and the callback function that should be removed. Here is an example:
const events = require('events'); const eventEmitter = new events.EventEmitter(); const ringBell = () => { console.log('ring ring ring'); }; eventEmitter.on('doorOpen', ringBell); eventEmitter.emit('doorOpen');
Can I limit the number of listeners for events in EventEmitter?
Yes, you can use the EventEmitter
method of the setMaxListeners
instance to limit the number of listeners for events. This method allows you to specify the maximum number of listeners that can be added for events.
How to get the number of listeners for events in EventEmitter?
To get the number of listeners for an event, you can use the EventEmitter
method of the listenerCount
instance. This method allows you to specify an event name and return the number of listeners for that event.
Can I issue and handle errors in EventEmitter?
Yes, you can issue and handle errors in EventEmitter
. If EventEmitter
does not register at least a listener for error
event and the error
event is emitted, an error is thrown, a stack trace is printed, and the Node.js process will exit.
Can I use EventEmitter in my browser?
Although EventEmitter
is a Node.js module and is mainly used for server-side applications, there are also available browser versions. These can be used like the Node.js version, allowing you to use the same event-driven architecture in your client code.
The above is the detailed content of Node.js Events and EventEmitter. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

The following points should be noted when processing dates and time in JavaScript: 1. There are many ways to create Date objects. It is recommended to use ISO format strings to ensure compatibility; 2. Get and set time information can be obtained and set methods, and note that the month starts from 0; 3. Manually formatting dates requires strings, and third-party libraries can also be used; 4. It is recommended to use libraries that support time zones, such as Luxon. Mastering these key points can effectively avoid common mistakes.

PlacingtagsatthebottomofablogpostorwebpageservespracticalpurposesforSEO,userexperience,anddesign.1.IthelpswithSEObyallowingsearchenginestoaccesskeyword-relevanttagswithoutclutteringthemaincontent.2.Itimprovesuserexperiencebykeepingthefocusonthearticl

Event capture and bubble are two stages of event propagation in DOM. Capture is from the top layer to the target element, and bubble is from the target element to the top layer. 1. Event capture is implemented by setting the useCapture parameter of addEventListener to true; 2. Event bubble is the default behavior, useCapture is set to false or omitted; 3. Event propagation can be used to prevent event propagation; 4. Event bubbling supports event delegation to improve dynamic content processing efficiency; 5. Capture can be used to intercept events in advance, such as logging or error processing. Understanding these two phases helps to accurately control the timing and how JavaScript responds to user operations.

If JavaScript applications load slowly and have poor performance, the problem is that the payload is too large. Solutions include: 1. Use code splitting (CodeSplitting), split the large bundle into multiple small files through React.lazy() or build tools, and load it as needed to reduce the first download; 2. Remove unused code (TreeShaking), use the ES6 module mechanism to clear "dead code" to ensure that the introduced libraries support this feature; 3. Compress and merge resource files, enable Gzip/Brotli and Terser to compress JS, reasonably merge files and optimize static resources; 4. Replace heavy-duty dependencies and choose lightweight libraries such as day.js and fetch

The main difference between ES module and CommonJS is the loading method and usage scenario. 1.CommonJS is synchronously loaded, suitable for Node.js server-side environment; 2.ES module is asynchronously loaded, suitable for network environments such as browsers; 3. Syntax, ES module uses import/export and must be located in the top-level scope, while CommonJS uses require/module.exports, which can be called dynamically at runtime; 4.CommonJS is widely used in old versions of Node.js and libraries that rely on it such as Express, while ES modules are suitable for modern front-end frameworks and Node.jsv14; 5. Although it can be mixed, it can easily cause problems.

There are three common ways to initiate HTTP requests in Node.js: use built-in modules, axios, and node-fetch. 1. Use the built-in http/https module without dependencies, which is suitable for basic scenarios, but requires manual processing of data stitching and error monitoring, such as using https.get() to obtain data or send POST requests through .write(); 2.axios is a third-party library based on Promise. It has concise syntax and powerful functions, supports async/await, automatic JSON conversion, interceptor, etc. It is recommended to simplify asynchronous request operations; 3.node-fetch provides a style similar to browser fetch, based on Promise and simple syntax

To write clean and maintainable JavaScript code, the following four points should be followed: 1. Use clear and consistent naming specifications, variable names are used with nouns such as count, function names are started with verbs such as fetchData(), and class names are used with PascalCase such as UserProfile; 2. Avoid excessively long functions and side effects, each function only does one thing, such as splitting update user information into formatUser, saveUser and renderUser; 3. Use modularity and componentization reasonably, such as splitting the page into UserProfile, UserStats and other widgets in React; 4. Write comments and documents until the time, focusing on explaining the key logic and algorithm selection

JavaScript's garbage collection mechanism automatically manages memory through a tag-clearing algorithm to reduce the risk of memory leakage. The engine traverses and marks the active object from the root object, and unmarked is treated as garbage and cleared. For example, when the object is no longer referenced (such as setting the variable to null), it will be released in the next round of recycling. Common causes of memory leaks include: ① Uncleared timers or event listeners; ② References to external variables in closures; ③ Global variables continue to hold a large amount of data. The V8 engine optimizes recycling efficiency through strategies such as generational recycling, incremental marking, parallel/concurrent recycling, and reduces the main thread blocking time. During development, unnecessary global references should be avoided and object associations should be promptly decorated to improve performance and stability.
