How to use nodejs launcher

Node.js is a JavaScript runtime environment for scalable network applications that uses an event-driven, non-blocking I/O model. Node emitter (Emitter) is an interactive data communication module provided by Node.js. It can implement the observer mode between objects, so that an object can be monitored by multiple users at the same time, and automatically when the object status changes. Notify all users. In this article, we will introduce how to use Node.js's emitter to implement object monitoring and event triggering.

First of all, we need to understand some basic concepts of launchers. Emitter is a built-in module of Node.js that implements event management mechanism. It is an event trigger that provides the functions of registering events, emitting events and removing event listeners. In Node.js, most modules inherit from the EventEmitter class, so they all have an emitter instance that they can use to implement event handling.

Before using the launcher, we need to understand some basic knowledge. First, to create an emitter instance, we need to introduce the events module and create an EventEmitter object. The code is as follows:

const EventEmitter = require('events');
const emitter = new EventEmitter();

Here we use the require function to introduce the built-in module of Node.js events, and created an EventEmitter object emitter for event emission and monitoring.

Then, we need to implement event registration, processing and removal. For event registration and removal, we can use the on and off methods, and for event processing, we can use the emit method. Here are some basic examples:

function callback() {
  console.log('event is emitted');
}

// 注册事件监听器，监听event事件
emitter.on('event', callback);

// 触发event事件，会自动调用callback函数
emitter.emit('event');

// 移除event事件监听器，不再监听event事件
emitter.off('event', callback);

In the above example, we used the on method to register an event listener. When the event 'event' is emitted, the callback function is automatically called. Then, we used the emit method to emit the event 'event', which triggered the callback function. Finally, we use the off method to remove the callback function's monitoring of the event 'event'.

In addition to the on, off, and emit methods, we can also use some other methods to operate the emitter. Among them, the once method is similar to the on method. The difference is that the once method will automatically delete the event listener after the event is emitted once. In addition, the prependListener method can insert the event listener to the front of the event listener list, and the prependOnceListener method provides the event listener to the front of the listener list. and will only be called once.

// 在所有事件监听器之前添加事件监听器
emitter.prependListener('event', callback);

// 在所有事件监听器之前添加事件监听器，只会调用一次
emitter.prependOnceListener('event', callback);

Of course, we can also use the eventNames method to get the name list of all events registered on the current object.

const eventNames = emitter.eventNames();
console.log(eventNames);
// ['event']

Overall, using Node.js’ emitters is a very useful way to communicate. Emitters allow us to establish loose coupling relationships between objects, reducing application complexity and maintenance costs. Using emitters, we can easily implement data communication and event handling between objects, thereby improving the reliability and scalability of our applications.

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