Node.js composition and Events module

1 Node.js module and package

1.1 module

Node.js itself provides many modules. Each module implements a function, such as file operation module fs, HTTP module for building HTTP services, etc. each module is a javascript file and can write its own module.

Each node js is a node js module, including JavaScript file (. js), json text file (. json) and binary module file (. Node)

1.1.1 use of modules

Module. Is used in the module Exports and exports provide external interfaces to facilitate the introduction of modules to be used in other modules and the creation of new modules JS and provide external interfaces.

Use exports to provide an interface

function hello() {
    console.log("Hello");
}

function world() {
    console.log("World");
}

exports.hello = hello;
exports.world = world;

Use module Exports provides an interface

function Hello() {
    this.hello = function() {
        console.log("Hello");
    };

    this.world = function() {
        console.log("World");
    };
}

module.exports = Hello;
1.1.2 reference module

Use the require() reference to create a module, and the parameter is the path of the module.

Refer to the module with the interface provided by exports

var hello = require("./test.js");

hello.hello();  //==> Hello
hello.world();  //==> World

Reference module Exports modules that provide interfaces

//module.exports exports the Hello function and saves it in the Test variable
var Test = require("./test.js");

//Use Test() as a constructor and create an object to use
var demo = new Test();
demo.hello();    //==> Hello
demo.world();  //==> World

1.2 module. Differences between exports and exports

Each module has a module object, which is a reference to the current module. module. The exports object is built by the module system, and exports can be regarded as a module References to the exports object.

When requiring () a module, use module The value of exports shall prevail. In some cases, module The values of exports and exports are different.

// module.exports is the same as exports
var m = {};        // Represents a module
var e = m.e = {};  // e stands for exports and m.e stands for module exports

m.e.a = 5;
e.b = 6;

console.log(m.e);  // Object { a: 5, b: 6 }
console.log(e);    // Object { a: 5, b: 6 }
// module.exports and exports are different
var m = {};        // Represents a module
var e = m.e = {};  // e stands for exports and m.e stands for module exports

m.e = { c: 9 };    // m. The object referenced by E (module.exports) has been changed
e.d = 10;

console.log(m.e);  // Object { c: 9 }
console.log(e);    // Object { d: 10 }

1.2 package

Packages can organize multiple modules with dependencies and encapsulate multiple modules for easy management.

Node.js adopts the CommonJS specification. A javascript file is a module. A package is a folder. The package must contain a file named package JSON file. The bin folder in the package stores binary files, the lib folder stores javascript files, the doc folder stores documents, and the test folder stores unit tests.

1.2.1 package directory

The package directory must contain package JSON file, CommonJS is used to describe the package, and should contain the following fields:

  • Name: the name of the package, which is unique and can only contain characters, underscores and numbers;
  • Version: the version number of the package;
  • Description: description of the package;
  • keywords: keyword array, used for searching;
  • Home page: project home page;
  • bugs: address of submitting bug;
  • liciense: license;
  • maintainers: maintainer array;
  • Contributors: contributors array;
  • repositories: project warehouse hosting address array;
  • Dependencies: package dependencies.

example:

{
    "name": "Kyxy",
    "description": "Kyxy test package.",
    "version": "0.1.0",
    "keywords": [
        "kyxy",
        "nodejs"
     ],
    "maintainers": [{
        "name": "test",
        "email": "test@kyxy.com"
    }],
    "contributors": [{
        "name": "test",
        "web": "http://www.kyxy.com/"
    }],
    "bugs": {
        "mail": "test@kyxy.com",
        "web": "http://www.kyxy.com/"
    },
    "licenses": [{
        "type": "Apache License v2",
        "url": "http://www.apache.org/licenses/apache2.html"
    }],
    "repositories": [{
        "type": "git",
        "url": "http://github.com/test/test.git"
    }],
    "dependencies": { 
        "webkit": "1.2",
        "ssl": { 
            "gnutls": ["1.0", "2.0"],
            "openssl": "0.9.8"
        }
    }
}

1.3 npm package management tool

npm is node JS package management tool, npm defines the package dependency standard, uses npm to download third-party packages and manage locally downloaded third-party packages.

Query package information: npm info express Installation package: sudo npm install express Update package: sudo npm update express Uninstall package: sudo npm uninstall express

2 Events module

Node.js, many objects emit events: FS An event is emitted when readstream opens a file. All emitted event objects are events An instance of EventEmitter can be accessed through require("events"); Get the events module.

Hump nomenclature is recommended for event naming. Add functions to the object. When the object sends an event, the corresponding functions are executed (these functions are called listeners); First, assign the function to the object (set it as a listener) and call the function when the object emits an event

2.1 Class: events.EventEmitter

Through require ("events") EventEmitter gets the EventEmitter class. When the EventEmitter object encounters an error, the error event is triggered;

The error event is on node JS is a special event (if no listener triggered by error event is set), print out the stack tracker by default and exit the program.

2.2 adding listeners

There are two ways to add a listener to an event, which have exactly the same functions: parameter event and handler (listener)

  • emitter.addListener(event, listener) :
  • emitter.on(event, listener): listen all the time
//import http module
var http = require("http");
//create a http server
var server = http.createServer();

//bind a listener for server`s request Event
server.on("request", function(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write("Tracy");
    console.log("Kyxy");
    res.end();
});

//listen ip:127.0.0.1 port:1337, if requested, execute the listener function
server.listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

Run the above code, enter 127.0.0.1:1337 in the browser address bar, you can see Tracy, and the console outputs Kyxy

2.3 listener that executes only once

Use emitter The event listener bound to the once (event, listener) method will execute only once, and then the event will be deleted.

var http = require('http');
var server = http.createServer();

// Bind the handler for the request event, and the event will be executed only once
server.once('request', function(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write('shiyanlou');
    console.log('shiyanlou');
    res.end();
});

server.listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

Run the above code, enter 127.0.0.1:1337 in the browser address bar, you can see Tracy, and the console outputs Kyxy. If you refresh the page again, it will not be displayed again, because the event will be executed only once.

2.4 removing the listener

Use emitter The removelistener (event, listener) method removes the handler bound to the request event of the object.

//import http module
var http = require("http");
//create a http server
var server = http.createServer();

function callback(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write("Hello World");
    console.log("Hello World");
    res.end();
}

//bind a listener for server`s request Event
server.on("request", callback);

//remove a listener for server`s request Event
server.removeListener("request", callback);

server.on("request", function(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/json' });
    res.write("I`m back");
    console.log("LMD");
    res.end();
});

//listen ip:127.0.0.1 port:1337, if requested, execute the listener function
server.listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

Run the above code, enter 127.0.0.1:1337 in the browser address bar, you can see Im back, and the console outputs LMD, but there is no Hello World, because the listener callback 'has been removed.

2.4 remove all listeners

Use emitter Removealllisteners ([event]) to remove all listeners listening for event events on the object.

//import http module
var http = require("http");
//create a http server
var server = http.createServer();

function talk(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write("haha");
    console.log("xixi");
    res.end();
}function fruit(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write("apple");
    console.log("leomon");
    res.end();
}

//bind  listeners for server`s request Event
server.on("request", talk);
server.on("request", fruit)
//remove  listeners from server`s request Event
server.removeAllListeners("request");

server.on("request", function(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/json' });
    res.write("I`m back");
    console.log("LMD");
    res.end();
});

//listen ip:127.0.0.1 port:1337, if requested, execute the listener function
server.listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

Remove all event listeners bound to the server, so only Im back is displayed and LMD 'is output.

2.5 set the maximum binding number of listeners

emitter.setMaxListeners(n) can set the maximum number of listeners that can be bound to the same event. By default, more than 10 will prompt warning messages (which can help find memory leaks). Not all event triggers are limited to 10. You can set this method. If it is set to 0, there is no limit.

2.6 user defined events

Use emitter emit(event, [arg1], [arg2], [...])， Custom events can be triggered.

//import http module
var http = require("http");
//create a http server
var server = http.createServer();

server.on("myevent", function(arg) {
    console.log(arg);
});

server.emit("myevent", "Kyxy");

server.on("request", function(req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.write("Tracy");
    res.end();
});

//listen ip:127.0.0.1 port:1337, if requested, execute the listener function
server.listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

2.7 viewing the number of event bound listeners

Use events EventEitter. Listener count (emitter, event) can view the number of event listeners.

var http = require("http");
var events = require("events");
var server = http.createServer();

server.on("request", function(req, res) {
    res.writeHead(200, {"Content-Type": "text/plain"});
    res.write("kyxy");
    console.log("kyxy");
    res.end();
});

server.on("request", function(req, res) {
    res.writeHead(200, {"Content-Type": "text/plain"});
    res.write("Tracy");
    console.log("Tracy");
    res.end();
});

server.listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

var num = events.EventEmitter.listenerCount(server, "request");
console.log(num); //2

The console interface outputs the number 2 because the server binds two listeners to the 'request' event.

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Added by Cascade on Wed, 05 Jan 2022 15:18:37 +0200

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