Cómo funciona la programación modular en Node.js

Los módulos son una de las características fundamentales de Node.js.

Cuando crea una aplicación, a medida que el código se vuelve cada vez más complejo, no puede poner todo el código en un solo archivo.

A medida que esto se vuelva inmanejable, puede usar el patrón de módulo de Node para escribir diferentes archivos y exportarlos (incluidas funciones, objetos y métodos) al archivo principal.

Ahora podría preguntar: ¿qué es exactamente un module?

En términos simples, a moduleno es más que un archivo JavaScript. Eso es.

Con la funcionalidad modular de Node, podemos importar nuestros propios archivos externos, módulos de nodo centrales (nativos) y módulos NPM. En este artículo, analizaremos cada uno de estos en detalle.

Cómo importar sus propios archivos

En este artículo, vamos a discutir cómo podemos exportar e importar nuestros propios archivos.

Básicamente, hay dos archivos: calculate.jsdesde donde exportaremos y main.jshacia donde importaremos ese archivo.

mduleexport.png

Tenemos ambos archivos en la misma carpeta para que sea más sencillo.

image.png

Cómo importar una función

//---- Exported file [calculate.js] ---- const add = (a,b)=>{ return a + b } module.exports = add 

Aquí exportamos una función llamada addusing module.exports. Luego, esta función se importa a un archivo diferente usando el requiremétodo.

En Node, cada archivo se denomina a module, y exportses una propiedad del módulo Object.

Ahora podemos invocar la función en el archivo diferente, es decir main.js, pasando los argumentos como se muestra a continuación.

//------ Main File[main.js] ---- const add = require('./calculate') //name of the desired file const result = add(2,4) console.log(result); //Output : 6 

Cómo importar un objeto

También podemos exportar un objeto completo y acceder a los diferentes métodos que contiene.

//---- Exported file [calculate.js] ---- const add = { result : (a,b)=>{ return a + b } } module.exports = add 

Exportamos el objeto addy lo importamos a nuestro archivo principal usando el requiremétodo.

Ahora podemos acceder al resultmétodo del addobjeto usando el .operador de punto:

//---- Main file[main.js] ---- const add = require('./calculate') const result = add.result(5,8) console.log(result) //Output : 13 

Otra forma en que podemos exportar el objeto anterior es exportando solo el método que necesitamos en lugar de todo el objeto.

//---- Exported file [calculate.js] ---- const add = { result : (a,b)=>{ return a + b } } module.exports = add.result 

Como puede ver, estamos importando el resultmétodo en el addobjeto. Entonces, este método se puede invocar directamente en el archivo principal.

Esta es una buena práctica si no necesita todo el objeto, pero solo necesita algunos métodos / funciones del mismo. También hace que nuestro código sea más seguro.

//---- Main file[main.js] ---- const add = require('./calculate') const result = add(5,8) console.log(result) //Output : 13 

Cómo importar un constructor de funciones:

Un constructor de funciones se utiliza básicamente para crear una nueva instancia de un objeto que posee las mismas propiedades que el objeto / función principal.

En el siguiente caso, creamos una nueva instancia del objeto 'Agregar' usando la newpalabra clave. Este proceso en el que creamos una instancia de un objeto se llama 'instanciación'.

Luego exportamos esta instancia usando module.exports:

//---- Exported file [calculate.js] ---- function Add (){ this.result = (a,b)=>{ return a + b } } module.exports = new Add() 

Ahora podemos importarlo a nuestro archivo principal y acceder al método 'resultado' dentro de él, para obtener nuestro valor calculado.

//---- Main file[main.js] ---- const add = require('./calculate2') const result = add.result(1,3) console.log(result); //Output : 4 

De esta forma podemos exportar e importar un constructor de funciones.

Hay otra forma de hacer esto, que es creando nuestra nueva instancia en el archivo principal en lugar de en el archivo exportado como se muestra arriba module.exports = new Add().

Veremos cómo funciona esto cuando exportamos clases de ES6 que funcionan de manera similar a los constructores de funciones.

Cómo importar clases de ES6

classes un tipo especial de función donde la classpalabra clave ayuda a inicializarlo. Utiliza el constructormétodo para almacenar las propiedades.

Ahora vamos a exportar todo classusando module.exports:

//---- Exported file [calculate.js] ---- const Add = class{ constructor(a,b){ this.a = a; this.b = b; } result(){ return this.a + this.b } } module.exports = Add; 

Ahora en nuestro archivo principal, creamos una nueva instancia usando la newpalabra clave y accedemos al resultmétodo para obtener nuestro valor calculado.

 //---- Main file[main.js] ---- const add = require('./calculate') const result = new add(2,5) console.log(result.result()); //Output : 7 

Cómo importar módulos de núcleo de nodo (nativos)

En lugar de crear nuestros propios módulos personalizados cada vez, Node proporciona un conjunto de módulos para hacernos la vida más fácil.

Vamos a discutir algunos de los módulos, pero puede encontrar la lista completa en el documento oficial de la API del nodo aquí.

Importar módulos de nodo es similar a cómo importa sus propios módulos. Utiliza la misma require()función para acceder a ella en su propio archivo.

But there are some modules which you may have used unknowingly which do not need to be imported. For example console.log() – we have used the console module many times without fetching it in our own local file as these methods are available globally.

Let's look at one of the Core Native Modules which is File System (fs).

There are n number of operations we can perform with the file system module such as reading a file, writing a file, and updating it, to name a few.

We are going to use the fs module to read a file. Even in this method, there are two ways we can perform this action: one by using the synchronous function fs.readFileSync(), and the other by asynchronous function fs.readFile().

We'll discuss synchronous-asynchronous Node functions in future posts.

Today, we'll use the asynchronous version, that is fs.readFile().

For this example, we have created two files: main.js, where we are going to perform the file reading operation, and file.txt which is the file we are going to read.

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Thefile.txt contains some text in it.

Hello World!

Now, we use the fs module to read the file, without importing it, as shown below:

fs.readFile('./file.txt','utf-8',(err,data)=>{ if (err) throw err console.log(data); })

It will throw an error as fs is not defined. That is because the file system fs module is not available globally like the console module is.

ReferenceError: fs is not defined at Object. (C:\Users\Sarvesh Kadam\Desktop\Training\blog\code snippets\Node Modular Pattern\main.js:3:1) at Module._compile (internal/modules/cjs/loader.js:1256:30) at Object.Module._extensions..js (internal/modules/cjs/loader.js:1277:10) at Module.load (internal/modules/cjs/loader.js:1105:32) at Function.Module._load (internal/modules/cjs/loader.js:967:14) at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:60:12) at internal/main/run_main_module.js:17:47 

Therefore, we need to import all the data from the file system module using the require() function and store all that data in a variable fs.

const fs = require('fs') fs.readFile('./file.txt','utf-8',(err,data)=>{ if (err) throw err console.log(data); })

Now you can name that variable anything. I named it fs for readability and it's the standard which most developers follow.

Using the fs variable we can access the readFile() method where we passed three arguments Those arguments are file path, character encoding utf-8, and the callback function to give an output.

You might ask why we're passing utf-8 as our argument in the readFile()?

Because it encodes the value and gives the text as an output rather than giving a buffer as shown below:

The callback function, in turn, has two arguments: an error (err) and the actual content in the file (data). Then we print that data in the console.

//Output: Hello World! 

How to Import NPM Modules

So what exactly is Node Package Manager?

El paquete es un fragmento de código administrado por el Administrador de paquetes. No es más que un software que gestiona la instalación y actualización de paquetes.

NPM según la documentación oficial:

NPM es el registro de software más grande del mundo. Los desarrolladores de código abierto de todos los continentes usan npm para compartir y tomar prestados paquetes y muchas organizaciones también usan npm para administrar el desarrollo privado.

Entonces, en NPM usamos el código de fuente abierta de otra persona administrado por NPM al importarlo a nuestro proyecto.

NPM generalmente viene con Node JS cuando lo descarga. Puede verificar si NPM está instalado en su máquina simplemente ejecutando el comando npm -ven su símbolo del sistema. Si devuelve algún número de versión, eso significa que NPM se instaló correctamente.

NPM tiene su registro en npmjs.com, donde puede descubrir paquetes que puede utilizar.

Let's look at one of the packages called chalk which is basically used for terminal styling.

chalknpm2.jpg

In the above figure, we can see the weekly downloads of the package which suggests how popular is it.

Also, you can see that this package has dependencies in it. So this module which will serve as a dependency on our project is itself dependent on other modules.

This entire management process is taken care of by the Package Manager.

Even the source code is which is present on GitHub is given to us. We can navigate to it and verify if there are any open issues present.

One more thing before moving forward: the NPM packages come in different versions. The pattern which the version follows is semantic versioning.

As you can see, the latest version of the chalk module when I wrote this article is 4.1.0.

It follows the semantic versioning Major_changes.Minor_changes.Patch pattern.

Major_changes, as the name stands, are the significant changes made on the module which might affect your existing code.

Minor_changes are new enhancements or features along with defect fixes that have been added which should not affect your existing code.

Patch is the small bug fixes that will not crash your existing code.

You can learn more about semantic versioning on semver.org.

How to Install NPM

Now to import any package from NPM, you first need to initialize NPM on your local project folder by running the command on the command prompt:

npm init 

Once you run the above command, it will ask you for some data as shown below such as package name, version, and so on.

Much of this data can be kept as default as mentioned in the Round brackets ().

Also, the fields such as author and license are for the folks who created those NPM packages.

On the other hand, we are just importing and using them to create our own application.

package name: (code_npm) code_npm version: (1.0.0) 1.0.0 description: npm demo entry point: (index.js) index.js test command: test git repository: keywords: npm test author: Sarvesh license: (ISC) 

Once you enter all the fields, it will create a JSON file with values that have the above properties, and it'll ask you for confirmation like this:

Is this OK? (yes) yes 

Once you've confirmed yes it will create a package.json file with all the data you entered as illustrated below:

{ "name": "code_npm", "version": "1.0.0", "description": "npm demo", "main": "index.js", "scripts": { "test": "echo \"Error: no test specified\" && exit 1" }, "keywords": [ "npm", "test" ], "author": "Sarvesh", "license": "ISC" } 

Also, you can see a script object that has a test property added. You can run it using the npm test command and it will give back the desired output like this:

"Error: no test specified" 

Now instead of doing this elongated method of initializing NPM and entering the custom properties values, you can simply run the command:

npm init -y 

Once you run this command, it will directly create a package.json file with the default values.

pkgjson.PNG

Now to install the latest version of the chalk package in your project, you need to execute the command:

npm install chalk 

You can also install any specific version you need of chalk by just adding @version number as shown below. Also instead of install you can simply put the short-hand i flag which stands for installation:

npm i [email protected] 

This will install two things, a node_modules folder, and a package-lock.json file.

folderdir.PNG

Also, it will add a new property called dependencies to our package.json file which contains the name of the package installed and its version.

"dependencies": { "chalk": "^4.0.0" } 

The node_module folder contains the packages folder and its dependency's folders. It gets modifies as and when the npm package gets installed.

The package-lock.json contains the code which makes NPM faster and more secure.

"chalk": { "version": "4.0.0", "resolved": "//registry.npmjs.org/chalk/-/chalk-4.0.0.tgz", "integrity": "sha512-N9oWFcegS0sFr9oh1oz2d7Npos6vNoWW9HvtCg5N1KRFpUhaAhvTv5Y58g880fZaEYSNm3qDz8SU1UrGvp+n7A==", "requires": { "ansi-styles": "^4.1.0", "supports-color": "^7.1.0" } 

It mainly contains properties such as version, which is the semantic version number.

The resolved property is the directory or location from which the package was fetched. In this case it was fetched from chalk.

The integrity property is to make sure that we get the same code if we install the dependency again.

The requires object property represents the dependency of the chalk package.

Note: Do not make any changes to these two files node_modules and package-lock.json

How to Use NPM

Now once we've installed chalk to our project, we can import it to our root project file using the require() method. Then we can store that module in a variable called chalk.

const chalk = require('chalk') console.log(chalk.red("Hello World"))

Using the red() method of the chalk package, we have styled the "Hello World" text color in red.

On running the command node index.js we get the following output:

chalkop.png

Now there are many ways you can style your command line output using the chalk package. For more information you can refer to the Chalk official document on NPM.

Also, you can install the NPM packages globally (that is, on our operating system) rather than installing it in your local project by adding the -g flag on the command line (which stands for global, as mentioned below):

npm i nodemon -g 

This global package will not affect our package.json in any way since it is not installed locally.

We have installed the nodemon package globally which is used for automatic restart of a Node application when file changes in the directory are observed.

You can refer to nodemon for more information.

We can use the nodemon package by running the application using this command:

nodemon index.js 

It works similarly to node index.js, except it keeps an eye on the file changes and it restarts the application once changes are detected.

[nodemon] 2.0.6 [nodemon] to restart at any time, enter `rs` [nodemon] watching path(s): *.* [nodemon] watching extensions: js,mjs,json [nodemon] starting `node index.js` Hello World 

Note: The chalk styling will probably not work when you used nodemon.

Finally, we will go through the dev dependencies. There are some NPM packages or modules which we won't need in our project's production environment, but only for our development requirements.

We can install these modules in our project using the dev flag as shown below:

 npm i nodemon --save-dev 

It then creates a new property in the package.json called devDependencies:

"devDependencies": { "nodemon": "^2.0.6" } 

Conclusion

Using Node's Module Pattern, we can import from our own files by exporting them in form of functions, objects, function constructors, and ES6 classes.

And Node has its own set of Core (Native) Modules which we can use. Some of them are available globally, while some of them need to be imported locally in your project/folder.

NPM is a package manager that manages 3rd party open source code which we can use in our project. Before using NPM modules, you need to initialize NPM locally using npm init on your command line in the root of your project folder.

Puede instalar cualquier paquete de NPM usando el comando npm i . Y puede instalar el paquete NPM globalmente usando la -gbandera. Además, el paquete puede hacerse dependiente del desarrollo utilizando la --save-devbandera.

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