Sådan fungerer modulær programmering i Node.js

Moduler er et af de grundlæggende træk ved Node.js.

Når du bygger en applikation, når koden bliver mere og mere kompleks, kan du ikke placere hele din kode i en enkelt fil.

Da dette ikke kan håndteres, kan du bruge Nodes modulmønster til at skrive forskellige filer og eksportere dem (inklusive funktioner, objekter og metoder) til hovedfilen.

Nu kan du spørge - hvad er egentlig en module?

Enkelt sagt er a moduleintet andet end en JavaScript-fil. Det er det.

Med Nodes modulære funktionalitet kan vi importere vores egne eksterne filer, kernemoduler (Native) og NPM-moduler. I denne artikel vil vi diskutere hver enkelt af disse detaljeret.

Sådan importeres dine egne filer

I denne artikel skal vi diskutere, hvordan vi kan eksportere og importere vores egne filer.

Dybest set er der to filer: calculate.jshvorfra vi eksporterer og main.jstil hvor vi importerer filen.

mduleexport.png

Vi har begge filer i samme mappe for at holde det enkelt.

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Sådan importeres en funktion

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

Her eksporterer vi en funktion kaldet addved hjælp af module.exports. Derefter importeres denne funktion til en anden fil ved hjælp af requiremetoden.

I node kaldes hver fil som en moduleog exportser en egenskab for modulet Objekt.

Vi kan nu påberåbe sig funktionen i den forskellige fil, dvs. main.jsved at videregive argumenterne som vist nedenfor.

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

Sådan importeres et objekt

Vi kan også eksportere et helt objekt og få adgang til de forskellige metoder i det.

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

Vi eksporterede objektet addog importerede det til vores hovedfil ved hjælp af requiremetoden.

Vi kan nu få adgang resulttil addobjektets metode ved hjælp af .dot-operatoren:

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

En anden måde, hvorpå vi kan eksportere ovenstående objekt, er kun at eksportere den metode, som vi har brug for, i stedet for hele objektet.

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

Som du kan se, importerer vi resultmetoden i addobjektet. Så denne metode kan direkte påberåbes i hovedfilen.

Dette er god praksis, hvis du ikke har brug for hele objektet, men kun har brug for nogle metoder / funktioner til det. Det gør også vores kode mere sikker.

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

Sådan importeres en funktionskonstruktør:

En funktionskonstruktør bruges grundlæggende til at oprette en ny forekomst af et objekt, der har de samme egenskaber som det for hovedobjektet / -funktionen.

I nedenstående tilfælde opretter vi en ny forekomst af 'Tilføj' -objektet ved hjælp af newnøgleordet. Denne proces, hvor vi opretter en forekomst af et objekt, kaldes 'instantiering'.

Derefter eksporterer vi denne forekomst ved hjælp af module.exports:

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

Nu kan vi importere den til vores hovedfil og få adgang til 'resultat'-metoden inde i den for at få vores beregnede værdi.

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

På denne måde kan vi eksportere og importere en funktionskonstruktør.

Der er en anden måde, vi kan gøre dette på, ved at oprette vores nye forekomst i hovedfilen snarere end i den eksporterede fil som vist ovenfor module.exports = new Add().

Vi får se, hvordan dette fungerer, når vi eksporterer ES6-klasser, der fungerer svarende til funktionskonstruktører.

Sådan importeres ES6-klasser

classer en særlig funktionstype, hvor classnøgleordet hjælper med at initialisere det. Det bruger constructormetoden til at gemme egenskaberne.

Nu skal vi eksportere hele classved hjælp af 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; 

Nu i vores hovedfil opretter vi en ny instans ved hjælp af newnøgleordet og får adgang til resultmetoden for at få vores beregnede værdi.

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

Sådan importeres nodekernemoduler (indfødte)

I stedet for at oprette vores egne brugerdefinerede moduler hver gang, leverer Node et sæt moduler for at gøre vores liv lettere.

Vi skal diskutere nogle af modulerne, men du kan finde hele listen i det officielle node API-dokument her.

Import af nodemoduler svarer til, hvordan du importerer dine egne moduler. Du bruger den samme require()funktion til at få adgang til den i din egen fil.

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?

Pakken er et stykke kode, der administreres af Package Manager. Det er intet andet end software, der styrer installationen og opdateringen af ​​pakker.

NPM i henhold til den officielle dokumentation:

NPM er verdens største software-registreringsdatabase. Open source-udviklere fra alle kontinenter bruger npm til at dele og låne pakker, og mange organisationer bruger npm til også at styre privat udvikling.

Så i NPM bruger vi andres open source-kode, der administreres af NPM ved at importere den til vores projekt.

NPM leveres normalt med Node JS, når du downloader den. Du kan kontrollere, om NPM er installeret på din maskine ved blot at køre kommandoen npm -vpå din kommandoprompt. Hvis det returnerer noget versionsnummer, betyder det, at NPM er installeret.

NPM har sit register på npmjs.com, hvor du kan finde pakker, som du kan bruge.

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.

Du kan installere enhver NPM-pakke ved hjælp af kommandoen npm i . Og du kan installere NPM-pakken globalt ved hjælp af -gflagget. Pakken kan også gøres udviklingsafhængig ved hjælp af --save-devflagget.

Tak fordi du læste! Hvis du kan lide denne artikel, skal du kontakte mig på Twitter, da jeg fortsætter med at dokumentere min læring.