REPL stands for Read Eval Print Loop and it represents a computer environment like a Windows console or Unix/Linux shell where a command is entered and the system responds with an output in an interactive mode. Node.js or Node comes bundled with a REPL environment. It performs the following tasks −
• Read − Reads user’s input, parses the input into JavaScript data-structure, and stores in memory.
• Eval − Takes and evaluates the data structure.
• Print − Prints the result.
• Loop − Loops the above command until the user presses ctrl-c twice.
The REPL feature of Node is very useful in experimenting with Node.js codes and to debug JavaScript codes.

Online REPL Terminal

To simplify your learning, we have set up an easy to use Node.js REPL environment online, where you can practice Node.js syntax − Launch Node.js REPL Terminal

Starting REPL

REPL can be started by simply running node on shell/console without any arguments as follows.
$ node
You will see the REPL Command prompt > where you can type any Node.js command −
$ node
>

Simple Expression

Let’s try a simple mathematics at the Node.js REPL command prompt −

$ node
> 1 + 3
4
> 1 + ( 2 * 3 ) - 4
3
>

Use Variables

You can make use variables to store values and print later like any conventional script. If var keyword is not used, then the value is stored in the variable and printed. Whereas if var keyword is used, then the value is stored but not printed. You can print variables using console.log().
$ node

> x = 10
10
> var y = 10
undefined
> x + y
20
> console.log(“Hello World”)
Hello World
undefined

Multiline Expression

Node REPL supports multiline expression similar to JavaScript. Let’s check the following do-while loop in action −

$ node
> var x = 0
undefined
> do {
... x++;
... console.log("x: " + x);
... } 
while ( x < 5 );
x: 1
x: 2
x: 3
x: 4
x: 5
undefined
>

… comes automatically when you press Enter after the opening bracket. Node automatically checks the continuity of expressions.

Underscore Variable

You can use underscore (_) to get the last result −

$ node
> var x = 10
undefined
> var y = 20
undefined
> x + y
30
> var sum = _
undefined
> console.log(sum)
30
undefined
>

REPL Commands

• ctrl + c − terminate the current command.
• ctrl + c twice − terminate the Node REPL.
• ctrl + d − terminate the Node REPL.
• Up/Down Keys − see command history and modify previous commands.
• tab Keys − list of current commands.
• .help − list of all commands.
• .break − exit from multiline expression.
• .clear − exit from multiline expression.
• .save filename − save the current Node REPL session to a file.
• .load filename − load file content in current Node REPL session.

Stopping REPL

As mentioned above, you will need to use ctrl-c twice to come out of Node.js REPL.

$ node
>
(^C again to quit)
>

Node.js – NPM

Node Package Manager (NPM) provides two main functionalities −
• Online repositories for node.js packages/modules which are searchable on search.nodejs.org
• Command line utility to install Node.js packages, do version management and dependency management of Node.js packages.
NPM comes bundled with Node.js installables after v0.6.3 version. To verify the same, open console and type the following command and see the result −
$ npm –version
2.7.1
If you are running an old version of NPM then it is quite easy to update it to the latest version. Just use the following command from root −
$ sudo npm install npm -g
/usr/bin/npm -> /usr/lib/node_modules/npm/bin/npm-cli.js
npm@2.7.1 /usr/lib/node_modules/npm

Installing Modules using NPM

There is a simple syntax to install any Node.js module −
$ npm install <Module Name>
For example, following is the command to install a famous Node.js web framework module called express −
$ npm install express
Now you can use this module in your js file as following −
var express = require(‘express’);

Global vs Local Installation

By default, NPM installs any dependency in the local mode. Here local mode refers to the package installation in node_modules directory lying in the folder where Node application is present. Locally deployed packages are accessible via require() method. For example, when we installed express module, it created node_modules directory in the current directory where it installed the express module.
$ ls -l
total 0
drwxr-xr-x 3 root root 20 Mar 17 02:23 node_modules
Alternatively, you can use npm ls command to list down all the locally installed modules.
Globally installed packages/dependencies are stored in system directory. Such dependencies can be used in CLI (Command Line Interface) function of any node.js but cannot be imported using require() in Node application directly. Now let’s try installing the express module using global installation.
$ npm install express -g
This will produce a similar result but the module will be installed globally. Here, the first line shows the module version and the location where it is getting installed.
express@4.12.2 /usr/lib/node_modules/express
├── merge-descriptors@1.0.0
├── utils-merge@1.0.0
├── cookie-signature@1.0.6
├── methods@1.1.1
├── fresh@0.2.4
├── cookie@0.1.2
├── escape-html@1.0.1
├── range-parser@1.0.2
├── content-type@1.0.1
├── finalhandler@0.3.3
├── vary@1.0.0
├── parseurl@1.3.0
├── content-disposition@0.5.0
├── path-to-regexp@0.1.3
├── depd@1.0.0
├── qs@2.3.3
├── on-finished@2.2.0 (ee-first@1.1.0)
├── etag@1.5.1 (crc@3.2.1)
├── debug@2.1.3 (ms@0.7.0)
├── proxy-addr@1.0.7 (forwarded@0.1.0, ipaddr.js@0.1.9)
├── send@0.12.1 (destroy@1.0.3, ms@0.7.0, mime@1.3.4)
├── serve-static@1.9.2 (send@0.12.2)
├── accepts@1.2.5 (negotiator@0.5.1, mime-types@2.0.10)
└── type-is@1.6.1 (media-typer@0.3.0, mime-types@2.0.10)
You can use the following command to check all the modules installed globally −
$ npm ls -g

Using package.json

package.json is present in the root directory of any Node application/module and is used to define the properties of a package. Let’s open package.json of express package present in node_modules/express/

{
"name": "express",
"description": "Fast, unopinionated, minimalist web framework",
"version": "4.11.2",
"author": {

"name": "TJ Holowaychuk",
"email": "tj@vision-media.ca"
},

"contributors": [{
"name": "Aaron Heckmann",
"email": "aaron.heckmann+github@gmail.com"
}, 

{
"name": "Ciaran Jessup",
"email": "ciaranj@gmail.com"
},

{
"name": "Douglas Christopher Wilson",
"email": "doug@somethingdoug.com"
},

{
"name": "Guillermo Rauch",
"email": "rauchg@gmail.com"
},

{
"name": "Jonathan Ong",
"email": "me@jongleberry.com"
},

{
"name": "Roman Shtylman",
"email": "shtylman+expressjs@gmail.com"
},

{
"name": "Young Jae Sim",
"email": "hanul@hanul.me"
} ],

"license": "MIT", "repository": {
"type": "git",
"url": "https://github.com/strongloop/express"
},

"homepage": "https://expressjs.com/", "keywords": [
"express",
"framework",
"sinatra",
"web",
"rest",
"restful",
"router",
"app",
"api"
],

"dependencies": {
"accepts": "~1.2.3",
"content-disposition": "0.5.0",
"cookie-signature": "1.0.5",
"debug": "~2.1.1",
"depd": "~1.0.0",
"escape-html": "1.0.1",
"etag": "~1.5.1",
"finalhandler": "0.3.3",
"fresh": "0.2.4",
"media-typer": "0.3.0",
"methods": "~1.1.1",
"on-finished": "~2.2.0",
"parseurl": "~1.3.0",
"path-to-regexp": "0.1.3",
"proxy-addr": "~1.0.6",
"qs": "2.3.3",
"range-parser": "~1.0.2",
"send": "0.11.1",
"serve-static": "~1.8.1",
"type-is": "~1.5.6",
"vary": "~1.0.0",
"cookie": "0.1.2",
"merge-descriptors": "0.0.2",
"utils-merge": "1.0.0"
},

"devDependencies": {
"after": "0.8.1",
"ejs": "2.1.4",
"istanbul": "0.3.5",
"marked": "0.3.3",
"mocha": "~2.1.0",
"should": "~4.6.2",
"supertest": "~0.15.0",
"hjs": "~0.0.6",
"body-parser": "~1.11.0",
"connect-redis": "~2.2.0",
"cookie-parser": "~1.3.3",
"express-session": "~1.10.2",
"jade": "~1.9.1",
"method-override": "~2.3.1",
"morgan": "~1.5.1",
"multiparty": "~4.1.1",
"vhost": "~3.0.0"
},

"engines": {
"node": ">= 0.10.0"
},

"files": [
"LICENSE",
"History.md",
"Readme.md",
"index.js",
"lib/"
],

"scripts": {
"test": "mocha --require test/support/env 
--reporter spec --bail --check-leaks test/ test/acceptance/",
"test-cov": "istanbul cover node_modules/mocha/bin/_mocha 
-- --require test/support/env --reporter dot --check-leaks test/ test/acceptance/",
"test-tap": "mocha --require test/support/env 
--reporter tap --check-leaks test/ test/acceptance/",
"test-travis": "istanbul cover node_modules/mocha/bin/_mocha 
--report lcovonly -- --require test/support/env 
--reporter spec --check-leaks test/ test/acceptance/"
},

"gitHead": "63ab25579bda70b4927a179b580a9c580b6c7ada",
"bugs": {
"url": "https://github.com/strongloop/express/issues"
},

"_id": "express@4.11.2",
"_shasum": "8df3d5a9ac848585f00a0777601823faecd3b148",
"_from": "express@*",
"_npmVersion": "1.4.28",
"_npmUser": {
"name": "dougwilson",
"email": "doug@somethingdoug.com"
},

"maintainers": [{
"name": "tjholowaychuk",
"email": "tj@vision-media.ca"
},

{
"name": "jongleberry",
"email": "jonathanrichardong@gmail.com"
},

{
"name": "shtylman",
"email": "shtylman@gmail.com"
},

{
"name": "dougwilson",
"email": "doug@somethingdoug.com"
},

{
"name": "aredridel",
"email": "aredridel@nbtsc.org"
},

{
"name": "strongloop",
"email": "callback@strongloop.com"
},

{
"name": "rfeng",
"email": "enjoyjava@gmail.com"
}],

"dist": {
"shasum": "8df3d5a9ac848585f00a0777601823faecd3b148",
"tarball": "https://registry.npmjs.org/express/-/express-4.11.2.tgz"
},

"directories": {},
"_resolved": "https://registry.npmjs.org/express/-/express-4.11.2.tgz",
"readme": "ERROR: No README data found!"
}

Attributes of Package.json

• name − name of the package
• version − version of the package
• description − description of the package
• homepage − homepage of the package
• author − author of the package
• contributors − name of the contributors to the package
• dependencies − list of dependencies. NPM automatically installs all the dependencies mentioned here in the node_module folder of the package.
• repository − repository type and URL of the package
• main − entry point of the package
• keywords − keywords

Uninstalling a Module

Use the following command to uninstall a Node.js module.
$ npm uninstall express
Once NPM uninstalls the package, you can verify it by looking at the content of /node_modules/ directory or type the following command −
$ npm ls

Updating a Module

Update package.json and change the version of the dependency to be updated and run the following command.
$ npm update express

Search a Module

Search a package name using NPM.
$ npm search express

Create a Module

Creating a module requires package.json to be generated. Let’s generate package.json using NPM, which will generate the basic skeleton of the package.json.
$ npm init
This utility will walk you through creating a package.json file.
It only covers the most common items, and tries to guess sane defaults.

See ‘npm help json’ for definitive documentation on these fields
and exactly what they do.

Use ‘npm install <pkg> –save’ afterwards to install a package and
save it as a dependency in the package.json file.

Press ^C at any time to quit.
name: (webmaster)
You will need to provide all the required information about your module. You can take help from the above-mentioned package.json file to understand the meanings of various information demanded. Once package.json is generated, use the following command to register yourself with NPM repository site using a valid email address.
$ npm adduser
Username: mcmohd
Password:
Email: (this IS public) mcmohd@gmail.com
It is time now to publish your module −
$ npm publish
If everything is fine with your module, then it will be published in the repository and will be accessible to install using NPM like any other Node.js module.

So, this brings us to the end of blog. This Tecklearn ‘How to use Node Package Manager and REPL Terminal’ blog helps you with commonly asked questions if you are looking out for a job in NodeJS Programming. If you wish to learn NodeJS and build a career in NodeJS Programming domain, then check out our interactive, Node.js Training, that comes with 24*7 support to guide you throughout your learning period.

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• Node.js Modules
• Import your own Package
• Node Package Manager (NPM)
• Local and Global Packages

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• REST API
• REST API in Express
• Postman
• MongoDB Driver API
• Express Router
• Mongoose API
• GraphQL
• GraphQL Playground

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• ES6 variables
• Functions with ES6
• Import and Export withES6
• Async/Await
• Introduction to Babel
• Rest API with ES6
• Browsing HTTP Requests with Fetch
• Processing Query String
• Creating API using ES6
• Building Dashboard API
• Creating dashboard UI with EJS
• ES6 Aside: Default Function Parameters
• Data Validation and Sanitization

User Authentication and Application Security

• Authentication
• Types of Authentication
• Session Vs Tokens
• JSON Web Tokens
• Bcrypt
• Node-local storage

Understand Buffers, Streams, and Events

• Using buffers for binary data
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• Streaming I/O from files and other sources
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• Timestamps
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• Why Microservices?
• What is Microservices?
• Why Docker?
• What is Docker?
• Terminologies in Docker
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