How To Use the Node.js REPL

Introduction

The Node.js Read-Eval-Print-Loop (REPL) is an interactive shell that processes Node.js expressions. The shell reads JavaScript code the user enters, evaluates the result of interpreting the line of code, prints the result to the user, and loops until the user signals to quit.

The REPL is bundled with with every Node.js installation and allows you to quickly test and explore JavaScript code within the Node environment without having to store it in a file.

Prerequisites

To complete this tutorial, you will need:

• Node.js installed on your development machine. This tutorial uses version 10.16.0.
• A basic knowledge of JavaScript.

Step 1 — Starting and Stopping the REPL

If you have node installed, then you also have the Node.js REPL. To start it, simply enter node in your command line shell:

node

This results in the REPL prompt:

>

The > symbol lets you know that you can enter JavaScript code to be immediately evaluated.

For an example, try adding two numbers in the REPL by typing this:

> 2 + 2

When you press ENTER, the REPL will evaluate the expression and return:

4

To exit the REPL, you can type .exit, or press CTRL+D once, or press CTRL+C twice, which will return you to the shell prompt.

With starting and stopping out of the way, let’s take a look at how you can use the REPL to execute simple JavaScript code.

Step 2 — Executing Code in the Node.js REPL

The REPL is a quick way to test JavaScript code without having to create a file. Almost every valid JavaScript or Node.js expression can be executed in the REPL.

In the previous step you already tried out addition of two numbers, now let’s try division. To do so, start a new REPL:

node

In the prompt type:

> 10 / 5

Press ENTER , and the output will be 2, as expected:

2

The REPL can also process operations on strings. Concatenate the following strings in your REPL by typing:

> "Hello " + "World"

Again, press ENTER, and the string expression is evaluated:

'Hello World'

Note: You may have noticed that the output used single quotes instead of double quotes. In JavaScript, the quotes used for a string do not affect its value. If the string you entered used a single quote, the REPL is smart enough to use double quotes in the output.

Calling Functions

When writing Node.js code, it’s common to print messages via the global console.log method or a similar function. Type the following at the prompt:

> console.log("Hi")

Pressing ENTER yields the following output:

Hi
undefined

The first result is the output from console.log, which prints a message to the stdout stream (the screen). Because console.log prints a string instead of returning a string, the message is seen without quotes. The undefined is the return value of the function.

Creating Variables

Rarely do you just work with literals in JavaScript. Creating a variable in the REPL works in the same fashion as working with .js files. Type the following at the prompt:

> let age = 30

Pressing ENTER results in:

undefined

Like before, with console.log, the return value of this command is undefined. The age variable will be available until you exit the REPL session. For example, you can multiply age by two. Type the following at the prompt and press ENTER:

> age * 2

The result is:

60

Because the REPL returns values, you don’t need to use console.log or similar functions to see the output on the screen. By default, any returned value will appear on the screen.

Multi-line Blocks

Multi-line blocks of code are supported as well. For example, you can create a function that adds 3 to a given number. Start the function by typing the following:

> const add3 = (num) => {

Then, pressing ENTER will change the prompt to:

...

The REPL noticed an open curly bracket and therefore assumes you’re writing more than one line of code, which needs to be indented. To make it easier to read, the REPL adds 3 dots and a space on the next line, so the following code appears to be indented.

Enter the second and third lines of the function, one at a time, pressing ENTER after each:

... return num + 3;
... }

Pressing ENTER after the closing curly bracket will display an undefined, which is the “return value” of the function assignment to a variable. The ... prompt is now gone and the > prompt returns:

undefined
>

Now, call add3() on a value:

> add3(10)

As expected, the output is:

13

You can use the REPL to try out bits of JavaScript code before including them into your programs. The REPL also includes some handy shortcuts to make that process easier.

Step 3 — Mastering REPL Shortcuts

The REPL provides shortcuts to decrease coding time when possible. It keeps a history of all the entered commands and allows us to cycle through them and repeat a command if necessary.

For an example, enter the following string:

"The answer to life the universe and everything is 32"

This results in:

'The answer to life the universe and everything is 32'

If we’d like to edit the string and change the “32” to “42”, at the prompt, use the UP arrow key to return to the previous command:

> "The answer to life the universe and everything is 32"

Move the cursor to the left, delete 3, enter 4, and press ENTER again:

'The answer to life the universe and everything is 42'

Continue to press the UP arrow key, and you’ll go further back through your history until the first used command in the current REPL session. In contrast, pressing DOWN will iterate towards the more recent commands in the history.

When you are done maneuvering through your command history, press DOWN repeatedly until you have exhausted your recent command history and are once again seeing the prompt.

To quickly get the last evaluated value, use the underscore character. At the prompt, type _ and press ENTER:

> _

The previously entered string will appear again:

'The answer to life the universe and everything is 42'

The REPL also has an autocompletion for functions, variables, and keywords. If you wanted to find the square root of a number using the Math.sqrt function, enter the first few letters, like so:

> Math.sq

Then press the TAB key and the REPL will autocomplete the function:

> Math.sqrt

When there are multiple possibilities for autocompletion, you’re prompted with all the available options. For an example, enter just:

> Math.

And press TAB twice. You’re greeted with the possible autocompletions:

> Math.
Math.__defineGetter__      Math.__defineSetter__      Math.__lookupGetter__
Math.__lookupSetter__      Math.__proto__             Math.constructor
Math.hasOwnProperty        Math.isPrototypeOf         Math.propertyIsEnumerable
Math.toLocaleString        Math.toString              Math.valueOf

Math.E                     Math.LN10                  Math.LN2
Math.LOG10E                Math.LOG2E                 Math.PI
Math.SQRT1_2               Math.SQRT2                 Math.abs
Math.acos                  Math.acosh                 Math.asin
Math.asinh                 Math.atan                  Math.atan2
Math.atanh                 Math.cbrt                  Math.ceil
Math.clz32                 Math.cos                   Math.cosh
Math.exp                   Math.expm1                 Math.floor
Math.fround                Math.hypot                 Math.imul
Math.log                   Math.log10                 Math.log1p
Math.log2                  Math.max                   Math.min
Math.pow                   Math.random                Math.round
Math.sign                  Math.sin                   Math.sinh
Math.sqrt                  Math.tan                   Math.tanh
Math.trunc

Depending on the screen size of your shell, the output may be displayed with a different number of rows and columns. This is a list of all the functions and properties that are available in the Math module.

Press CTRL+C to get to a new line in the prompt without executing what is in the current line.

Knowing the REPL shortcuts makes you more efficient when using it. Though, there’s another thing REPL provides for increased productivity—The REPL commands.

Step 4 — Using REPL Commands

The REPL has specific keywords to help control its behavior. Each command begins with a dot ..

.help

To list all the available commands, use the .help command:

> .help

There aren’t many, but they’re useful for getting things done in the REPL:

.break    Sometimes you get stuck, this gets you out
.clear    Alias for .break
.editor   Enter editor mode
.exit     Exit the repl
.help     Print this help message
.load     Load JS from a file into the REPL session
.save     Save all evaluated commands in this REPL session to a file

Press ^C to abort current expression, ^D to exit the repl

If ever you forget a command, you can always refer to .help to see what it does.

.break/.clear

Using .break or .clear, it’s easy to exit a multi-line expression. For example, begin a for loop as follows:

> for (let i = 0; i < 100000000; i++) {

To exit from entering any more lines, instead of entering the next one, use the .break or .clear command to break out:

... .break

You’ll see a new prompt:

>

The REPL will move on to a new line without executing any code, similar to pressing CTRL+C.

.save and .load

The .save command stores all the code you ran since starting the REPL, into a file. The .load command runs all the JavaScript code from a file inside the REPL.

Quit the session using the .exit command or with the CTRL+D shortcut. Now start a new REPL with node. Now only the code you are about to write will be saved.

Create an array with fruits:

> fruits = ['banana', 'apple', 'mango']

In the next line, the REPL will display:

[ 'banana', 'apple', 'mango' ]

Save this variable to a new file, fruits.js:

> .save fruits.js

We’re greeted with the confirmation:

Session saved to: fruits.js

The file is saved in the same directory where you opened the Node.js REPL. For example, if you opened the Node.js REPL in your home directory, then your file will be saved in your home directory.

Exit the session and start a new REPL with node. At the prompt, load the fruits.js file by entering:

> .load fruits.js

This results in:

fruits = ['banana', 'apple', 'mango']

[ 'banana', 'apple', 'mango' ]

The .load command reads each line of code and executes it, as expected of a JavaScript interpreter. You can now use the fruits variable as if it was available in the current session all the time.

Type the following command and press ENTER:

> fruits[1]

The REPL will output:

'apple'

You can load any JavaScript file with the .load command, not only items you saved. Let’s quickly demonstrate by opening your preferred code editor or nano, a command line editor, and create a new file called peanuts.js:

nano peanuts.js

Now that the file is open, type the following:

console.log('I love peanuts!');

Save and exit nano by pressing CTRL+X.

In the same directory where you saved peanuts.js, start the Node.js REPL with node. Load peanuts.js in your session:

> .load peanuts.js

The .load command will execute the single console statement and display the following output:

console.log('I love peanuts!');

I love peanuts!
undefined
>

When your REPL usage goes longer than expected, or you believe you have an interesting code snippet worth sharing or explore in more depth, you can use the .save and .load commands to make both those goals possible.

Conclusion

The REPL is an interactive environment that allows you to execute JavaScript code without first having to write it to a file.

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Why use Node.js for Web Development? Benefits and Examples of Apps

Front-end web development has been overwhelmed by JavaScript highlights for quite a long time. Google, Facebook, Wikipedia, and most of all online pages use JS for customer side activities. As of late, it additionally made a shift to cross-platform mobile development as a main technology in React Native, Nativescript, Apache Cordova, and other crossover devices. 

Throughout the most recent couple of years, Node.js moved to backend development as well. Designers need to utilize a similar tech stack for the whole web project without learning another language for server-side development. Node.js is a device that adjusts JS usefulness and syntax to the backend. 

What is Node.js? 

Node.js isn’t a language, or library, or system. It’s a runtime situation: commonly JavaScript needs a program to work, however Node.js makes appropriate settings for JS to run outside of the program. It’s based on a JavaScript V8 motor that can run in Chrome, different programs, or independently. 

The extent of V8 is to change JS program situated code into machine code — so JS turns into a broadly useful language and can be perceived by servers. This is one of the advantages of utilizing Node.js in web application development: it expands the usefulness of JavaScript, permitting designers to coordinate the language with APIs, different languages, and outside libraries.

What Are the Advantages of Node.js Web Application Development? 

Of late, organizations have been effectively changing from their backend tech stacks to Node.js. LinkedIn picked Node.js over Ruby on Rails since it took care of expanding responsibility better and decreased the quantity of servers by multiple times. PayPal and Netflix did something comparative, just they had a goal to change their design to microservices. We should investigate the motivations to pick Node.JS for web application development and when we are planning to hire node js developers. 

Amazing Tech Stack for Web Development 

The principal thing that makes Node.js a go-to environment for web development is its JavaScript legacy. It’s the most well known language right now with a great many free devices and a functioning local area. Node.js, because of its association with JS, immediately rose in ubiquity — presently it has in excess of 368 million downloads and a great many free tools in the bundle module. 

Alongside prevalence, Node.js additionally acquired the fundamental JS benefits: 

• quick execution and information preparing; 
• exceptionally reusable code; 
• the code is not difficult to learn, compose, read, and keep up; 
• tremendous asset library, a huge number of free aides, and a functioning local area. 

In addition, it’s a piece of a well known MEAN tech stack (the blend of MongoDB, Express.js, Angular, and Node.js — four tools that handle all vital parts of web application development). 

Designers Can Utilize JavaScript for the Whole Undertaking 

This is perhaps the most clear advantage of Node.js web application development. JavaScript is an unquestionable requirement for web development. Regardless of whether you construct a multi-page or single-page application, you need to know JS well. On the off chance that you are now OK with JavaScript, learning Node.js won’t be an issue. Grammar, fundamental usefulness, primary standards — every one of these things are comparable. 

In the event that you have JS designers in your group, it will be simpler for them to learn JS-based Node than a totally new dialect. What’s more, the front-end and back-end codebase will be basically the same, simple to peruse, and keep up — in light of the fact that they are both JS-based. 

A Quick Environment for Microservice Development 

There’s another motivation behind why Node.js got famous so rapidly. The environment suits well the idea of microservice development (spilling stone monument usefulness into handfuls or many more modest administrations). 

Microservices need to speak with one another rapidly — and Node.js is probably the quickest device in information handling. Among the fundamental Node.js benefits for programming development are its non-obstructing algorithms.

Node.js measures a few demands all at once without trusting that the first will be concluded. Many microservices can send messages to one another, and they will be gotten and addressed all the while. 

Versatile Web Application Development 

Node.js was worked in view of adaptability — its name really says it. The environment permits numerous hubs to run all the while and speak with one another. Here’s the reason Node.js adaptability is better than other web backend development arrangements. 

Node.js has a module that is liable for load adjusting for each running CPU center. This is one of numerous Node.js module benefits: you can run various hubs all at once, and the environment will naturally adjust the responsibility. 

Node.js permits even apportioning: you can part your application into various situations. You show various forms of the application to different clients, in light of their age, interests, area, language, and so on. This builds personalization and diminishes responsibility. Hub accomplishes this with kid measures — tasks that rapidly speak with one another and share a similar root. 

What’s more, Node’s non-hindering solicitation handling framework adds to fast, letting applications measure a great many solicitations. 

Control Stream Highlights

Numerous designers consider nonconcurrent to be one of the two impediments and benefits of Node.js web application development. In Node, at whatever point the capacity is executed, the code consequently sends a callback. As the quantity of capacities develops, so does the number of callbacks — and you end up in a circumstance known as the callback damnation. 

In any case, Node.js offers an exit plan. You can utilize systems that will plan capacities and sort through callbacks. Systems will associate comparable capacities consequently — so you can track down an essential component via search or in an envelope. At that point, there’s no compelling reason to look through callbacks.

 

Final Words

So, these are some of the top benefits of Nodejs in web application development. This is how Nodejs is contributing a lot to the field of web application development. 

I hope now you are totally aware of the whole process of how Nodejs is really important for your web project. If you are looking to hire a node js development company in India then I would suggest that you take a little consultancy too whenever you call. 

Good Luck!

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Node.js is a prominent tech trend in the space of web and mobile application development. It has been proven very efficient and useful for a variety of application development. Thus, all business owners are eager to leverage this technology for creating their applications.

Are you striving to develop an application using Node.js? But can’t decide which company to hire for NodeJS app development? Well! Don’t stress over it, as the following list of NodeJS app development companies is going to help you find the best partner.

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I believe this list will help you out in choosing the best NodeJS service provider company. So, now let’s explore the top NodeJS developer companies to choose from in 2021.

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