Understanding the Basics

When getting started with Node.js, it’s important to understand how to write and run basic scripts, as well as how to use the Node.js REPL (Read-Eval-Print Loop) for quick experimentation and debugging. In this section, you’ll learn how to write your first “Hello World” application, run Node.js scripts from the command line, and explore the Node.js REPL.

Writing Your First “Hello World” Application

The “Hello World” application is the most basic example you can write when learning a new programming language or environment. In Node.js, this involves creating a simple script that outputs “Hello, World!” to the console.

Step 1: Create a New File

Open your text editor (e.g., VS Code).
Create a new file named hello.js in your project directory.

Step 2: Write the “Hello World” Script

In the hello.js file, add the following code:

				
					console.log("Hello, World!");

This script uses Node.js’s console.log() function to print “Hello, World!” to the console.

Running Node.js Scripts from the Command Line

Once you’ve written your first Node.js script, you can run it from the command line.

Step 1: Open Your Terminal or Command Prompt

Open the terminal (macOS/Linux) or command prompt (Windows).
Navigate to the directory where your hello.js file is located using the cd command.

Step 2: Run the Script

Run the script using the node command:

				
					node hello.js

When you run this command, Node.js executes the hello.js file, and you should see the output:

				
					Hello, World!

This confirms that your script ran successfully.

Introduction to the Node.js REPL (Read-Eval-Print Loop)

The Node.js REPL is an interactive shell that allows you to write and execute JavaScript code in real-time. It’s a useful tool for experimenting with JavaScript and testing small snippets of code.

Step 1: Start the Node.js REPL

To start the REPL, simply open your terminal or command prompt and type:

				
					node

You’ll see a prompt that looks like this:

This prompt indicates that the REPL is ready to accept your input.

Step 2: Experiment with JavaScript in the REPL

You can now type JavaScript code directly into the REPL and see the results immediately. For example:

				
					> console.log("Hello, World!");
Hello, World!
undefined

Here’s what happens:

You entered console.log("Hello, World!");.
The REPL executed the code and printed Hello, World! to the console.
The undefined output indicates that console.log() doesn’t return a value.

You can also perform other JavaScript operations, such as:

Basic Arithmetic:

				
					> 5 + 3
8

Variable Declaration and Usage:

				
					> let name = "Node.js";
undefined
> name
'Node.js'

Step 3: Exit the REPL

To exit the REPL, press Ctrl+C twice or type .exit and press Enter:

				
					> .exit

This will return you to the regular terminal prompt.

Conclusion

Writing your first “Hello World” application in Node.js, running Node.js scripts from the command line, and using the Node.js REPL are foundational skills that will help you get comfortable with Node.js development. The REPL is particularly useful for quickly testing code snippets and understanding how Node.js and JavaScript work in real-time. With these basics under your belt, you’re ready to start exploring more advanced features of Node.js and building more complex applications.

Contents

Understanding the Basics

Writing Your First “Hello World” Application

Running Node.js Scripts from the Command Line

Introduction to the Node.js REPL (Read-Eval-Print Loop)

Conclusion

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