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Introduction

The Node.js Read-Eval-Print Loop (REPL) is an interactive shell that processes Node.js expressions. It allows you to quickly test snippets of JavaScript code and see the results immediately. This is particularly useful for debugging, experimenting with new features, or learning the basics of Node.js.

Key Concepts

Read: The REPL reads the user's input, parses it into JavaScript data structures, and stores it in memory.
Eval: The REPL evaluates the data structure.
Print: The REPL prints the result of the evaluation.
Loop: The REPL repeats the process until the user exits.

Starting the REPL

To start the Node.js REPL, open your terminal or command prompt and type:

node

You should see a prompt like this:

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

Basic Usage

Simple Expressions

You can type any valid JavaScript expression, and the REPL will evaluate it and print the result.

> 1 + 1
2
> console.log("Hello, Node.js!")
Hello, Node.js!
undefined

Variable Declarations

You can declare variables and use them in subsequent expressions.

> let x = 10
undefined
> x * 2
20

Multiline Expressions

To enter a multiline expression, use the backslash (\) at the end of the line.

> let sum = (a, b) => { \
... return a + b; \
... }
undefined
> sum(5, 3)
8

Special Commands

The REPL provides several special commands that are not part of the JavaScript language but are useful for working in the REPL environment.

.help: Displays a list of special commands.
.exit: Exits the REPL.
.save <filename>: Saves the current REPL session to a file.
.load <filename>: Loads a file into the current REPL session.

> .help
.break    Sometimes you get stuck, this gets you out
.clear    Alias for .break
.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

Practical Examples

Example 1: Simple Arithmetic

> let a = 5
undefined
> let b = 10
undefined
> a + b
15

Example 2: Working with Arrays

> let arr = [1, 2, 3, 4, 5]
undefined
> arr.map(x => x * 2)
[ 2, 4, 6, 8, 10 ]

Example 3: Using Node.js Modules

You can require Node.js modules and use them in the REPL.

> const fs = require('fs')
undefined
> fs.readFileSync('example.txt', 'utf8')
'This is an example file.'

Exercises

Exercise 1: Basic Arithmetic

Start the Node.js REPL.
Declare two variables, x and y, and assign them the values 7 and 3, respectively.
Calculate the sum, difference, product, and quotient of x and y.

Solution:

> let x = 7
undefined
> let y = 3
undefined
> x + y
10
> x - y
4
> x * y
21
> x / y
2.3333333333333335

Exercise 2: Working with Strings

Start the Node.js REPL.
Declare a variable str and assign it the value "Node.js is awesome!".
Use the toUpperCase method to convert the string to uppercase.
Use the split method to split the string into an array of words.

Solution:

> let str = "Node.js is awesome!"
undefined
> str.toUpperCase()
'NODE.JS IS AWESOME!'
> str.split(' ')
[ 'Node.js', 'is', 'awesome!' ]

Exercise 3: Using Built-in Modules

Start the Node.js REPL.
Require the os module.
Use the os.platform method to get the operating system platform.
Use the os.cpus method to get information about the system's CPUs.

Solution:

> const os = require('os')
undefined
> os.platform()
'linux' // or 'win32', 'darwin', etc.
> os.cpus()
[
  {
    model: 'Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz',
    speed: 1992,
    times: { user: 252000, nice: 0, sys: 88000, idle: 1230000, irq: 0 }
  },
  // More CPU information...
]

Conclusion

The Node.js REPL is a powerful tool for quickly testing and debugging JavaScript code. It allows you to experiment with Node.js features and modules in an interactive environment. By mastering the REPL, you can enhance your productivity and deepen your understanding of Node.js. In the next module, we will dive into the core concepts of Node.js, starting with its architecture.

Node.js REPL

Introduction

Key Concepts

Starting the REPL

Basic Usage

Simple Expressions

Variable Declarations

Multiline Expressions

Special Commands

Practical Examples

Example 1: Simple Arithmetic

Example 2: Working with Arrays

Example 3: Using Node.js Modules

Exercises

Exercise 1: Basic Arithmetic

Exercise 2: Working with Strings

Exercise 3: Using Built-in Modules

Conclusion

Node.js Course

Module 1: Introduction to Node.js

Module 2: Core Concepts

Module 3: File System and I/O

Module 4: HTTP and Web Servers

Module 5: NPM and Package Management

Module 6: Express.js Framework

Module 7: Databases and ORMs

Module 8: Authentication and Authorization

Module 9: Testing and Debugging

Module 10: Advanced Topics

Module 11: Deployment and DevOps

Module 12: Real-World Projects