Node.js REPL & CLI

What is REPL?

REPL stands for Read-Eval-Print-Loop. It's an interactive environment that reads input, evaluates code, prints result, and loops for the next command.

Why Use REPL?

# Running a file (non-interactive)
node app.js
# Starting REPL (interactive)
node

Getting Started with REPL

Start REPL

node
>

First REPL Commands

> 2 + 2
4
> let name = "John"
undefined
> name
'John'
> let fruits = ["apple", "banana", "orange"]
undefined
> fruits[1]
'banana'
> function greet(name) { return `Hello ${name}!` }
undefined
> greet("Bob")
'Hello Bob!'

Exit REPL

// Ctrl+C twice, Ctrl+D, .exit, or process.exit()
> .exit

REPL Features & Commands

Underscore Variable

> 5 + 3
8
> _ + 2
10

Tab Auto-completion

> Math. // Press Tab to view methods

Advanced REPL Usage

Custom REPL

const repl = require('repl');
const r = repl.start({ prompt: 'myapp> ' });
r.context.version = '1.0.0';
r.context.add = (a, b) => a + b;

REPL History

// Up/Down arrow for history
console.log(process.env.HOME + '/.node_repl_history')

REPL Options

repl.start({
  prompt: 'custom> ',
  useColors: true,
  ignoreUndefined: true
});

Creating CLI Applications

CLI Entry Pattern

// cli.js
console.log('Args:', process.argv.slice(2));
// node cli.js hello --name John

Working with process.argv

const args = process.argv.slice(2);
let name = 'World';
for (let i = 0; i < args.length; i++) {
  if (args[i] === '--name' || args[i] === '-n') name = args[++i];
}
console.log(`Hello, ${name}!`);

Use process.env

const env = process.env.NODE_ENV || 'development';
const port = process.env.PORT || 8080;

Using Readline Module

Basic Readline

const readline = require('readline');
const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
rl.question('What is your name? ', (answer) => {
  console.log(`Hello, ${answer}!`);
  rl.close();
});

Multiple Questions (Promise style)

function askQuestion(q, rl) {
  return new Promise(resolve => rl.question(q, resolve));
}

Building Interactive CLI Tools

Build menu systems, todo tools, and command-based apps with readline and filesystem/network modules.

Menu Example Pattern

// show options - read input - switch case - repeat prompt
switch(choice) {
  case '1': console.log('Hello'); break;
  case '4': rl.close(); break;
}

CLI Frameworks & Libraries

Popular Libraries

npm install commander
npm install yargs
npm install inquirer
npm install chalk
npm install ora

Practical Examples

1) File Explorer CLI

// Commands: ls, cd, cat, pwd, exit
const fs = require('fs');
const path = require('path');

2) Backup CLI Tool

// Use commander + ora + fs to backup/restore files
program.command('backup <file>')

3) Password Generator CLI

const crypto = require('crypto');
function generatePassword(length = 12) {
  const chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@#$%^&*';
  let out = '';
  for (let i = 0; i < length; i++) out += chars[crypto.randomInt(0, chars.length)];
  return out;
}

Best Practices

• Use proper exit codes (0 success, non-zero errors).
• Handle signals (SIGINT, SIGTERM) gracefully.
• Add centralized error handling for uncaught exceptions/rejections.
• Show progress indicators for long-running commands.

Contextual Learning Links

• HTML Tutorial - structure and semantics for web pages
• CSS Tutorial - styling concepts used in full-stack apps
• Bootstrap Tutorial - fast responsive UI building
• JavaScript Tutorial - core language foundation for Node.js