process

process.argv is an array where index 0 is the Node.js executable path and index 1 is the script file path. Arguments passed by the user are stored at index 2 and beyond.

argv.js

// Check command-line arguments
console.log('All args:', process.argv);

// User-passed arguments start at index 2
var args = process.argv.slice(2);
console.log('User args:', args);

// Use default values if no arguments provided
var name   = args[0] || 'Okabe Rintaro';
var target = args[1] || 'the future';

console.log('Sending D-Mail from ' + name + ' to ' + target);

node argv.js "Makise Kurisu" "the past"
All args: [ '/usr/local/bin/node', '/home/okabe/argv.js', 'Makise Kurisu', 'the past' ]
User args: [ 'Makise Kurisu', 'the past' ]
Sending D-Mail from Makise Kurisu to the past

Environment variables are accessed as properties of an object. Keys that do not exist return undefined, so they are commonly used with default values. See the process.env (environment variables) page for details.

env.js

// Get environment variables (use defaults if not set)
var host = process.env.HOST || 'localhost';
var port = process.env.PORT || '3000';
var mode = process.env.NODE_ENV || 'development';

console.log('Host:', host);
console.log('Port:', port);
console.log('Mode:', mode);

node env.js
Host: localhost
Port: 3000
Mode: development

cwd_exit.js

// Get the current working directory
console.log('Current directory:', process.cwd());

// Check if a required file exists; exit with error if not found
var fs = require('fs');
var configPath = process.cwd() + '/config.json';

if (!fs.existsSync(configPath)) {
    // Write a message to standard error
    process.stderr.write('Error: config.json not found\n');
    // Exit with code 1 (abnormal exit)
    // 0 = normal exit, 1 or higher = abnormal exit (detectable in shell scripts)
    process.exit(1);
}

console.log('Configuration file loaded');
process.exit(0);  // Normal exit

node cwd_exit.js
Current directory: /home/okabe/future-gadget-lab
Error: config.json not found

process_info.js

// Display basic process information
console.log('Process ID:', process.pid);
console.log('Parent PID:', process.ppid);
console.log('OS:', process.platform);
console.log('Node.js version:', process.version);
console.log('Uptime:', process.uptime().toFixed(2) + 's');

// Branch based on OS
if (process.platform === 'win32') {
    console.log('Shiina Mayuri: Running on Windows~!');
} else {
    console.log('Hashida Itaru: Running on a Unix-based OS');
}

node process_info.js
Process ID: 12345
Parent PID: 12300
OS: linux
Node.js version: v20.11.0
Uptime: 0.05s
Hashida Itaru: Running on a Unix-based OS

stdin.js

// Read data from standard input
// Set character encoding to UTF-8
process.stdin.setEncoding('utf8');

process.stdout.write('Enter the time machine destination: ');

// The 'data' event fires each time data is entered
process.stdin.on('data', function(input) {
    // Remove trailing newline
    var destination = input.trim();
    console.log('Amane Suzuha: Heading to ' + destination + '. El Psy Kongroo');
    // Stop reading input
    process.stdin.pause();
});

node stdin.js
Enter the time machine destination: 1975
Amane Suzuha: Heading to 1975. El Psy Kongroo

process_report.js

// A report that displays process status all at once
function printReport() {
    var mem = process.memoryUsage();

    console.log('=== Future Gadget Lab Process Report ===');
    console.log('PID       :', process.pid);
    console.log('Platform  :', process.platform);
    console.log('Node.js   :', process.version);
    console.log('CWD       :', process.cwd());
    console.log('Uptime    :', process.uptime().toFixed(3) + 's');

    // Convert memory usage to MB and display
    console.log('RSS       :', (mem.rss / 1024 / 1024).toFixed(2) + ' MB');
    console.log('Heap Used :', (mem.heapUsed / 1024 / 1024).toFixed(2) + ' MB');

    // Register a listener for the exit event
    process.on('exit', function(code) {
        console.log('Exit code:', code, '-- Okabe Rintaro: D-Mail sent successfully');
    });
}

printReport();
process.exit(0);

node process_report.js
=== Future Gadget Lab Process Report ===
PID       : 98765
Platform  : linux
Node.js   : v20.11.0
CWD       : /home/okabe/future-gadget-lab
Uptime    : 0.031s
RSS       : 28.47 MB
Heap Used : 5.83 MB
Exit code: 0 -- Okabe Rintaro: D-Mail sent successfully

The process object is a global interface for operating on and monitoring the Node.js process itself. The pattern of receiving command-line arguments with process.argv, accessing environment variables with process.env, and exiting with a specified exit code via process.exit() is especially common when implementing CLI tools.

process.stdin, process.stdout, and process.stderr are the streams for standard input, standard output, and standard error output respectively. console.log() internally calls process.stdout.write().

Common Mistake 1: Calling process.exit() in the middle of an async operation causes data to not be written

process.exit() immediately stops the event loop. It does not wait for pending asynchronous operations (file writes, network communication, etc.) to complete.

NG (calling exit before the write completes):

var fs = require('fs');

fs.writeFile('dmail.txt', 'D-Mail to Makise Kurisu', function(err) {
    // This callback has not executed yet
});

process.exit(0);  // Process exits before the write completes

OK (call exit inside the callback):

var fs = require('fs');

fs.writeFile('dmail.txt', 'D-Mail to Makise Kurisu', function(err) {
    if (err) {
        process.stderr.write('Write error: ' + err.message + '\n');
        process.exit(1);
        return;
    }
    console.log('D-Mail sent');
    process.exit(0);  // Exit after the write completes
});

Common Mistake 2: Forgetting that process.env values are always strings causes numeric comparisons to fail

All environment variables are strings. Even if they look like numbers, process.env.PORT is '3000' (a string), not 3000 (a number).

NG (comparing a string against a number):

// When run with PORT=3000
if (process.env.PORT == 3000) {
    console.log('Starting on port 3000');  // == does type coercion, so this may accidentally pass
}

if (process.env.PORT === 3000) {
    console.log('This line is never reached');  // === compares types too, so this is false
}

OK (explicitly convert to a number before comparing):

// Convert to number with Number()
if (Number(process.env.PORT) === 3000) {
    console.log('Starting on port 3000');
}

// Or compare as a string
if (process.env.PORT === '3000') {
    console.log('Starting on port 3000');
}

// Always convert when using the port number
var port = Number(process.env.PORT) || 3000;