Node.js Reference

// console.log automatically appends a newline to the output
console.log("Hello, World!");

# Execute a JavaScript file using Node.js
node main.js

// Single-line comment for brief annotations

/* Multi-line
   comment block for more extensive notes */

const fs = require('fs'); // File system module
const path = require('path'); // Path utility module
const { parseArgs } = require('util'); // Selective import from utility module

// Terminate the process immediately
// exit(0) for success, non-zero for error (e.g., 1)
process.exit(1); 

try {
    // Manually throw a new Error object
    throw new Error("something went wrong");
} catch (e) {
    // Catch and print the error message
    console.log(e.message);
} finally {
    // Always execute this block
    console.log("Cleanup");
}

let x = 5;       // Block-scoped mutable variable
const Y = 10;    // Block-scoped immutable constant
var z = 1;       // Function-scoped variable (legacy)

// Maximum safe integer for standard Number type
console.log(Number.MAX_SAFE_INTEGER); 
// BigInt literal defined by 'n' suffix
let large = 9007199254740991n; 

let i = Math.floor(3.14); // Explicit truncation to integer
let n = Number("1.23");   // String to numeric conversion
let s = String(100);      // Numeric to string conversion
let p = parseInt("10px"); // Parse integer from string (result: 10)

// Access an environment variable via process.env
const path = process.env.PATH;
// Set/Update an environment variable for the current process
process.env.APP_ENV = "production";

// High-resolution performance measurement in nanoseconds
const start = process.hrtime.bigint();
const elapsed = process.hrtime.bigint() - start;

// Date and time formatting
const now = new Date();
console.log(now.toISOString()); // ISO 8601 string

let a = 10 / 3; // Standard division (3.333)
let b = Math.floor(10 / 3); // Floor division (3)
let c = 10 ** 3; // Exponentiation (1000)
let d = true && false; // Logical AND (false)

const re = /\$(\d+)/; // Regex pattern to match '$' followed by digits
const match = re.exec("Price: $100");
if (match) {
    console.log(match[1]); // Print first capture group: "100"
}

const { parseArgs } = require('util');

// Configuration for expected CLI flags
const options = {
    verbose: { type: 'boolean', short: 'v' },
    output: { type: 'string', short: 'o' }
};

// Parse process.argv
const { values, positionals } = parseArgs({ options, strict: false });
// console.log(values.verbose);

// Standard function with default parameter
function add(a, b = 0) { return a + b; }

// Concise arrow function
const multiply = (a, b) => a * b;

// Multi-return via Object destructuring
function getStatus() { return { code: 200, msg: "OK" }; }
const { code, msg } = getStatus();

function modify(o) {
    // Directly modifies the property of the passed object
    o.val += 1;
}

let myObj = { val: 10 };
modify(myObj); // myObj.val is now 11

const x = 10;
if (x > 5) {
    console.log("Greater");
} else if (x === 5) {
    console.log("Equal");
} else {
    console.log("Less");
}

const val = 2;
switch (val) {
    case 1:
        console.log("One");
        break;
    case 2:
    case 3:
        console.log("Two or Three");
        break;
    default:
        console.log("Other");
}

// Standard for loop (0 to 2)
for (let i = 0; i < 3; i++) {
    console.log(`for: ${i}`);
}

let x = 0;
while (x < 3) {
    console.log(`while: ${x}`);
    x++;
}

for (let x = 0; x < 10; x++) {
    if (x < 2) continue; // Skip iteration
    if (x === 5) break;    // Exit loop
    console.log(x);
}

// Iterating over Array values
for (const val of [10, 20]) {
    console.log(`val: ${val}`);
}

// Iterating over Object entries
const obj = { a: 1, b: 2 };
for (const [key, val] of Object.entries(obj)) {
    console.log(`${key}: ${val}`);
}

let s = " Hello ";
s = s.trim().toUpperCase(); // Result: "HELLO"
let sub = s.substring(0, 2); // Result: "HE"
let joined = ["a", "b"].join("-"); // Result: "a-b"

const path = require('path');
// Join path segments using OS-specific separators
const p = path.join("/", "usr", "bin");

const fs = require('fs');
// Create a readable stream with a 4KB buffer
const rs = fs.createReadStream("data.bin", { highWaterMark: 4096 });
rs.on('data', (chunk) => {
    // 'chunk' is a Buffer containing the data
});

const fs = require('fs');
// Synchronous read to Buffer (binary)
const buffer = fs.readFileSync("file.bin");
// Synchronous read to UTF-8 String
const text = fs.readFileSync("file.txt", "utf8");

const { Buffer } = require('buffer');
// Allocate a 12-byte buffer
const buf = Buffer.alloc(12);

// Pack a 32-bit Big-Endian integer
buf.writeInt32BE(42, 0);
// Pack a 64-bit Big-Endian double
buf.writeDoubleBE(3.14, 4);

let res = 0b1010 & 0b1100; // Bitwise AND
res = 0b1010 | 0b1100;      // Bitwise OR
res = 0b1010 ^ 0b1100;      // Bitwise XOR
res = ~0b1010;              // Bitwise NOT (complement)
res = 0b0001 << 2;          // Bitwise Shift Left (0b0100)

// Convert integer to hex string
let s = (255).toString(16); // "ff"
// Parse hex string to integer
let val = parseInt("ff", 16); // 255

let l = [1, 2]; l.push(3); // Array (Dynamic List)
let m = new Map(); m.set("k", 1); // Key-Value Map
let s = new Set([1, 1, 2]); // Unique values Set ({1, 2})

// Parse JSON string into JavaScript object
const data = JSON.parse('{"id": 42}');
// Serialize JavaScript object to JSON string
const s = JSON.stringify(data);

const net = require('net');

// Simple TCP Echo Server
const srv = net.createServer((c) => {
    c.on('data', (d) => c.write(d));
});
srv.listen(8080);

// TCP Client connection
const cli = net.createConnection({ port: 80 }, "example.com", () => {
    cli.write("GET / HTTP/1.0\r\n\r\n");
});

const dgram = require('dgram');
// Create a UDP IPv4 socket
const s = dgram.createSocket('udp4');
// Send datagram payload to address:port
s.send("ping", 8080, "127.0.0.1");

async function main() {
    // Pause execution until the promise resolves
    const result = await Promise.resolve("Done");
    console.log(result);
}
main();

const { execSync } = require('child_process');

// Execute sqlite3 CLI command synchronously
const out = execSync('sqlite3 :memory: "SELECT 42;"');
console.log(out.toString().trim());