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JavaScript Reference

0. Basics

0.1 Hello World

// console.log prints to stdout with a trailing newline
console.log("Hello, World!");

TERMINAL OUTPUT

Hello, World!

0.2 Building & Running

JavaScript is an interpreted language typically run in a browser or a runtime like Node.js.

# Execute a script using Node.js
node script.js

# For browsers, include via HTML
# <script type="module" src="script.js"></script>

TERMINAL OUTPUT

Hello, World!

0.3 Comments

// This is a single-line comment

/* This is a 
   multi-line comment */

TERMINAL OUTPUT

(No output)

0.4 Imports (import/export)

Modern JavaScript uses ECMAScript Modules (ESM).

// math.js
export const add = (a, b) => a + b;
export default add;

// main.js
import { add } from './math.js';
import addAlias from './math.js'; // Default import

TERMINAL OUTPUT

(No output)

0.5 Exit Codes

Exit codes are specific to the environment (e.g., Node.js).

// Terminate the process with a status code (0 for success, non-zero for error)
// process.exit(1);

TERMINAL OUTPUT

(Process exits with status 1)

0.6 Error Handling

try {
    throw new Error("Something went wrong");
} catch (error) {
    console.log(`Caught: ${error.message}`);
} finally {
    console.log("Cleanup complete");
}

TERMINAL OUTPUT

Caught: Something went wrong
Cleanup complete

1. Environment & Time

1.1 Variable Declaration

const constant = 10; // Block-scoped, immutable reference
let mutable = 5;      // Block-scoped, mutable variable
var legacy = 1;       // Function-scoped (avoid in modern JS)

TERMINAL OUTPUT

(No output)

1.2 Integer Types (Number/BigInt)

JavaScript Number is a 64-bit float. BigInt handles arbitrary-precision integers.

const num = 42;
const large = 9007199254740991n; // BigInt literal
const maxSafe = Number.MAX_SAFE_INTEGER;
console.log(maxSafe);

TERMINAL OUTPUT

9007199254740991

1.3 Casting

const s = "123";
const n = Number(s);     // 123
const b = Boolean(1);    // true
const str = String(42);  // "42"
const i = parseInt("10px", 10); // 10

TERMINAL OUTPUT

(No output)

1.4 Truthiness

Values that are “falsy”: false, 0, -0, 0n, "", null, undefined, and NaN. Everything else is “truthy”.

if ("") console.log("True"); else console.log("False");
if ([]) console.log("Empty array is truthy");

TERMINAL OUTPUT

False
Empty array is truthy

1.5 Environment Vars

Environment variables are accessed via process.env in Node.js.

// In Node.js environment
const apiKey = process.env.API_KEY || "default_key";
console.log(apiKey);

TERMINAL OUTPUT

default_key (or environment value)

1.6 Time

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

const timestamp = Date.now(); // Current time in milliseconds
console.log(typeof timestamp);

TERMINAL OUTPUT

2024-01-01T00:00:00.000Z (example)
number

2. Operators & Regex

2.1 Arithmetic/Logic

let x = 10 + 5 * 2; // 20
let pow = 2 ** 3;    // 8
let logic = true && (false || !false); // true
let nullish = null ?? "default"; // "default"

TERMINAL OUTPUT

(No output)

2.2 Regex

const regex = /ID: (\d+)/i;
const match = "User ID: 123".match(regex);
if (match) {
    console.log(match[1]); // 123
}

TERMINAL OUTPUT

3. Argument Parsing

3.1 CLI Argument Parsing

In Node.js, process.argv contains the command-line arguments.

// node script.js --user=admin
const args = process.argv.slice(2);
console.log(args);

TERMINAL OUTPUT

[ '--user=admin' ]

4. Functions & Memory

4.1 Declaration/Returns

function greet(name) {
    return `Hello, ${name}`;
}

const square = (x) => x * x; // Arrow function

console.log(greet("World"));
console.log(square(4));

TERMINAL OUTPUT

Hello, World!
16

4.2 References/Pointers

Objects and Arrays are passed by reference. Primitives are passed by value.

const obj = { val: 10 };
const modify = (o) => { o.val = 20; };
modify(obj);
console.log(obj.val);

TERMINAL OUTPUT

5. Loops & Iteration

5.1 Basic Loops

for (let i = 0; i < 3; i++) {
    console.log(i);
}

let j = 0;
while (j < 2) {
    console.log(j++);
}

TERMINAL OUTPUT

5.2 Break/Continue

for (let i = 0; i < 5; i++) {
    if (i === 1) continue;
    if (i === 3) break;
    console.log(i);
}

TERMINAL OUTPUT

0
2

5.3 Iterating Collections

const arr = ["a", "b"];
arr.forEach((val, index) => console.log(`${index}: ${val}`));

for (const val of arr) {
    console.log(val);
}

TERMINAL OUTPUT

0: a
1: b
a
b

6. Strings & Files

6.1 Manipulation

const s = "JavaScript";
console.log(s.slice(0, 4));    // "Java"
console.log(s.toUpperCase());  // "JAVASCRIPT"
console.log(`${s} is fun`);   // Template literal

TERMINAL OUTPUT

Java
JAVASCRIPT
JavaScript is fun

6.2 Path Joining (generic)

Using the URL API for cross-platform path handling or Node’s path.

// Generic approach using URL
const base = "https://example.com/api/";
const endpoint = "users";
const full = new URL(endpoint, base).href;
console.log(full);

TERMINAL OUTPUT

https://example.com/api/users

6.3 Streaming I/O (generic Streams API)

Modern JavaScript uses the Web Streams API.

const stream = new ReadableStream({
    start(controller) {
        controller.enqueue("Chunk 1");
        controller.close();
    }
});

TERMINAL OUTPUT

(No output)

6.4 Read to Memory

Using fetch (Web API) or fs (Node.js).

// Node.js example (CommonJS or ESM with prefix)
// import { readFile } from 'node:fs/promises';
// const content = await readFile('file.txt', 'utf8');

TERMINAL OUTPUT

(No output)

7. Binary & Bitwise

7.1 Allocation/Packing

const buffer = new ArrayBuffer(8); // 8 bytes
const view = new DataView(buffer);
view.setInt32(0, 42); // Set int32 at offset 0
const uint8 = new Uint8Array(buffer);
console.log(uint8[0]);

TERMINAL OUTPUT

0 (or 42 depending on endianness/offset)

7.2 Bitwise Ops

console.log(5 & 1);  // AND (1)
console.log(5 | 1);  // OR  (5)
console.log(1 << 2); // Shift Left (4)

TERMINAL OUTPUT

1
5
4

7.3 Hex Conversion

const hex = (255).toString(16); // "ff"
const num = parseInt("ff", 16); // 255
console.log(hex, num);

TERMINAL OUTPUT

ff 255

8. Collections & JSON

8.1 Lists/Maps/Sets

const list = [1, 2, 3];
const map = new Map([["key", "value"]]);
const set = new Set([1, 1, 2]); // {1, 2}

console.log(map.get("key"));
console.log(set.size);

TERMINAL OUTPUT

value
2

8.2 JSON Parsing

const json = '{"id": 1, "name": "JS"}';
const obj = JSON.parse(json);
const str = JSON.stringify(obj);
console.log(obj.name);

TERMINAL OUTPUT

JS

9. Systems & Networking

9.1 TCP

Requires a runtime like Node.js (net module).

// Node.js
// import net from 'node:net';
// const server = net.createServer((socket) => { ... }).listen(8080);

TERMINAL OUTPUT

(No output)

9.2 UDP

Requires a runtime like Node.js (dgram module).

// Node.js
// import dgram from 'node:dgram';
// const client = dgram.createSocket('udp4');

TERMINAL OUTPUT

(No output)

9.3 Concurrency (Promises, Async/Await)

const delay = (ms) => new Promise(res => setTimeout(res, ms));

async function run() {
    console.log("Start");
    await delay(10);
    console.log("End");
}
run();

TERMINAL OUTPUT

Start
End

9.4 SQLite (generic SQL interop)

Requires a library (e.g., sqlite3 or better-sqlite3).

// import Database from 'better-sqlite3';
// const db = new Database(':memory:');
// const row = db.prepare('SELECT "JS" as name').get();

TERMINAL OUTPUT

(No output)