var x = 10;              // Function-scoped
let y = 20;              // Block-scoped
const z = 30;            // Block-scoped, immutable

let num = 42;            // Number
let str = "Hello";       // String
let bool = true;         // Boolean
let nothing = null;      // Null
let undef = undefined;   // Undefined
let sym = Symbol();      // Symbol
let big = 10n;           // BigInt

let obj = { key: 'value' };
let arr = [1, 2, 3];
let func = function() {};

+ - * / % **
++ --

== != === !==
< > <= >=

&& || !

condition ? true : false

arr.push(item)           // Add to end
arr.pop()                // Remove from end
arr.unshift(item)        // Add to start
arr.shift()              // Remove from start
arr.splice(index, count) // Remove/add
arr.slice(start, end)    // Copy portion
arr.concat(arr2)         // Merge arrays
arr.join(',')            // Join to string
arr.reverse()            // Reverse
arr.sort()               // Sort
 
// Iteration
arr.forEach(item => {})
arr.map(item => item * 2)
arr.filter(item => item > 5)
arr.reduce((acc, item) => acc + item, 0)
arr.find(item => item > 5)
arr.findIndex(item => item > 5)
arr.some(item => item > 5)
arr.every(item => item > 0)

obj.key                  // Access
obj['key']               // Access
obj.key = 'value'        // Set
delete obj.key           // Delete
'key' in obj             // Check existence
Object.keys(obj)         // All keys
Object.values(obj)       // All values
Object.entries(obj)      // Key-value pairs
Object.assign({}, obj)   // Copy
{...obj}                 // Spread operator

// Function declaration
function func(a, b) {
    return a + b;
}
 
// Function expression
const func = function(a, b) {
    return a + b;
};
 
// Arrow function
const func = (a, b) => a + b;
const func = a => a * 2;
const func = () => console.log('hi');
 
// Default parameters
function func(a, b = 10) {
    return a + b;
}
 
// Rest parameters
function func(...args) {
    return args.reduce((a, b) => a + b);
}

if (condition) {
    // code
} else if (condition) {
    // code
} else {
    // code
}

switch (value) {
    case 1:
        break;
    case 2:
        break;
    default:
        break;
}

// For loop
for (let i = 0; i < 10; i++) {
    console.log(i);
}
 
// For...of (values)
for (const item of array) {
    console.log(item);
}
 
// For...in (keys)
for (const key in object) {
    console.log(key);
}
 
// While
while (condition) {
    // code
}
 
// Do-while
do {
    // code
} while (condition);

// Array
const [a, b] = [1, 2];
const [first, ...rest] = [1, 2, 3, 4];
 
// Object
const {name, age} = person;
const {name: n, age: a} = person;

// Spread
const arr = [...arr1, ...arr2];
const obj = {...obj1, ...obj2};
 
// Rest
function func(...args) {}

const name = "John";
const greeting = `Hello, ${name}!`;
const multi = `Line 1
Line 2`;

const promise = new Promise((resolve, reject) => {
    if (success) resolve(value);
    else reject(error);
});
 
promise
    .then(result => {})
    .catch(error => {})
    .finally(() => {});
 
// Async/await
async function func() {
    try {
        const result = await promise;
    } catch (error) {
        console.error(error);
    }
}

class MyClass {
    constructor(value) {
        this.value = value;
    }
    
    method() {
        return this.value;
    }
    
    static staticMethod() {
        return 'static';
    }
}
 
class Child extends MyClass {
    constructor(value) {
        super(value);
    }
}

// CommonJS
const module = require('module');
module.exports = {};
exports.func = () => {};
 
// ES Modules
import module from 'module';
import { func } from 'module';
export default {};
export const func = () => {};

const fs = require('fs');
 
// Read file
fs.readFile('file.txt', 'utf8', (err, data) => {});
const data = fs.readFileSync('file.txt', 'utf8');
 
// Write file
fs.writeFile('file.txt', 'data', (err) => {});
fs.writeFileSync('file.txt', 'data');
 
// Promises
const fs = require('fs').promises;
await fs.readFile('file.txt', 'utf8');

const path = require('path');
 
path.join('/a', 'b', 'c');
path.resolve('file.txt');
path.dirname('/a/b/c.txt');
path.basename('/a/b/c.txt');
path.extname('file.txt');

const http = require('http');
 
const server = http.createServer((req, res) => {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
});
 
server.listen(3000);

const express = require('express');
const app = express();
 
app.get('/', (req, res) => {
    res.send('Hello World');
});
 
app.post('/api', (req, res) => {
    res.json({ message: 'Success' });
});
 
app.listen(3000);

npm init                 # Initialize project
npm install package      # Install package
npm install -g package   # Install globally
npm install --save-dev package  # Dev dependency
npm uninstall package    # Remove package
npm update               # Update packages
npm outdated             # Check outdated
npm list                 # List installed

{
  "scripts": {
    "start": "node index.js",
    "dev": "nodemon index.js",
    "test": "jest"
  }
}

npm run script-name
npm start
npm test