JavaScript Cheat Sheet

// Output ke console
console.log('Hello World');
 
// Dengan multiple values
console.log('Nilai:', 42);

// let - bisa di-reassign
let name = 'John';
name = 'Jane'; // OK
 
// const - gabisa di-reassign
const PI = 3.14159;
// PI = 3.14; // Error!
 
// var - old way (hindari kalo bisa)
var oldSchool = 'avoid this';

// Integer (angka bulat)
let age = 25;
 
// Float (angka desimal)
let price = 99.99;
 
// Operasi matematika
let sum = 10 + 5;    // 15
let

// Bikin string
let single = 'Hello';
let double = "World";
let template = `Hello ${name}`;
 
// Method-method string
let text =

const name = 'Budi';
const age = 25;
 
// String multi-line
const message = `
 Halo, nama gw ${name}.
 Umur gw ${age} tahun.
 Tahun depan gw ${age + 1} tahun.

if (age >= 18) {
 console.log('Udah dewasa');
}
 
if (score >= 90) {
 console.log('Mantap!');
} else if (score >= 70) {

// kondisi ? true : false
const status = age >= 18 ? 'Adult' : 'Minor';
 
// Nested ternary (hati-hati, bisa bikin pusing)
const grade = score >= 90 ? 'A' : score >= 80 ? 'B' :

// Persamaan
5 == '5'  // true (cuma nilai)
5 === '5' // false (nilai + tipe)
5 != '5'  // false
5 !== '5' // true
 
// Perbandingan
5 > 3   // true
5 < 3   // false

// AND (&&) - semua harus true
const canDrive = age >= 17 && hasLicense;
 
// OR (||) - salah satu true
const isWeekend = day === 'Saturday' || day === 'Sunday';
 
// NOT (!) - balik nilai boolean
const isNotLoggedIn

switch (day) {
 case 'Monday':
  console.log('Mulai kerja');
  break;
 case 'Friday':
  console.log('Weekend soon!');
  break;
 case 'Saturday':
 case

function greet(name) {
 return `Hello, ${name}!`;
}
 
greet('Budi'); // "Hello, Budi!"

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

// Arrow function dasar
const greet = (name) => {
 return `Hello, ${name}!`;
};
 
// Versi pendek (return otomatis)
const greet = (name) => `Hello, ${name

function greet(name = 'Guest') {
 return `Hello, ${name}!`;
}
 
greet();    // "Hello, Guest!"
greet('Budi'); // "Hello, Budi!"

function sum(...numbers) {
 return numbers.reduce((total, num) => total + num, 0);
}
 
sum(1, 2, 3, 4

const fruits = ['Apple', 'Banana', 'Orange'];
const numbers = [1, 2, 3, 4, 5];
const mixed = [1, 'two'

const arr = [1, 2, 3, 4, 5];
 
// Tambah/Hapus
arr.push(6);    // Tambah di akhir [1,2,3,4,5,6]

const numbers = [1, 2, 3, 4, 5];
 
numbers.forEach((num) => {
 console.log(num * 2);
});
// Output: 2, 4, 6, 8, 10

// Transform array
const doubled = numbers.map(num => num * 2);
// [2, 4, 6, 8, 10]
 
const names = ['john', 'jane'];
const capitalized = names.map

// Filter array
const evens = numbers.filter(num => num % 2 === 0);
// [2, 4]
 
const adults = users.filter(user => user.age >= 18);

// Jumlahkan semua angka
const sum = numbers.reduce((total, num) => total + num, 0);
// 15
 
// Hitung jumlah kemunculan
const fruits = ['apple', 

const users = [
 { id: 1, name: 'John' },
 { id: 2, name: 'Jane' }
];
 
const user = users.find(u => u.id === 2);
// { id: 2, name: 'Jane' }

const numbers = [1, 2, 3, 4, 5];
 
// some - ada yang true?
numbers.some(num => num > 4); // true
 
// every - semua true?
numbers.every

// For loop klasik
for (let i = 0; i < 5; i++) {
 console.log(i); // 0, 1, 2, 3, 4
}
 
// Loop terbalik
for (let i = 5; i > 0

let i = 0;
while (i < 5) {
 console.log(i);
 i++;
}

let i = 0;
do {
 console.log(i);
 i++;
} while (i < 5);

// Loop lewat array values
const fruits = ['Apple', 'Banana', 'Orange'];
for (const fruit of fruits) {
 console.log(fruit);
}

// Loop lewat object keys
const person = { name: 'John', age: 30 };
for (const key in person) {
 console.log(`${key}: ${person[key]}`);
}

// Break - keluar dari loop
for (let i = 0; i < 10; i++) {
 if (i === 5) break;
 console.log(i); // 0, 1, 2, 3, 4
}
 
// Continue - skip iterasi

// Object literal
const person = {
 name: 'John',
 age: 30,
 city: 'Jakarta'
};
 
// New Object
const obj = new Object();
obj.name = 'John';

// Dot notation
person.name; // 'John'
 
// Bracket notation
person['age']; // 30
 
// Property dinamis
const prop = 'city';
person[prop]; // 'Jakarta'

const person = {
 name: 'John',
 age: 30,
 greet: function() {
  return `Hello, I'm ${this.name}`;
 },
 // Shorthand method
 sayHi() {
  return

const person = { name: 'John', age: 30, city: 'Jakarta' };
 
// Destructure
const { name, age } = person;
console.log(name); // 'John'
 
// Ganti nama variabel
const

const person = { name: 'John', age: 30 };
 
// Copy object
const copy = { ...person };
 
// Gabung object
const address = { city: 'Jakarta', country: 'Indonesia' };
const

const person = { name: 'John', age: 30 };
 
// Ambil key
Object.keys(person);  // ['name', 'age']
 
// Ambil value
Object.values(person); // ['John', 30]
 
// Ambil entries
Object.entries(person); // [['name', 'John'], ['age', 30]]

class Person {
 constructor(name, age) {
  this.name = name;
  this.age = age;
 }
 
 greet() {
  return `Hello, I'm ${this.name}`;

class Animal {
 constructor(name) {
  this.name = name;
 }
 
 speak() {
  return `${this.name} makes a sound`;
 }
}
 

class MathHelper {
 static add(a, b) {
  return a + b;
 }
 
 static PI = 3.14159;
}
 
MathHelper.add(5, 3); 

class Circle {
 constructor(radius) {
  this._radius = radius;
 }
 
 get radius() {
  return this._radius;
 }
 
 set radius(value) {

const promise = new Promise((resolve, reject) => {
 setTimeout(() => {
  const success = true;
  if (success) {
   resolve('Success!');
  } 

promise
 .then(result => {
  console.log(result); // 'Success!'
 })
 .catch(error => {
  console.error(error);
 })
 .finally(() => {

const promise1 = Promise.resolve(3);
const promise2 = Promise.resolve(42);
const promise3 = Promise.resolve('foo');
 
Promise

async function fetchData() {
 try {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return data;
 } catch (error) {

// Array destructuring
const [first, second, ...rest] = [1, 2, 3, 4, 5];
// first: 1, second: 2, rest: [3, 4, 5]
 
// Object destructuring
const { name, 

const user = {
 name: 'John',
 address: {
  city: 'Jakarta'
 }
};
 
// Without optional chaining
const city = user.address ? user.address.city : undefined;
 
// With optional chaining

// Old way
const name = username || 'Guest';
 
// Problem: '' and 0 are also replaced
const count = userCount || 10; // if userCount is 0, still returns 10
 
// New way with ??
const count = userCount ?? 10; 

// Export
export const name = 'John';
export function greet() { }
 
// Default export
export default class Person { }
 
// Import
import Person, { name, greet } from './module.js';
 
// Import all