Dasar

Variabel

let name = "Alice";    // reassignable
const PI = 3.14;       // constant
var old = "avoid";     // function-scoped (legacy)

Tipe Data

string	Teks: `"hello"` atau `'hello'`
number	Integer atau float: `42`, `3.14`
boolean	`true` / `false`
null	Nilai kosong yang disengaja
undefined	Dideklarasikan tapi belum diisi
object	Pasangan key-value: `{ a: 1 }`
array	List terurut: `[1, 2, 3]`
symbol	Identifier unik

Pengecekan & Konversi Tipe

typeof "hello"     // "string"
typeof 42          // "number"
Number("42")       // 42
String(100)        // "100"
parseInt("3.9")    // 3
parseFloat("3.14") // 3.14

String

Template Literal

const name = "Alice";
`Hello, ${name}!`       // Hello, Alice!
`Total: ${2 + 3}`       // Total: 5
`Multi
 line string`

Method String

s.length	Jumlah karakter
s.toUpperCase()	Salinan HURUF BESAR
s.toLowerCase()	Salinan huruf kecil
s.trim()	Hapus spasi di awal/akhir
s.split(",")	Pecah menjadi array
s.includes("x")	Cek keberadaan → bool
s.indexOf("x")	Index pertama (-1 jika tidak ada)
s.slice(1, 4)	Substring berdasarkan index
s.replace(a, b)	Ganti kemunculan pertama
s.replaceAll(a, b)	Ganti semua kemunculan
s.startsWith(x)	Cek awalan → bool
s.endsWith(x)	Cek akhiran → bool
s.padStart(n, c)	Padding di awal hingga panjang n

Array

Membuat & Mengakses

const fruits = ["apple", "banana", "cherry"];
fruits[0]          // "apple"
fruits.length      // 3
fruits.at(-1)      // "cherry"

Method Mutasi

arr.push(x)	Tambah di akhir
arr.pop()	Hapus & kembalikan item terakhir
arr.unshift(x)	Tambah di awal
arr.shift()	Hapus & kembalikan item pertama
arr.splice(i, n)	Hapus n item di index i
arr.sort()	Urutkan di tempat (leksikografis)
arr.reverse()	Balik urutan di tempat

Method Non-Mutasi

arr.map(fn)	Transformasi setiap elemen
arr.filter(fn)	Pertahankan elemen jika fn bernilai true
arr.reduce(fn, init)	Akumulasi menjadi satu nilai
arr.find(fn)	Kemunculan pertama atau undefined
arr.findIndex(fn)	Index kemunculan pertama (-1)
arr.includes(x)	Cek keberadaan → bool
arr.slice(a, b)	Salinan sebagian (shallow)
arr.join(",")	Gabung menjadi string
arr.forEach(fn)	Iterasi (tanpa return value)
[...a, ...b]	Gabung array (spread)

Object

Membuat & Mengakses

const user = { name: "Alice", age: 20 };
user.name          // "Alice"
user["age"]        // 20
user.gpa = 3.85;   // tambah/update

Destructuring & Spread

const { name, age } = user;
const copy = { ...user, age: 21 };

Method Object

Object.keys(o)	Array key
Object.values(o)	Array value
Object.entries(o)	Array pasangan [key, value]
Object.assign(t, s)	Salin properti s → t
"k" in obj	Key ada? → bool
delete obj.k	Hapus properti
Object.freeze(o)	Buat immutable (shallow)

Alur Kontrol

if / else if / else

if (score >= 90) {
  grade = "A";
} else if (score >= 80) {
  grade = "B";
} else {
  grade = "C";
}

Ternary & Nullish Coalescing

const status = score >= 60 ? "pass" : "fail";
const name = user.name ?? "Anonymous";

switch

switch (color) {
  case "red":   stop();   break;
  case "green": go();     break;
  default:      wait();
}

Loop

for / for...of / for...in

for (let i = 0; i < 5; i++) { }

for (const item of ["a", "b"]) { }  // arrays

for (const key in obj) { }  // object keys

while / do...while

while (count < 10) { count++; }

do { count++; } while (count < 10);

break & continue

for (let i = 0; i < 10; i++) {
  if (i === 5) break;       // stop loop
  if (i % 2 === 0) continue; // skip
}

Fungsi

Deklarasi Fungsi & Arrow

function greet(name) {
  return `Hello, ${name}!`;
}
const greet = (name) => `Hello, ${name}!`;
const square = x => x * x; // single param

Parameter Default & Rest

function greet(name = "World") { }

function sum(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}

Callback

[1, 2, 3].map(x => x * 2);    // [2, 4, 6]
[1, 2, 3].filter(x => x > 1); // [2, 3]
setTimeout(() => console.log("done"), 1000);

Class

class Dog {
  constructor(name, breed) {
    this.name = name;
    this.breed = breed;
  }
  bark() { return `${this.name} says Woof!`; }
}

class Puppy extends Dog {
  constructor(name, breed, toy) {
    super(name, breed);
    this.toy = toy;
  }
}

Penanganan Error

try {
  JSON.parse("bad json");
} catch (err) {
  console.error(err.message);
} finally {
  console.log("always runs");
}

Melempar Error

throw new Error("Something went wrong");

DOM

Memilih Elemen

document.querySelector(".cls")    // first match
document.querySelectorAll("li")   // all matches
document.getElementById("main")

Memodifikasi Elemen

el.textContent = "new text";
el.innerHTML = "bold";
el.style.color = "red";
el.classList.add("active");
el.classList.toggle("hidden");
el.setAttribute("data-id", "42");

Event

btn.addEventListener("click", (e) => {
  console.log(e.target);
});

Membuat Elemen

const li = document.createElement("li");
li.textContent = "New item";
ul.appendChild(li);
el.remove(); // remove element

Fetch API

GET Request

fetch("https://api.example.com/data")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

POST Request

fetch(url, {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ key: "value" }),
});

Async / Await

async function loadData() {
  try {
    const res = await fetch(url);
    const data = await res.json();
    return data;
  } catch (err) {
    console.error(err);
  }
}

Request Paralel

const [users, posts] = await Promise.all([
  fetch("/users").then(r => r.json()),
  fetch("/posts").then(r => r.json()),
]);

Modul

Named Export

// math.js
export const PI = 3.14;
export function add(a, b) { return a + b; }

// main.js
import { PI, add } from "./math.js";

Default Export

// logger.js
export default function log(msg) { }

// main.js
import log from "./logger.js";

REFERENSI CEPAT JAVASCRIPT