JavaScript Cheat Sheet

Essential JavaScript syntax for everyday development. Covers modern ES6+ features you use daily.

Variables

const x = 10;       // Cannot reassign
let y = 20;         // Can reassign
var z = 30;         // Old style, avoid

Data Types

const str = "hello";
const num = 42;
const bool = true;
const arr = [1, 2, 3];
const obj = { name: "Alice", age: 30 };
const nil = null;
const undef = undefined;

Template Literals

const name = "Alice";
const msg = `Hello, ${name}! You are ${30 + 5} years old.`;

Destructuring

// Array
const [a, b, ...rest] = [1, 2, 3, 4, 5];

// Object
const { name, age, city = "NYC" } = person;

Arrow Functions

const add = (a, b) => a + b;
const square = n => n * n;
const greet = name => {
  return `Hello, ${name}`;
};

Array Methods

const nums = [1, 2, 3, 4, 5];

nums.map(n => n * 2);          // [2,4,6,8,10]
nums.filter(n => n > 3);       // [4,5]
nums.reduce((sum, n) => sum + n, 0); // 15
nums.find(n => n > 3);         // 4
nums.some(n => n > 4);         // true
nums.every(n => n > 0);        // true
nums.includes(3);              // true
nums.sort((a, b) => a - b);   // sorted
nums.flat();                    // flatten nested

Objects

const person = { name: "Alice", age: 30 };
Object.keys(person);    // ["name", "age"]
Object.values(person);  // ["Alice", 30]
Object.entries(person); // [["name","Alice"],["age",30]]
const copy = { ...person, city: "NYC" };

Promises and Async/Await

// Promise
fetch("/api/data")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

// Async/Await
async function getData() {
  try {
    const res = await fetch("/api/data");
    const data = await res.json();
    return data;
  } catch (err) {
    console.error(err);
  }
}

Classes

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

class Dog extends Animal {
  speak() {
    return `${this.name} barks`;
  }
}

Modules

// Export
export const PI = 3.14;
export default function calc() {}

// Import
import calc, { PI } from './math.js';

Nullish Coalescing and Optional Chaining

const value = null ?? "default";  // "default"
const city = user?.address?.city;  // undefined if null