August 22, 2025

JavaScript Cheat Sheet

JavaScript Cheat Sheet — your quick coding sidekick

1 Basics

Variables: let, const (prefer), (avoid var)
Types: number, string, boolean, null, undefined, object, bigint, symbol
Template literals: `Hello ${name}`
Strict equality: === / !==
Modules: export, import

const name = "Ada";
const age = 37;
console.log(`Hi ${name}, ${age} next year: ${age + 1}`);

2 Operators

Arithmetic: + - * / % **
Assignment: += -= *= /= %= **=
Comparison: === !== < <= > >=
Logical: &&, ||, !, ?? (nullish)
Optional chaining: user?.profile?.email
Type: typeof x, Array.isArray(x)

3 Strings

Length: str.length
Case: toUpperCase(), toLowerCase()
Search: includes(), indexOf(), startsWith(), endsWith()
Cut/replace: slice(), substring(), replace(), trim()
Split/join: split(",") ⇄ Array (join is on arrays)

"  hello  ".trim().toUpperCase(); // "HELLO"

4 Arrays

Stack/queue: push, pop, shift, unshift
Search: indexOf, includes, find
Slice/mutate: slice (copy), splice (mutate)
Iterate: forEach, map, filter, reduce
Test/order: some, every, sort, reverse

const odds = [1,2,3,4,5].filter(n => n % 2);
const sum  = odds.reduce((a,b) => a + b, 0);

5 Functions

Declarations: function greet(){}
Expressions: const f = function(){}
Arrow: const f = (x) => x * 2
Defaults: function f(x = 1){}
Rest/spread: function f(...xs){}, f(...arr)
Closures & higher-order functions

const once = fn => { let ran=false, val; return (...a)=> ran ? val : (ran=true, val=fn(...a)); };

6 Objects

Create: { a:1 }, Object.create(proto)
Read/write: obj.key, obj["key"], delete obj.key
Utilities: Object.keys, values, entries, hasOwn
Merge/clone: {...a, ...b}, Object.assign({}, a, b)
Destructuring: const {x, y:yy=0} = pt

const user = {id:1, name:"Ada"};
const {name} = user; // "Ada"

7 Conditionals & Loops

if / else, switch, ternary cond ? A : B
Loops: for, for...of (iterables), for...in (object keys)
while, do...while, break/continue
Error handling: try/catch/finally

8 DOM Manipulation

Select: getElementById(), querySelector(), querySelectorAll()
Create/insert: document.createElement(), append()/appendChild()
Events: el.addEventListener('click', fn)
Classes: el.classList.add/remove/toggle
Content: textContent, innerHTML

const btn = document.querySelector('#buy');
btn?.addEventListener('click', () => alert('Thanks!'));

9 ES6+ Features

let/const, arrow functions, template literals
Destructuring & rest/spread ...
Modules import/export
Iterables & generators: function* g(){}
Optional chaining ?., nullish coalescing ??

10 Promises & Async

Create: new Promise((res, rej) => {...})
Consume: .then(...).catch(...).finally(...)
async/await for linear style
Combinators: Promise.all, race, allSettled, any
Fetch: await fetch(url).then(r => r.json())

async function getUser(id){
  const r = await fetch(`/api/users/${id}`);
  if(!r.ok) throw new Error('HTTP ' + r.status);
  return r.json();
}

11 Common Built‑ins

Math: Math.max, min, floor, ceil, round, random
Date: Date.now(), new Date(), getFullYear()
JSON: JSON.stringify() / JSON.parse()
Number: parseInt, parseFloat, Number.isNaN
Timers: setTimeout, setInterval, clear*
URL utils: URL, URLSearchParams

const q = new URLSearchParams(location.search);
const page = Number(q.get('page') ?? 1);

12 Patterns & Tips

Immutability: prefer map/filter/slice over mutation
Guard clause: return early to reduce nesting
Pure functions & small modules
Use try/catch at boundaries (I/O, network)
Lint/format: ESLint + Prettier

function price(cents){
  if(cents == null || cents < 0) throw new Error('bad');
  return `$${(cents/100).toFixed(2)}`;
}

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