JavaScript Cheat Sheet — Complete Reference (75+ snippets, 2026)

JavaScript Cheatsheet

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Showing 79 of 79 snippets

Modern JavaScript. Patterns need your application variables; DOM needs a browser and import/export a module. Complete examples include their own data.

Block-scoped constant

Variables · Pattern / syntax

const x = 10

Block-scoped variable

Variables · Pattern / syntax

let y = 20

Get type as string

Variables · Pattern / syntax

typeof x

Object destructuring

Variables · Pattern / syntax

const { a, b } = obj

Array destructuring

Variables · Pattern / syntax

const [a, b] = arr

Spread/merge objects

Variables · Pattern / syntax

const merged = { ...obj1, ...obj2 }

Spread/copy array

Variables · Pattern / syntax

const copy = [...arr]

Nullish coalescing (null/undefined only)

Variables · Pattern / syntax

x ?? fallback

Optional chaining

Variables · Pattern / syntax

obj?.prop?.nested

Create an array

Arrays · Pattern / syntax

[1, 2, 3]

Add item to end

Arrays · Pattern / syntax

arr.push(item)

Remove and return last item

Arrays · Pattern / syntax

arr.pop()

Remove first / add to start

Arrays · Pattern / syntax

arr.shift();
arr.unshift(item);

Two separate operations; both mutate arr.

Transform each element

Arrays · Pattern / syntax

arr.map(x => x * 2)

Reduce to single value

Arrays · Pattern / syntax

arr.reduce((acc, x) => acc + x, 0)

Find first matching element

Arrays · Pattern / syntax

arr.find(x => x.id === 1)

Find index of first match

Arrays · Pattern / syntax

arr.findIndex(x => x > 5)

Check if any element matches

Arrays · Pattern / syntax

arr.some(x => x > 0)

Check if all elements match

Arrays · Pattern / syntax

arr.every(x => x > 0)

Check if item exists

Arrays · Pattern / syntax

arr.includes(item)

Flatten nested arrays

Arrays · Pattern / syntax

arr.flat(Infinity)

Sort numerically ascending

Arrays · Complete example

const numbers = [10, 2, 1];
const sorted = [...numbers].sort((a, b) => a - b);
console.log(sorted); // [1, 2, 10]
console.log(numbers); // [10, 2, 1]

sort() mutates its receiver. Copy first to preserve the input; without a comparator, sorting uses strings.

Documentation

Get subarray (non-mutating)

Arrays · Pattern / syntax

arr.slice(1, 3)

Remove/insert at index (mutating)

Arrays · Pattern / syntax

arr.splice(1, 2, ...items)

Remove duplicates

Arrays · Pattern / syntax

[...new Set(arr)]

Create array from range

Arrays · Pattern / syntax

Array.from({ length: 5 }, (_, i) => i)

Create an object

Objects · Pattern / syntax

const obj = { key: "value" }

Get array of keys

Objects · Pattern / syntax

Object.keys(obj)

Get array of values

Objects · Pattern / syntax

Object.values(obj)

Get array of [key, value] pairs

Objects · Pattern / syntax

Object.entries(obj)

Merge objects (shallow)

Objects · Pattern / syntax

Object.assign({}, obj1, obj2)

Deep clone an object

Objects · Pattern / syntax

structuredClone(obj)

Supports structured-cloneable values, not functions or DOM nodes; unsupported values throw DataCloneError.

Documentation

Make object immutable (shallow)

Objects · Pattern / syntax

Object.freeze(obj)

Check if key exists

Objects · Pattern / syntax

"key" in obj

Delete a property

Objects · Pattern / syntax

delete obj.key

Omit a key via destructuring

Objects · Pattern / syntax

const { unwanted, ...rest } = obj

Function declaration

Functions · Pattern / syntax

function fn(x) { return x }

Arrow function

Functions · Pattern / syntax

const fn = (x) => x * 2

Default parameter

Functions · Pattern / syntax

const fn = (x = 10) => x

Rest parameters

Functions · Pattern / syntax

const fn = (...args) => args

Spread arguments

Functions · Pattern / syntax

fn(1, ...arr)

Delay execution by ms

Functions · Pattern / syntax

setTimeout(fn, 1000)

Repeat execution every ms

Functions · Pattern / syntax

setInterval(fn, 1000)

Generator function

Functions · Pattern / syntax

function* gen() { yield 1 }

Async/await function

Promises · Pattern / syntax

async function fetchJson(url) {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}

Pass a JSON endpoint and handle rejection at the call site. Browser requests must satisfy CORS.

Documentation

Fetch with .then() chain

Promises · Pattern / syntax

fetch(url)
  .then(r => {
    if (!r.ok) throw new Error(`HTTP ${r.status}`);
    return r.json();
  })
  .then(console.log)
  .catch(console.error);

Wait for all promises

Promises · Pattern / syntax

Promise.all([p1, p2])

First promise to resolve/reject

Promises · Pattern / syntax

Promise.race([p1, p2])

Wait for all (resolved or rejected)

Promises · Pattern / syntax

Promise.allSettled([p1, p2])

Create a new promise

Promises · Complete example

const promise = new Promise(resolve => {
  setTimeout(() => resolve(42), 100);
});
promise.then(console.log); // 42

Async error handling

Promises · Pattern / syntax

try {
  await fn();
} catch (e) {
  console.error(e);
}

Template literal

Strings · Pattern / syntax

`Hello ${name}`

Check if substring exists

Strings · Pattern / syntax

s.includes("sub")

Split into array

Strings · Pattern / syntax

s.split(",")

Remove whitespace from both ends

Strings · Pattern / syntax

s.trim()

Replace with regex (global)

Strings · Pattern / syntax

s.replace(/regex/g, "new")

Pad start to length

Strings · Pattern / syntax

s.padStart(5, "0")

Repeat string N times

Strings · Pattern / syntax

s.repeat(3)

Find all regex matches

Strings · Pattern / syntax

s.match(/pattern/g)

Get last character

Strings · Pattern / syntax

s.at(-1)

Select first matching element

DOM · Pattern / syntax

document.querySelector(".class")

Select all matching elements

DOM · Pattern / syntax

document.querySelectorAll("div")

Add event listener

DOM · Pattern / syntax

el.addEventListener("click", fn)

Add CSS class

DOM · Pattern / syntax

el.classList.add("active")

Toggle CSS class

DOM · Pattern / syntax

el.classList.toggle("hidden")

Set text content (safe)

DOM · Pattern / syntax

el.textContent = "text"

Set HTML attribute

DOM · Pattern / syntax

el.setAttribute("data-id", "5")

Set inline style

DOM · Pattern / syntax

el.style.color = "red"

Remove element from DOM

DOM · Pattern / syntax

el.remove()

Create a Map (any key type)

ES6+ · Pattern / syntax

const m = new Map()

Create a Set (unique values)

ES6+ · Pattern / syntax

const s = new Set([1, 2, 3])

Iterate over Map entries

ES6+ · Pattern / syntax

for (const [k, v] of map) {}

for...of loop

ES6+ · Pattern / syntax

for (const item of iterable) {}

Create unique symbol

ES6+ · Pattern / syntax

Symbol("desc")

ES module import

ES6+ · Pattern / syntax

import { fn } from "./module.js"

Default export

ES6+ · Pattern / syntax

export default fn

Create a Proxy

ES6+ · Pattern / syntax

const proxy = new Proxy(target, handler)

JavaScript Cheat Sheet — JS Reference & Syntax Guide

Find JavaScript syntax for arrays, objects, functions, promises, strings and the DOM. Search categories in English or Spanish, copy code or link directly to an entry. Complete examples include data; patterns need variables from your application.

Common pitfalls in JavaScript

Numeric sorting needs a comparator. sort() changes the array it is called on; the complete example copies the input first. fetch() can fulfill on an HTTP error, so check response.ok before reading JSON. DOM snippets run in a browser, while import and export require a module. Spread and Object.assign make shallow copies.

75+ practical JavaScript snippets
9 categories including DOM and ES6+
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By Marco B. · Last verified September 2026 — runs in your browser