JavaScript Cheat Sheet

A compact JavaScript reference for everyday syntax, APIs, and gotchas.

Last updated: May 28, 2026

Variables & Types

Syntax	Use	Example
const	Block-scoped value that should not be reassigned	const id = 42;
let	Block-scoped variable that can be reassigned	let count = 0;
typeof	Check primitive type at runtime	typeof value === "string"
Array.isArray()	Detect arrays reliably	Array.isArray(items)
?.	Optional chaining for nullable paths	user.profile?.email
??	Nullish fallback for null or undefined	label ?? "Untitled"

Functions

Pattern	Use	Example
function	Named function declaration	function add(a, b) { return a + b; }
=>	Arrow function expression	const add = (a, b) => a + b;
default param	Fallback argument value	function greet(name = "there") {}
rest param	Collect remaining arguments	function sum(...nums) {}
async/await	Work with promises using linear syntax	const data = await fetch(url).then(r => r.json());

Arrays

Method	Use	Example
map	Transform each item	users.map(user => user.name)
filter	Keep matching items	users.filter(user => user.active)
find	Return first matching item	users.find(user => user.id === id)
some	Check if any item matches	items.some(Boolean)
every	Check if all items match	items.every(item => item.valid)
reduce	Accumulate a value	items.reduce((sum, n) => sum + n, 0)
flatMap	Map and flatten one level	rows.flatMap(row => row.cells)
toSorted	Sort without mutating original	users.toSorted((a, b) => a.name.localeCompare(b.name))

Objects

Pattern	Use	Example
destructure	Extract properties into variables	const { id, name } = user;
spread	Copy or merge object properties	const next = { ...user, active: true };
Object.keys	Get property names	Object.keys(config)
Object.values	Get property values	Object.values(scores)
Object.entries	Iterate key/value pairs	Object.entries(env).map(([key, value]) => ...)
Object.fromEntries	Build object from pairs	Object.fromEntries(params)
structuredClone	Deep clone supported values	const copy = structuredClone(state);

Promises & Errors

API	Use	Example
Promise.all	Run promises in parallel and fail fast	await Promise.all([loadUser(), loadPosts()])
Promise.allSettled	Collect successes and failures	await Promise.allSettled(tasks)
try/catch	Handle thrown errors	try { await save(); } catch (error) {}
throw new Error	Create a standard error	throw new Error("Missing id");
finally	Run cleanup after success or failure	try {} finally { setLoading(false); }

DOM & Browser

API	Use	Example
querySelector	Find first matching element	document.querySelector("[data-menu]")
addEventListener	Attach event handler	button.addEventListener("click", onClick)
classList	Add, remove, toggle classes	el.classList.toggle("open")
dataset	Read data-* attributes	el.dataset.userId
localStorage	Persist small string data	localStorage.setItem("theme", "dark")
URLSearchParams	Parse or build query strings	new URLSearchParams(location.search).get("q")

Quick Gotchas

Gotcha	Why it matters	Safer pattern
== vs ===	Loose equality coerces types	Use === and !==
NaN	NaN is not equal to itself	Number.isNaN(value)
parseInt	Always pass radix for clarity	parseInt(value, 10)
Date parsing	Ambiguous date strings vary by engine	Use ISO strings or explicit constructors
Floating point	Decimal math can be imprecise	Use integer cents for money

Client-Side Processing

Input Data Stays on Device

Instant Local Execution

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What is JavaScript Cheat Sheet?

This JavaScript cheat sheet summarizes practical language features and browser APIs that developers look up often: variables, functions, arrays, objects, promises, DOM operations, and runtime pitfalls. It is designed for quick lookup while writing or reviewing JavaScript.

How to Use JavaScript Cheat Sheet

Scan the sections or use browser search to find a method, syntax pattern, or gotcha. Each row includes a short explanation and a copyable example-style snippet.

Common Use Cases

Looking up array method syntax.
Checking promise utility differences.
Remembering object helper APIs.
Reviewing common JavaScript pitfalls.

Example Input and Output

Common array transformation patterns.

Need

Filter active users and get names

Snippet

users.filter(user => user.active).map(user => user.name)

Tip

Use browser search on this page to jump directly to a method such as map, reduce, Promise.all, or querySelector.

Frequently Asked Questions

Is this a complete JavaScript reference?

No. It focuses on high-frequency syntax and APIs that are useful during everyday development.

Does this include browser APIs?

Yes. It includes a small DOM and browser API section for common front-end tasks.

Is this tool interactive?

It is a reference page. Use the linked beautifier, minifier, validator, and highlighter tools for interactive workflows.

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