Language cheatsheet

JavaScript 2025 Cheatsheet

Modern JavaScript for arrays, strings, objects, async code, the DOM and modules, in copy-ready snippets.

Arrays

Transform every item — Create a new array by applying a function to each item.
Keep matching items — Create a new array with the items that pass a test.
Combine items into one value — Add up a property across an array. The second argument is the starting value.
Find the first match — Return the first item that passes a test, or undefined if none does.
Test items against a condition — Check whether any item, or every item, passes a test.
Check if an array contains a value — Return true if the value is in the array.
Map and flatten in one step — Collect the tags of every post into a single array.
Sort numbers without changing the original — Sort ascending. sort() compares as text by default and changes the array, so toSorted with a comparator is safer.
Copy an array — Make a shallow copy with spread.
Get the last item — Use a negative index with at().
Create a range of numbers — Build an array of the numbers 0 to 4.
Remove duplicates — Convert to a Set and back to an array.
Group items by a property — Return an object whose keys are the group names. Available in current browsers and Node.js 21 and later.

Insert values into a string — Use backticks and ${} to build strings, including multi-line ones.
Split and join — Turn a string into an array, and back again.
Replace every occurrence — Replace all matches, not just the first.
Pad a string — Pad on the left, for example to make 7 into 07.
Check the start, end or contents — Test a string without a regular expression.
Capitalize a word — Uppercase the first letter and keep the rest.
Sort text correctly — Sort strings using the language's rules, including accents and capitals.
Format numbers and currency — Format with the user's locale conventions.
Format a date — Format a date for a locale.

Destructure an object — Pull properties into variables, with a default for a missing one.
Swap two variables — Use array destructuring to swap without a temporary variable.
Merge objects — Combine objects. Later properties win.
Loop over an object — Iterate over keys and values.
Build an object from pairs — Turn an array of [key, value] pairs into an object.
Read nested properties safely — Return undefined instead of throwing when something in the chain is null or undefined.
Use a default only for null or undefined — Unlike ||, it keeps 0 and empty strings.
Deep copy an object — Copy nested data, including dates and maps.
Convert to and from JSON — Pretty-print an object as JSON, and parse JSON text back.

Fetch JSON — Request JSON and check for HTTP errors. fetch only rejects on network failures.
POST JSON — Send a JSON body with the right header.
Time out a request — Abort a request that takes longer than five seconds.
Handle errors in async code — Catch rejected promises and thrown errors with try and catch.
Run promises in parallel — Wait for all promises. It rejects as soon as one fails.
Wait for all promises, whatever happens — Get the outcome of every promise, including failures.
Pause for a while — A promise that resolves after a delay.
Debounce a function — Run a function only after calls stop for a moment, for example on search input.

Find elements — Get the first element that matches a CSS selector, or all matches.
Listen for events — Run a function when the user clicks.
Handle events for many elements — Listen once on a parent and check which child was clicked.
Add, remove or toggle a class — Change CSS classes on an element.
Create and add an element — Build an element and append it to the page.
Set text safely — textContent treats the value as plain text. innerHTML parses HTML.
Read data attributes — Read data-user-id from an element.
Read a submitted form — Stop the page reload and read the form fields as an object.
Store data in the browser — Save and load JSON in localStorage. It only stores strings.
Run code when the page is ready — Wait for the HTML to be parsed. Scripts with defer or type=module don't need this.

Export and import by name — Share functions between files with ES modules.
Default export — Export one main value per file.
Load a module on demand — Import code only when it is needed.
Load a module in HTML — Modules are deferred and run in strict mode automatically.
Import a Node.js built-in — Use the node: prefix for built-in modules.

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CSS 3 cheatsheet — Layout with flexbox and grid, responsive design, custom properties, animations and modern selectors.
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