Detecting an element for change in dimensions using JS can be done with the ResizeObserver interface. This is supported in all modern browsers.

Device dark mode can be checked in Javascript by using window.matchMedia() to check a match for the prefers-color-scheme: dark CSS media query.

Property value in a deeply nested Javascript object can be found using the Optional Chaining Operator, introduced in ES2020.

Failed HTTP responses can be handled in fetch() by checking whether the returned Promise was rejected due to a network error. Subsequently the Response.ok property needs to be checked for an invalid HTTP status code.

A responsive slider / carousel can be created with CSS flexbox using the order property. Items can be repeated infinitely in a circular loop by changing the order property each time.

A text file on a server can be read with Javascript by downloading the file with Fetch / XHR and reading the response as text.

Javascript fetch method can be used to upload files to server. fetch is Promise-based, so we can use async / await to write synchronous styled functions.

With newer CSS properties not available in all browsers, it is sometimes required to check whether a CSS property & its value are supported in the current browser. This can be detected in Javascript using the CSS.supports() method.

Random numbers that are cryptographically strong, can be generated in Javascript using the Web Crypto API.

The newly introduced formdata event can enable the Javscript FormData object to participate in form submissions. When trying to submit a form via AJAX, this is helpful to include custom elements in the request.

An image can be rotated with Javascript by dynamically changing its CSS transform property. The point around which rotation needs to happen can be specified with the transform-origin property.

The Javascript URL object can be used to get and set hash value of a given url. To detect a change in hash in the current url, the hashchange event can be listened to.

The binary data of a local file selected by the user can be retrieved using the readAsBinaryString method of a FileReader object.

Without cookies being enabled in the browser, some web applications may not work as expected. To prevent such cases navigator.cookieEnabled property can be used to do a prior check whether cookies are enabled or not.

Browsers allow user-sensitive and low-level web APIs to execute only when the page is in a secure context. In addition, the window.isSecureContext property can be used to check whether the current environment is safe or not.

The Array.fill() method can be used to fill an array with a specific value, starting from a given index and ending upto another index.

Flattening an array refers to the process taking an input array, which may itself contain n-dimensional arrays as its elements, to a single array. This can be done in Javascript using the flat() method.

The Intl.NumberFormat object can be used to format a number as a currency using Javascript. Various options can be set to customize format of currency code, comma / decimal separators etc.

The finally() callback can be used to know whether the Promise was settled. We don't care whether it was resolved or rejected.

The nullish coalescing operator has been newly introduced in Javascript. This can be used to give default values if a variable is either null or undefined.