The logical nullish assignment operator (??=) works like the assignment operator (=) but assigns only when the left-hand side is a nullish value.

The logical AND assignment operator represented by &&= works like the assignment operator (=), but assigns only when the left-hand side is a truthy value.

The logical OR assignment operator (||=) works like the assignment operator (=), but assigns only when the left-hand side is a falsy value.

Javascript provides multiple methods that can check whether a sub-string is contained in a string, depending on various possible use-cases.

The new.target property can be used to detect whether a function was invoked as a constructor using the new operator, or just called normally.

Currently it is not possible to get file upload progress for fetch. The alternate solution for now is to use XMLHttpRequest.

Checking whether an element is visible or hidden in the full page, even through it may not be in screen view, can be done through an Intersection Observer. The trick is to set the root element to document.documentElement.

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.