@javascript Cucumber scenarios run with Selenium and can slow down test runs; cucumber --tags ~@javascript skips them when needed.

Using self as a JavaScript variable can collide with the global window alias and cause confusing bugs when capturing this in callbacks.

Automated tests need a way to enter text into JavaScript prompt dialogs in Capybara/Selenium. Browser dialog handling can block workflows unless the prompt input is filled correctly.

Client-side language detection can route visitors to their preferred locale on the root path without a server-side component, with JavaScript and a meta refresh fallback.

Blank and missing values in JavaScript need explicit handling; Unpoly provides up.util.isBlank(), up.util.isPresent(), up.util.isMissing() and up.util.isGiven() for common and custom types.

Always-on UI behaviors like polling, animations, autocomplete, and lazy loading can make E2E tests flaky; feature flags disable them by default and re-enable them only where needed.

Failed @javascript Cucumber steps can close the Selenium browser before the page state is inspected. Keeping the window open on failure helps debug the created data.

JavaScript parse and execution can delay app interactivity long after download, making heavy startup code a common cause of slow page load.

Warns users before leaving a page with unsaved form data or dirty CKEditor content, helping prevent accidental loss during navigation or submit handling.

Selenium tests can fail when pending AJAX calls are aborted as Capybara closes the browser tab. Blocking new requests and waiting for in-flight work reduces flaky teardown errors.

Bootstrap 4 switches from Less to Sass, adds flexbox-based layout options and cards, and modernizes customization, JavaScript, and browser support.

Pseudo-element styles are hard to inspect directly, but getComputedStyle can read ::before and ::after values for testing and color checks.

Cross-origin web pages cannot normally communicate, but <script> requests can pull executable JavaScript around same-origin limits. JSONP uses this loophole to load dynamic data from other origins.

CoffeeScript in checks array membership, not object properties; JavaScript in tests whether an object has a property. Use of for property lookup in CoffeeScript.

Nested drag-and-drop for reorderable trees is hard when items can be leaves or containers; precise drop targets need custom JavaScript, CSS, and testing support.

Automatically wraps Bower packages as gems for the Rails asset pipeline, but duplicate JavaScript copies can conflict when mixed with other package sources.

Jasmine spyOnProperty() fails on plain value fields; a helper can convert them into accessors so they can be stubbed or asserted in tests.

ES5-era JavaScript best practices have stale alternatives in modern code, and older patterns can become outdated as the language evolves quickly.

event.target points to the clicked descendant, while event.currentTarget points to the element handling the listener; currentTarget is usually the safer choice.

Promises provide a chainable way to handle asynchronous work and separate control flow from callbacks, with built-in support in modern browsers and polyfills for older ones.

iPhone and iPad browsers do not allow JavaScript to open the on-screen keyboard programmatically. The keyboard appears only after a user gesture in an editable field.

Unicode symbols can be hard to enter reliably across tools, and astral characters or non-breaking spaces often need escapes or entities instead of pasted text.

jQuery .animate() can feel choppy because it relies on JavaScript timers, while CSS transitions deliver smoother motion with nearly the same API.

Build RegExp objects from strings, but literal characters are not escaped automatically. Escaping input avoids unintended matches, and flags are passed as a second argument.