Event monitoring mode

The event listener pattern is the core mechanism for handling user interactions in JavaScript. Using the `addEventListener` method, event handlers can be registered. Event propagation consists of three phases: capture, target, and bubbling. Event delegation leverages the bubbling mechanism to handle child element events on a parent element. Custom events can be created and triggered. Asynchronous event handling uses `AbortController` to enable cancellable listeners. Performance optimization techniques include debouncing and throttling. Cross-browser compatibility requires consideration of legacy IE differences. The event object contains rich information such as coordinates and the triggering element. Modern frameworks like React and Vue encapsulate native events. Mobile support includes touch gesture events. Keyboard events enable precise input control. HTML5 provides native drag-and-drop functionality. Page lifecycle events monitor loading and unloading states. Network events detect connection changes. Media queries enable responsive design.

Asynchronous flow control

JavaScript asynchronous flow control is the core mechanism for handling non-blocking operations in a single-threaded language. Early implementations used callback functions but often led to callback hell, making code difficult to maintain. Promises provided a more elegant chaining solution. The ES2017 introduction of async/await syntax allowed asynchronous code to be written with synchronous-like readability. Parallel control can be achieved using Promise.all to execute multiple asynchronous operations simultaneously. Advanced scenarios requiring strategies like throttling or retries can leverage libraries like async.js or custom implementations. The event-driven pattern is suitable for handling multiple asynchronous events. Generator functions combined with Promises can achieve coroutine-like effects. Error handling requires global catching and middleware interception. Performance optimization must consider concurrency control and batch processing. These techniques are widely used in web development, including API calls, database operations, and file processing scenarios.

Asynchronous error handling

JavaScript asynchronous error handling faces numerous challenges, including the distinct mechanisms of callbacks, Promises, and async/await. Uncaught errors may lead to silent failures. Callbacks use the error-first pattern, with common issues such as forgetting to check error parameters and callback hell. Promises capture errors via the `catch` method, and unhandled rejections generate warnings. Async/await requires `try-catch` for error handling, while global error handlers can catch uncaught exceptions. Custom error types enable precise handling. In production, errors should be logged to monitoring systems. Testing asynchronous errors requires validating throws and handling. Performance considerations call for optimizing hot-path error handling. Differences exist between browser and Node.js environments, and third-party libraries follow their own conventions. Error recovery strategies, such as retry mechanisms, can be employed. Large-scale applications may design a centralized error-handling architecture.

Conditional rule group

In CSS3, conditional rule groups enhance the adaptability of stylesheets by applying different style rules based on specific conditions. Media queries are the most commonly used conditional rule groups, capable of detecting viewport width, device orientation, resolution, and other parameters. Feature queries check browser support for specific CSS properties to achieve progressive enhancement. Document rules apply styles based on the document URL, but browser support is limited. Conditional rule groups can be nested to create complex logic. Custom media queries allow defining reusable conditions. In practice, they are often used for responsive design breakpoints and high-DPI device optimization, with performance considerations to avoid excessive redundant conditions. Combining them with CSS preprocessors can improve the writing experience. Future developments include container queries and richer feature detection.

Variables and theme switching

CSS3 custom properties (CSS Variables) provide a flexible and efficient solution for theme switching. By defining variables on the root element and modifying their values in different scenarios, dynamic theme switching can be easily achieved without duplicating large amounts of style code. CSS variables start with a double hyphen (--) and are invoked via the `var()` function, supporting default value settings. Modifying root element variable values with JavaScript enables real-time theme switching. The article details foundational usage, dynamic switching implementation, multi-theme management systems, responsive adaptation, performance optimization, animation effects, browser compatibility, component library applications, and user experience considerations. It also explores advanced techniques, including variable grouping management, preprocessor usage, transition animation design, persistent storage, system preference detection, and dynamic theme generation. This comprehensive guide offers developers practical insights into theme-switching implementations.

Attribute selector

The CSS3 attribute selectors provide seven matching patterns through square bracket syntax, including the existence selector to check attribute presence, the equality selector for exact value matching, the inclusion selector for space-separated words, the prefix selector for matching beginnings, the suffix selector for matching endings, the substring selector for matching any position, and the hyphen selector for handling specific formats. These selectors can be combined, support case sensitivity control, and integrate with features like pseudo-classes and CSS variables. They are practically applied in scenarios such as form enhancement, link hints, and image processing. When using them, attention should be paid to performance optimization and browser compatibility. They are also applicable in JavaScript. The CSS4 draft proposes new matching patterns for attribute selectors, significantly improving the precision and flexibility of style control.

Pseudo-classes and pseudo-elements

In CSS, pseudo-classes and pseudo-elements are essential tools for style control. Pseudo-classes, denoted by a single colon, are used to select specific states of an element, such as user interaction states (link, visited, hover, focus) or document structure positions (first-child, nth-child, etc.). Pseudo-elements, denoted by double colons, are used to create virtual elements that do not exist in the DOM, such as inserting content before or after, styling the first letter or line of text, or text selection highlights. Both can be combined to achieve complex effects, such as custom checkboxes, breadcrumb navigation, etc. In practical applications, attention must be paid to performance optimization and browser compatibility. Pseudo-elements must have the `content` property set and are inline by default. Pseudo-classes like `hover` may behave differently on touch devices. Creative applications include gradient underlines, custom list markers, and complex shape creation. Pseudo-elements can also be combined with CSS variables to dynamically generate content. Proper use of these features can enhance visual effects while keeping the code concise.

Counter and list styles

The CSS3 counter feature provides flexible control for numbering list items by initializing counters with `counter-reset`, incrementing values with `counter-increment`, and displaying counts with the `content` property. It supports multi-level nested numbering, list-style-type extensions including custom characters and predefined values like Hebrew numbering or Chinese counting. Combined with multi-column properties, it can create newspaper-style lists. Using `@counter-style` rules, custom counting systems can be fully defined, even enabling animated counting effects. It allows setting appropriate numbering systems for different languages, creating multi-level numbering for legal documents, controlling list-style positioning, and adapting styles responsively based on viewports. Automatic numbering can be added before table rows, and print optimization ensures visibility across page breaks. Understanding counter scopes enables creative markers using CSS gradients and shapes. These techniques offer rich possibilities for web list design.

Responsive component design

Responsive component design is a development approach that enables interface elements to dynamically adjust their layout and styles based on device characteristics. The core principle lies in components' ability to perceive environmental changes and adapt accordingly. Modern CSS3 features like Flexbox and Grid layout systems provide robust support. Container queries allow components to adjust styles based on their own dimensions rather than viewport size. CSS custom properties enable centralized management of dynamic styles. Responsive spacing utilizes the `clamp` function to create smoothly varying spacing values. Responsive typography achieves dual text size adjustments through multiple CSS solutions. Conditional layout patterns leverage CSS Grid and media queries to switch layouts for different scenarios. Responsive image handling combines the `picture` element with CSS to load appropriate resources. Component breakpoint design adopts a content-first methodology. Performance optimization techniques include reducing repaint areas and lazy loading. Testing strategies account for various conditions like dark mode and touch devices. Design system integration requires clear component behaviors and configurable properties. Accessibility considerations ensure components remain usable across all environments. Modern CSS function combinations create refined responsive effects, while JavaScript handles more complex interaction logic to enhance responsive functionality.

Direction-aware layout

Orientation-aware layout is a technique that dynamically adjusts page layouts based on device orientation. CSS3 provides tools like media queries, viewport units, and flexible layouts to achieve orientation detection. Media queries use the `orientation` feature to distinguish between landscape and portrait styles. Viewport units (vw, vh, vmin, vmax) allow flexible adaptation to orientation changes. Flexible layouts can quickly reorganize content by modifying `flex-direction`. Grid layouts enable entirely different grid structures. JavaScript can assist with complex interactions and orientation detection. Practical examples demonstrate orientation-adaptive designs, such as e-commerce product pages. Text layout must account for orientation changes, and some applications may require forcing a specific orientation. Mobile browser toolbars can affect viewport dimensions. Smooth transitions can be added for orientation changes, and different interaction modes may be needed for different orientations. Responsive images can load different proportions based on orientation. Testing should validate various transition scenarios, and performance optimization must consider layout recalculations. Modern front-end frameworks can create orientation-aware higher-order components.