Interview Questions
The primary methods for DOM selection include getElementById (fastest, returns single element), querySelector (flexible, returns first match), querySelectorAll (returns static NodeList), getElementsByClassName and getElementsByTagName (return live HTMLCollections). Each method has different performance characteristics. getElementById provides the best performance for single element selection due to unique ID requirements. querySelector and querySelectorAll offer more flexibility with CSS selector syntax but may be slower for complex selectors. Consider caching DOM selections for frequently accessed elements to optimize performance.
DOM updates should be batched to minimize browser reflows and repaints. Use DocumentFragment for multiple insertions, modify elements off-DOM using cloneNode, and apply multiple style changes through CSS class toggles rather than individual style properties. Measure critical dimensions before making changes that could trigger reflow. Cache computed styles when needed for multiple reads. Consider using requestAnimationFrame for visual updates to ensure smooth animations and prevent layout thrashing. Implement proper cleanup for removed elements and event listeners.
Event delegation involves attaching event listeners to parent elements to handle events on descendants, leveraging event bubbling. This pattern improves performance by reducing the number of event listeners, handles dynamically added elements automatically, and reduces memory usage. Implement proper event target filtering using matches() or closest(). Consider event propagation and stopping mechanisms. Handle cleanup properly when removing parent elements. Document delegation patterns and requirements for maintainability.
When removing DOM elements, ensure proper cleanup by removing event listeners, clearing references in JavaScript, and handling any associated resources. Use removeEventListener for explicit event handler removal, clean up any intersection or mutation observers, and handle cleanup of third-party library integrations. Consider memory leak prevention, especially with closures and cached element references. Document cleanup requirements and implement proper error handling during cleanup processes.
innerHTML parses content as HTML, allowing element creation but presenting security risks with unsanitized input. textContent provides raw text access, ignoring styling and hidden elements, offering better performance. innerText respects CSS styling and visibility, triggering reflow. Consider security implications when using innerHTML, especially with user input. Use textContent for performance-critical text updates. Document content handling approaches and sanitization requirements.
Efficient DOM traversal utilizes properties like parentNode, children, nextSibling, and previousSibling. Consider using childNodes vs children based on needs (text nodes vs elements). Implement proper caching of frequently accessed nodes. Use appropriate iteration methods for NodeList and HTMLCollection objects. Handle cross-browser compatibility issues. Document traversal patterns and performance considerations.
MutationObserver monitors DOM changes, useful for dynamic content updates and third-party script integration. IntersectionObserver tracks element visibility, ideal for lazy loading and infinite scroll implementations. Both provide efficient alternatives to polling or scroll event listeners. Implement proper cleanup and disconnection. Consider performance implications and observation options. Document observer usage patterns and requirements.
Handle cross-browser differences through feature detection rather than browser detection. Implement appropriate fallbacks for unsupported features. Consider polyfills for broader compatibility. Test across different browsers and versions. Document browser support requirements and limitations. Implement graceful degradation strategies. Handle vendor prefix differences appropriately.
Minimize DOM access and updates by caching references and batching changes. Use document fragments for multiple insertions. Avoid forced synchronous layouts. Implement proper event delegation. Consider repaint and reflow implications. Use requestAnimationFrame for visual updates. Document performance requirements and optimization strategies. Monitor and profile DOM operations.
Implement form validation using both HTML5 validation attributes and JavaScript validation. Handle form submission events properly. Implement custom validation UI and error messages. Consider accessibility requirements. Handle form reset and clear operations. Document validation rules and requirements. Implement proper error handling and user feedback.
Data attributes (data-*) store custom data within HTML elements, accessible via dataset property. Use for configuration data, state management, and element identification. Avoid storing sensitive information. Consider naming conventions and documentation. Implement proper validation of data attribute values. Handle updates and changes appropriately.
Implement focus management considering accessibility requirements. Handle focus trapping in modals. Implement proper tabindex usage. Consider keyboard navigation patterns. Handle focus restoration after dynamic updates. Document focus management requirements. Implement proper error handling for focus operations.
Handle view updates efficiently without full page reloads. Implement proper cleanup between view changes. Consider memory management and event listener cleanup. Handle browser history and navigation. Document view lifecycle requirements. Implement proper error handling for view transitions.
Implement efficient content loading strategies using fetch or XMLHttpRequest. Handle loading states appropriately. Consider progressive enhancement. Implement proper error handling and retry logic. Document loading requirements and dependencies. Handle cleanup of loaded content when necessary.
Prevent XSS attacks by sanitizing input before insertion. Avoid eval() and innerHTML with untrusted content. Implement proper Content Security Policy. Consider iframe sandbox requirements. Document security requirements and practices. Implement proper input validation and sanitization.
Use HTML5 Drag and Drop API or implement custom drag and drop logic. Handle drag events properly. Implement proper visual feedback. Consider accessibility requirements. Handle cross-browser differences. Document drag and drop requirements. Implement proper cleanup after drag operations.
Use media queries and matchMedia for responsive behavior. Handle resize events efficiently. Implement proper breakpoint management. Consider device capabilities. Document responsive requirements. Implement proper cleanup of responsive handlers. Handle orientation changes appropriately.
Use CSS transitions when possible, falling back to JavaScript animations when needed. Implement proper animation cleanup. Consider performance implications. Handle animation completion and cancellation. Document animation requirements. Implement proper error handling for animations.
Implement scroll event throttling or debouncing. Use IntersectionObserver when possible. Handle scroll position restoration. Consider scroll performance optimization. Document scroll handling requirements. Implement proper cleanup of scroll handlers.
Render only visible elements while maintaining scroll position and size. Implement efficient item recycling. Handle dynamic content heights. Consider memory usage optimization. Document virtual scrolling requirements. Implement proper cleanup and memory management.
Handle touch events for mobile devices. Implement proper gesture recognition. Consider multi-touch interactions. Handle touch event delegation properly. Document touch interaction requirements. Implement proper cleanup of touch handlers.
Implement proper keyboard navigation. Handle keyboard shortcuts appropriately. Consider focus management. Document keyboard interaction requirements. Implement proper error handling for keyboard events. Handle modifier keys appropriately.
Handle positioning and visibility. Implement proper event handling. Consider accessibility requirements. Handle cleanup on removal. Document tooltip behavior requirements. Implement proper error handling for positioning calculations.
Handle DOM operation failures gracefully. Implement proper error recovery. Consider user feedback mechanisms. Document error handling requirements. Implement proper cleanup after errors. Handle async operation failures appropriately.
Handle cross-origin communication properly. Implement proper sandbox restrictions. Consider security implications. Handle iframe loading states. Document iframe requirements. Implement proper cleanup when removing iframes.
Synchronize DOM with application state efficiently. Handle state transitions properly. Consider optimistic updates. Implement proper error recovery. Document state management requirements. Handle cleanup during state changes.
Implement proper DOM virtualization. Handle memory management efficiently. Consider performance optimization. Document large DOM requirements. Implement proper cleanup strategies. Handle updates efficiently.
Implement proper unit and integration tests. Handle async operations in tests. Consider test isolation. Document testing requirements. Implement proper cleanup after tests. Handle test environment differences.