Front-End Developer interview questions

Best practices for scalable and maintainable HTML/CSS include using semantic HTML elements, adopting a modular CSS architecture such as BEM or SMACSS, leveraging pre-processors like SASS or LESS, minimizing specificity, and enforcing naming conventions. Code should be organized logically, reused wherever possible, and consistently documented. Version control and code review processes also enhance maintainability.

Ensuring cross-browser compatibility involves using standard-compliant HTML5 and CSS3, leveraging feature detection libraries such as Modernizr, employing appropriate CSS vendor prefixes, and testing rigorously on multiple browsers and devices. Polyfills and fallbacks should be used for unsupported features. Utilizing CSS resets or normalize.css can address default browser styling inconsistencies.

Advanced JavaScript concepts essential for efficient front-end development include closures, asynchronous programming (Promises, async/await), prototypal inheritance, module patterns, event delegation, and understanding the event loop. Deep knowledge of ES6+ features, functional programming paradigms, and performance optimizations are also necessary at a senior level.

Front-end web performance optimization involves minifying HTML, CSS, and JavaScript, leveraging lazy loading for images and assets, reducing the number of HTTP requests, using code splitting, deferring non-critical JS/CSS, enabling browser caching, and utilizing Content Delivery Networks (CDNs). Accessibility and responsiveness should be preserved alongside performance improvements.

Structuring CSS for large-scale responsive applications is facilitated by a combination of methodologies such as BEM for naming conventions, component-driven architecture, and use of pre-processors to abstract variables and mixins. Media queries should be modularized and follow a 'mobile-first' approach for responsiveness, ensuring scalability as the codebase grows.

Building responsive layouts requires the use of flexible grids (CSS Grid, Flexbox), relative units (em, rem, %, vw, vh), and scalable media (responsive images, SVGs). Accessibility, touch targets, and load times on different devices must be considered. Testing on various breakpoints and devices ensures consistency in user experience.

In large JavaScript applications, state management and data flow are addressed using architectural patterns like Flux, Redux, or Context API (for React applications). Immutability, centralized state storage, predictable state transitions, and middleware for handling side effects are important for maintainability, testability, and scalability.

Modularizing JavaScript code is achieved by breaking functionality into reusable modules or components, using ES6 modules or module bundlers like Webpack. Each module should have a single responsibility, clear interfaces, and minimal dependencies. Consistent directory structure and documentation facilitate collaboration and ease future maintenance.

Effective debugging techniques include utilizing browser DevTools for inspecting the DOM, network activity, and JavaScript execution; setting breakpoints; logging; tracing asynchronous code paths; using linters and static analyzers; and writing unit and integration tests. Collaborative debugging and pair programming can also expedite problem resolution.

Progressive enhancement focuses on building a baseline experience that works for the least-capable browsers and incrementally adding advanced features for browsers that support them. Graceful degradation emphasizes building for modern browsers first and ensuring the application still remains functional in older or less capable browsers, albeit with fewer features or a reduced experience.