Frontend Developer Interview Questions & Answers (2026)

`var` is function-scoped and hoisted. `let` and `const` are block-scoped and not hoisted. `const` prevents reassignment but not mutation of objects/arrays. Best practice: always use `const` by default, `let` when you need to reassign, never `var`.

JavaScript is single-threaded. The event loop continuously checks the call stack — if it's empty, it moves tasks from the callback queue (or microtask queue for Promises) into the stack. Microtasks (Promises, queueMicrotask) run before macrotasks (setTimeout, setInterval). This allows async operations without blocking the main thread.

The virtual DOM is an in-memory representation of the real DOM. When state changes, React re-renders the virtual DOM, diffs it against the previous version (reconciliation), and applies only the minimal set of real DOM changes. This batching approach is faster than direct DOM manipulation for most UI patterns.

Hooks (useState, useEffect, useContext, etc.) let you use state and lifecycle features in functional components without writing classes. They were introduced in React 16.8 to solve problems with class components: confusing `this` binding, difficulty reusing stateful logic between components, and complex lifecycle methods.

Key strategies: React.memo to prevent unnecessary re-renders, useMemo/useCallback for expensive calculations and stable function references, code splitting with React.lazy and Suspense, virtualization for long lists (react-window), image optimization, avoiding anonymous functions in JSX, and profiling with React DevTools.

Specificity determines which CSS rule applies when multiple rules target the same element. It's calculated as (inline styles, IDs, classes/attributes/pseudo-classes, elements). Inline styles beat everything. IDs beat classes. Classes beat elements. Equal specificity = last rule wins. `!important` overrides all specificity.

`==` performs type coercion before comparing (`'5' == 5` is true). `===` compares value AND type without coercion (`'5' === 5` is false). Always use `===` to avoid unexpected type coercion bugs. The only common exception: `x == null` checks for both `null` and `undefined`.

Core Web Vitals are Google's user experience metrics: LCP (Largest Contentful Paint — loading, target <2.5s), FID/INP (interaction responsiveness, target <200ms), CLS (Cumulative Layout Shift — visual stability, target <0.1). Improve LCP: optimize images, use CDN, preload critical resources. Reduce CLS: set explicit dimensions on images and embeds. Reduce INP: avoid long tasks, use web workers.

Web accessibility ensures people with disabilities can use your app. Key practices: semantic HTML (use `<button>` not `<div onClick>`), ARIA labels for non-semantic elements, keyboard navigation (tabindex, focus management), sufficient color contrast (WCAG AA = 4.5:1), alt text on images, and testing with screen readers (NVDA, VoiceOver).

CSR (React default): browser downloads a minimal HTML shell, then JavaScript renders everything client-side. Slow initial load, good for highly interactive apps. SSR (Next.js getServerSideProps): server renders full HTML on each request. Fast initial load, good for SEO. SSG: pages rendered at build time. ISR: SSG with revalidation. Choose based on data freshness requirements vs performance trade-offs.