ReactJS Mastery Quiz.
85+ expert-level questions covering React Fiber, Modern Hooks, and Next.js 15 Server Components.
Course Overview: Frontend Systems Design
Module 01 // Core Internals & Fiber Architecture
Fiber allows React to break rendering work into small units and spread it over multiple frames (time-slicing). It enables pausing, resuming, and prioritizing work, which prevents UI blocking during heavy updates and is the foundation for Concurrent React.
Shadow DOM is a native browser API for scoping CSS and DOM in web components. Virtual DOM is a lightweight JavaScript representation of the real DOM used by React to efficiently compute updates through 'diffing'.
Concurrent Mode (now 'Concurrent React') allows React to work on multiple versions of the UI simultaneously. It enables the UI to remain responsive even during large renders by interrupting low-priority updates for high-priority user interactions.
Module 02 // Recruiter's Screening Room (Junior & Confirmed)
Functional components are simpler, use Hooks for state/lifecycle, and are the modern standard. Class components rely on ES6 classes and lifecycle methods like 'componentDidMount' but are being phased out in favor of the more composable Hooks approach.
1) Only call Hooks at the top level (not inside loops or conditions) to ensure they always run in the same order. 2) Only call Hooks from React functions (not standard JavaScript functions).
Props (short for properties) are passed to a component from its parent and are read-only. State is managed internally by the component and can change over time, usually triggering a re-render when it does.
Keys help React identify which items have changed, been added, or removed. This allows for stable component identity and efficient updates, preventing bugs like state being 'passed' to the wrong list item.
It's a 'draft' of the UI that React keeps in memory. When state changes, React updates the draft first, compares it with the old version (diffing), and then only updates the specific parts of the real DOM that actually changed.
Module 03 // Advanced Hooks & State Logic
Use 'useTransition' when you have control over the state update that triggers the transition. Use 'useDeferredValue' when you receive a value from a parent (like a prop) and want to defer the re-render of a component tree that depends on that value.
The rule ensures that all reactive values used inside an effect are declared as dependencies. Skipping them causes 'stale closures' where the effect function captures variables from a previous render, leading to bugs where the effect logic operates on old data.
It runs synchronously after all DOM mutations but before the browser paints. It blocks the browser from painting until the effect finishes, which can cause 'jank' or flickering if the logic is too heavy.
useMemo memoizes the *result* of a function calculation. useCallback memoizes the *function instance* itself, preventing unnecessary re-renders of child components that rely on reference equality for props.
Module 04 // Next-Gen Architecture (RSC & Next.js)
RSCs are components that render exclusively on the server. They reduce the JavaScript bundle size shipped to the client to nearly zero and allow direct access to server-side resources like databases without intermediate APIs.
It marks a boundary between Server and Client components. Any component with 'use client' (and its imports) will be part of the client-side bundle, enabling the use of state, effects, and browser APIs.
Streaming allows the server to send HTML chunks as they are ready. Hydration can then begin on those chunks immediately, rather than waiting for the entire page to download, significantly improving 'Largest Contentful Paint' and 'Time to Interactive'.
Advanced Insight: Next.js 15 PPR
Partial Prerendering (PPR) is the biggest breakthrough in Next.js 15. It combines the speed of static site generation with the flexibility of dynamic rendering in a single page request.
// Minimal PPR Implementation
export const experimental_ppr = true;
export default function Page() {
return (
<main>
<StaticHeader />
<Suspense fallback={<Skeleton />}>
<DynamicUserDashboard />
</Suspense>
</main>
);
}With PPR enabled, the StaticHeader is served instantly from the edge (CDN), while the DynamicUserDashboard is streamed in as soon as the server finishes rendering it. This eliminates the "All or Nothing" choice between Static and Dynamic rendering.
React Architecture FAQ
The React Compiler (formerly React Forget) statically analyzes your component code at build time and automatically inserts memoization where needed. This means developers no longer need to manually use useMemo, useCallback, or React.memo — the compiler determines optimal re-render boundaries by analyzing data flow dependencies and automatically caches expensive computations and stable callback references.
SSR renders the entire page to HTML on the server, then the client re-processes (hydrates) everything. Server Components render individual components on the server and stream their output as a serialized React tree. The critical difference: Server Components never ship their JavaScript to the client, reducing bundle size. They can also directly access databases, file systems, and APIs without exposing credentials to the browser.
Use useState for simple, independent state values (toggle, counter, input text). Use useReducer when: (1) state updates depend on previous state, (2) multiple sub-values are logically related (form with 5+ fields), or (3) state transitions follow predictable patterns that benefit from a centralized dispatch/action model. The reducer function is also easier to test in isolation since it's a pure function.
Server Actions are async functions marked with 'use server' that execute on the server when called from client components. They enable form submissions and data mutations without building separate API endpoints. Under the hood, Next.js creates a POST endpoint for each Server Action, serializes the arguments, executes the function on the server, and returns the result — all with automatic error handling and optimistic UI support.
Concurrent Rendering allows React to interrupt long-running renders to handle higher-priority updates (like user input). With features like useTransition and useDeferredValue, expensive re-renders (filtering 10,000 list items) can be deprioritized so the UI remains responsive. The key insight: React can now prepare new UI in the background without blocking the current screen from responding to user interactions.
The React ecosystem has consolidated: use React's built-in useState/useReducer + Context for app-level state, React Query (TanStack Query) for server state (caching, synchronization, background refetching), and Zustand for complex client-side global state that needs to be accessed outside React's component tree. Redux remains viable for large existing codebases but is no longer the default recommendation for new projects.
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