🔥 JavaScript Interview Series(17): React vs Vanilla JS — Common Interview Scenarios

Understanding how React compares to Vanilla JavaScript is one of the most common themes in front-end interviews. Hiring managers often test not just your coding knowledge, but also your ability to reason about trade-offs, performance, and maintainability. Below are 10 common interview questions that dive into practical differences and real-world scenarios between React and Vanilla JS.

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1. How does React handle the DOM differently from Vanilla JavaScript?

Concept Tested: Virtual DOM, reconciliation, and performance optimization.

Model Answer:
In Vanilla JavaScript, developers directly manipulate the DOM using methods like document.getElementById() or querySelector(), which can become inefficient when updates are frequent.
React, however, uses a Virtual DOM — an in-memory representation of the real DOM. When the state changes, React compares the new Virtual DOM with the previous one using a process called reconciliation, updating only the parts that changed. This results in improved rendering performance.

Example:

// Vanilla JS
document.getElementById("count").innerText = count;

// React
setCount(count + 1); // React automatically updates the DOM efficiently

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What are the performance drawbacks of directly manipulating the DOM?
2. How does React’s diffing algorithm decide which elements to update?
3. Can you name a situation where using Vanilla JS for DOM manipulation might be preferable?

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2. What is the difference between state management in React and Vanilla JS?

Concept Tested: React state, component re-rendering, and data binding.

Model Answer:
In Vanilla JS, state is managed through variables and manual DOM updates. In React, state is declarative — you define how the UI should look based on state, and React re-renders automatically when the state changes. Hooks like useState and useReducer simplify this process.

Example:

// React
const [count, setCount] = useState(0);
<button onClick={() => setCount(count + 1)}>Increment</button>

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How does React determine when to re-render a component?
2. Why shouldn’t you mutate state directly in React?
3. What are the pros and cons of using Redux or Context API instead of local state?

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3. How does event handling differ in React compared to Vanilla JS?

Concept Tested: Synthetic events, event delegation, and JSX syntax.

Model Answer:
React uses a SyntheticEvent system that wraps native browser events for consistent behavior across browsers. Handlers are defined directly in JSX (e.g., onClick={handleClick}), and React optimizes performance using event delegation at the root.

Example:

// React
<button onClick={handleClick}>Click Me</button>

// Vanilla JS
document.querySelector("button").addEventListener("click", handleClick);

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Why does React use Synthetic Events instead of native browser events?
2. How does event delegation improve React’s performance?
3. How do you stop event propagation in React?

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4. How does conditional rendering in React differ from Vanilla JS DOM manipulation?

Concept Tested: Declarative rendering, logic in JSX, and performance.

Model Answer:
In Vanilla JS, conditional rendering typically involves manually adding or removing elements from the DOM. React achieves the same declaratively using conditional expressions inside JSX, which keeps logic and UI tightly coupled and more predictable.

Example:

// React
{isLoggedIn ? <Dashboard /> : <Login />}

// Vanilla JS
if (isLoggedIn) showDashboard(); else showLogin();

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How does React minimize re-rendering during conditional rendering?
2. Can React render nothing conditionally?
3. How do conditional renderings affect performance in React applications?

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5. How does component reusability in React compare to Vanilla JS?

Concept Tested: Component architecture and modular design.

Model Answer:
React encourages component-based architecture, where UI and logic are encapsulated into reusable blocks. In Vanilla JS, developers often duplicate code or manually create modular functions, making maintenance harder. Components enhance scalability and reduce redundancy.

Example:

// React
function Button({ label }) {
  return <button>{label}</button>;
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How can React components communicate with each other?
2. What are some ways to make components more reusable?
3. How would you create reusable UI logic without React?

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6. What is the difference between React’s one-way data flow and Vanilla JS data manipulation?

Concept Tested: Unidirectional data flow, props, and state consistency.

Model Answer:
React enforces a unidirectional data flow, meaning data flows from parent to child via props. This ensures predictable state management. Vanilla JS allows free data manipulation, often leading to inconsistent UI and logic. React’s design simplifies debugging and improves stability.

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Why is unidirectional data flow easier to debug?
2. What is prop drilling, and how can it be avoided?
3. When might two-way data binding be beneficial?

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7. How does performance optimization differ in React vs Vanilla JS?

Concept Tested: Rendering optimization, memoization, and diffing.

Model Answer:
React optimizes performance through reconciliation, memoization, and selective re-rendering using React.memo, useMemo, and useCallback. In Vanilla JS, developers must manually control DOM updates and optimize event listeners. React abstracts much of this complexity.

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How can you prevent unnecessary re-renders in React?
2. What role does the key prop play in improving list rendering performance?
3. When might manual DOM manipulation outperform React’s diffing algorithm?

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8. How does React handle asynchronous operations compared to Vanilla JS?

Concept Tested: useEffect, async handling, and lifecycle behavior.

Model Answer:
React uses the useEffect hook to handle asynchronous side effects like API calls. It ensures state updates happen after the render cycle, keeping the UI consistent. Vanilla JS relies on callbacks or promises with manual DOM updates, which can easily become unstructured.

Example:

useEffect(() => {
  fetchData().then(setData);
}, []);

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Why should side effects not be placed directly inside React components?
2. How does React prevent race conditions when fetching data?
3. What happens if you update state after a component unmounts?

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9. How do you handle form inputs in React vs Vanilla JS?

Concept Tested: Controlled components, event binding, and data flow.

Model Answer:
In Vanilla JS, you directly read form values from the DOM using properties like input.value. React introduces controlled components, where form values are tied to component state. This ensures a single source of truth and easier validation.

Example:

// React Controlled Component
<input value={name} onChange={e => setName(e.target.value)} />

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What’s the difference between controlled and uncontrolled components?
2. How can you improve performance in forms with many inputs?
3. When would you use uncontrolled components instead?

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10. How would you decide when to use React over Vanilla JS?

Concept Tested: Project complexity, scalability, and team collaboration.

Model Answer:
React is best suited for large, interactive, and state-heavy UIs where maintainability, reusability, and scalability matter. Vanilla JS is preferable for lightweight, static, or small projects where a framework would add unnecessary overhead.

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What are the trade-offs of using React for small projects?
2. How does React improve developer productivity on large teams?
3. Can you replicate React’s component system using Vanilla JS?