11 Advanced React Interview Questions you should absolutely know (with detailed answers)

1. What is the React Virtual DOM?

Virtual DOM is a concept where a virtual representation of the real DOM is kept inside the memory and is synced with the actual DOM by a library such as ReactDOM.

The virtual DOM is an object that represents the real DOM in the memory. Since DOM updates are an integral part of any web app but are the costliest operation in the world of frontend, the virtual DOM is utilized to check for parts of the app that need to be updated & update only those parts, thus significantly boosting performance.

2. Why do we need to transpile React code?

React code is written in JSX, but no browser can execute JSX directly as they are built to read-only regular JavaScript.

Thus we require to use tools like Babel to transpile JSX to JavaScript so that the browser can execute it.

3. What is the significance of keys in React?

Keys in React is used to identify unique VDOM Elements with their corresponding data driving the UI; having them helps React optimize rendering by recycling existing DOM elements.

Key helps React identify which items have changed, are added, or are removed, enabling it to reuse already existing DOM elements, thus providing a performance boost.

For example:

const Todos = ({ todos }) => {
  return (
    <div>
      {todos.map((todo) => (
        <li>{todo.text}</li>
      ))}
    </div>
  );
};

This would cause new DOM Elements to be created everytime todos change, but adding the key prop (<li key={todo.id}>{todo.text}</li>) would result in "dragging" around the DOM Elements inside the ul tag & updating only the necessary lis.

4. What is the significance of refs in React?

Refs are variables that allow you to persist data between renders, just like state variables, but unlike state variables, updating refs does NOT cause the component to re-render.

Refs are usually used to, but not restricted to, store reference to DOM elements.

5. What are the most common approaches for styling a React application?

CSS Classes

React allows class names to be specified for a component like class names are set for a DOM element in HTML.

When developers first start using React after developing traditional web applications, they often opt for CSS classes as they are already familiar with the approach.

Inline CSS

Styling React elements using inline CSS allows styles to be completely scoped to an element. However, certain styling features are not available with inline styles. For example, the styling of pseudo-classes like :hover.

Pre-processors (Sass, Stylus, and Less)

Pre-processors are often used on React projects. This is because, like CSS, they are well understood by developers and are often already in use if React is being integrated into a legacy application.

CSS-in-JS Modules (Styled Components, Emotion, and Styled-jsx)

CSS-in-JS modules are a popular option for styling React applications because they integrate closely with React components. For example, they allow styles to change based on React props at runtime. Also, by default, most of these systems scope all styles to the respective component being styled.

6. What are some of the performance optimization strategies for React?

Using useMemo

useMemo is a React hook that is used for caching CPU-Expensive functions. A CPU-Expensive function called repeatedly due to re-renders of a component, can lead to slow rendering.

useMemo hook can be used to cache such functions. By using useMemo, the CPU-Expensive function gets called only when it is needed.

useCallback can be used to obtain a similar result.

Lazy Loading

Lazy loading is a technique used to reduce the load time of a React app. It helps reduce the risk of web app performances to a minimum, by loading up the components as the user navigates through the app.

7. What is prop drilling and how to avoid it?

Sometimes while developing React applications, there is a need to pass data from a component that is higher in the hierarchy to a component that is deeply nested. To pass data between such components, we pass props from a source component and keep passing the prop to the next component in the hierarchy till we reach the deeply nested component.

The disadvantage of using prop drilling is that the components that should otherwise be not aware of the data have access to the data, moreover, the code becomes harder to maintain.

Prop drilling can be avoided using the Context API or some form of State Management library.

8. What is the StrictMode component and why would you use it?

<StrictMode /> is a component included with React to provide additional visibility of potential issues in components. Suppose the application is running in development mode. In that case, any issues are logged to the development console, but these warnings are not shown if the application is running in production mode.

Developers use <StrictMode /> to find problems such as deprecated lifecycle methods and legacy patterns, to ensure that all React components follow current best practices.

<StrictMode /> can be applied at any level of an application component hierarchy, which allows it to be adopted incrementally within a codebase.

9. What are synthetic events in React?

Synthetic events combine the response of different browser's native events into one API, ensuring that the events are consistent across different browsers. The application is consistent regardless of the browser it is running in.

const Component = () => {
  const handleClick = (e) => {
    e.preventDefault(); // synthetic event
    console.log("link clicked");
  };
  return <a onClick={(e) => handleClick}>Click me</a>;
};

10. Why is it not advisable to update state directly, but use the setState call?

The conventional way to update state is to use the setState call. Without using it, the user would still be able to modify the state, but it would not update the DOM to reflect the new state.

const Component = () => {
  const [count, setCount] = useState(0);
  // let [count, setCount] = React.useState(0);
  const handleClickUpdate = () => {
    setCount((c) => c + 1);
    // count = count + 1; // will not update the DOM
  };
  return (
    <>
      {count}
      <button onClick={handleClickUpdate}>Click me</button>
    </>
  );
};

11. What are portals in React?

Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.

const Portal = ({ children }) => {
  // NOTE: it is advisable to create a new DOM node for the portal
  const portalRoot = document.getElementById("portal-root");
  return ReactDOM.createPortal(children, portalRoot);
};

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