ReactJS is one of the most widely used JavaScript libraries for developing dynamic and responsive user interfaces. If you are preparing for a ReactJS interview, it’s important to understand the fundamentals of React, as many interview questions will focus on core concepts.
In this blog, we’ll cover some of the common ReactJS interview questions for beginners, helping you get ready for your next interview.
ReactJS Interview Questions For Freshers
Q1. What is React?
React is a JavaScript library created by Facebook for crafting user interfaces, especially single-page applications (SPAs). It allows developers to build reusable UI components that dynamically update based on changes in state or data, offering an efficient solution for developing interactive and responsive web applications.
Q2. What are the key features of React?
Here are some of the key features of React:
- JSX (JavaScript XML): JSX allows you to write HTML elements and components in JavaScript code, which React then compiles to create virtual DOM elements.
- Components: React centres the concept of components. A component is a reusable, self-contained unit that encapsulates its own structure, behavior, and appearance.
- Virtual DOM: React uses something known as a virtual DOM to enhance performance. Instead of updating the real DOM every time the state changes. To reduce modifications and increase speed, React initially modifies the virtual DOM before comparing it to the real DOM.
- State and Props: React components have state (data that can change) and props (data passed from parent to child components).
- Unidirectional Data Flow: React follows one-way data flow, meaning data is passed from parent to child components, making it easier to debug and manage.
Q3. What is JSX?
JSX stands for JavaScript XML. Writing HTML elements in JavaScript code is made possible by this syntax enhancement. JSX makes React code more readable and concise, as it closely resembles HTML. React components use JSX to define how the UI should look.
Here’s an example of JSX syntax:
While JSX looks like HTML, it’s actually JavaScript, so it can be compiled into JavaScript functions.
Q4. What are props in React?
Props (short for “properties”) are the mechanism through which data is passed from a parent component to a child component. Props are immutable (i.e., they cannot be changed by the child component). They are used to configure a child component and allow for communication between components.
For example:
}
<Greeting name=”John” />
In this example, name is passed as a prop to the Greeting component, and it will be displayed as “Hello, John!”.
Q5. What is the difference between state and props in React?
- State: State refers to the internal data of a component that can change over time. It is mutable and is used to manage changes in the component’s behavior or appearance. When the state of a component changes, the component re-renders to reflect the updated state.
- Props:Props are immutable and are employed to transfer information between parent and child components. A component cannot modify its own props, but it can use props to render content.
In short, state is managed within a component,while components of the props are transmitted from parent to kid.
Q6. What is a component in React?
A component in React is a JavaScript function or class that accepts input (called props) and returns React elements describing what should appear on the screen. Components can be either:
Functional Components: These are simpler components written as JavaScript functions. They can use hooks like useState and useEffect to manage state and lifecycle events.
Example:
}
Class Components: These are written as ES6 classes and extend the React.Component class. They have lifecycle methods and can manage state.
Example:
return <h1>Hello, {this.props.name}</h1>;
}
}
Q7. What is the Virtual DOM?
The Virtual DOM (VDOM) is a lightweight copy of the actual DOM. It is a concept implemented by React to optimize UI updates. When the state of an object changes, React first updates the Virtual DOM. It then determines the most effective method of altering the real DOM by comparing the current Virtual DOM with an earlier version. This process, called reconciliation, makes React programs quick and effective.
Q8. What are React hooks?
React hooks are functions that allow you to use React state and lifecycle features in functional components. They were introduced in React 16.8 to provide a simpler and more intuitive way to manage state and side effects in functional components.
Some common hooks include:
useState: Allows you to add state to functional components.
Example:
useEffect: Enables you to carry out side effects in functional elements, like data retrieval.
Example:
}, [count]);
Q9. What is the difference between functional and class components in React?
Functional Components: These simple JavaScript functions take props as arguments and return React elements. They are stateless by default but can use hooks to manage state and lifecycle methods in modern React.
Example:
}
Class Components: These are ES6 classes that extend React.Component. They can have state and lifecycle methods. Class components were the traditional way to write React components, but with the introduction of hooks, functional components are now preferred.
Example:
return <h1>Hello, {this.props.name}</h1>;
}
}
Q10. What is the difference between componentDidMount() and useEffect()?
componentDidMount(): This is a lifecycle method in class components. It is called after a component is mounted (added to the DOM) and is used for one-time setup like fetching data or subscribing to events.
Example:
console.log(“Component mounted”);
}
render() {
return <h1>Hello</h1>;
}
}
useEffect(): This is the hook used in functional components to handle side effects like fetching data or DOM manipulation. It serves a similar purpose to componentDidMount() but in a more flexible way.
Example:
}, []); // Empty dependency array means it runs once when the component mounts
Q11. What are keys in React, and why are they important?
Keys are distinct identifiers that assist React in monitoring of components and efficiently updating the UI. They are especially important when rendering lists of elements. React uses keys to optimize re-rendering and reconciliation of the Virtual DOM with the actual DOM.
Example:
return (
<ul>
{items.map((item, index) => (
<li key={index}>{item}</li>
))}
</ul>
);
}
In this case, each <li> element needs a key to help React distinguish between elements in the list.
Q12. What is a controlled component in React?
A component that contains form data is said to be controlled (such as input fields) is controlled by React state. In a controlled component, the value of the input is bound to a state variable, and any changes to the input are handled by a function that updates the state.
Example:
const handleChange = (event) => {
setInputValue(event.target.value);
};
return (
<form>
<input type=”text” value={inputValue} onChange={handleChange} />
</form>
);
}
Here, the value of the input field is controlled by the inputValue state.
Q13. What are React fragments?
React fragments allow you to group a list of children without adding extra nodes to the DOM. This is useful when you want to return several components’ elements without enclosing them in a parent element like a <div>.
Example:
<>
<h1>Item List</h1>
<ul>
<li>Item 1</li>
<li>Item 2</li>
</ul>
</>
);
}
Here, we are returning two elements without adding an extra wrapping element to the DOM.
Q14. What are defaultProps in React?
defaultProps is used to specify default values for props if they are not passed by the parent component. This is useful to ensure that a component has default values for props, preventing errors when the parent doesn’t provide them.
Example:
}
Greeting.defaultProps = {
name: ‘Guest’,
};
If the name prop is not passed, it defaults to ‘Guest’.
Q15. What is prop drilling in React?
The technique of using intermediary components to transfer information from a parent component to a child component that is deeply nested is known as prop drilling. When multiple levels of components are involved, this might make maintaining the code challenging:
Example:
return <Child name={name} />;
}
function Child({ name }) {
return <GrandChild name={name} />;
}
function GrandChild({ name }) {
return <h1>Hello, {name}</h1>;
}
To avoid prop drilling, you can use tools like React Context API or state management libraries like Redux to share data without passing props through many layers.
Q16. What is the Context API in React?
The React Context API is a way to share data across the component tree without having to pass props down manually at every level. It’s useful for global data like themes, user authentication, or language preferences.
To use Context:
Create a Context:
Provider:
<ThemeContext.Provider value=”dark”>
<Child />
</ThemeContext.Provider>
);
}
Consume Context:
return <h1>Current theme: {theme}</h1>;
}
The Context API is a great alternative to prop drilling for passing data through deeply nested components.
Q17. What is useState in React?
You can add state to functional components with the useState React hook. The current state value and a function to change that value are the two elements of the array that is returned.
Example:
setCount(count + 1);
}
return <button onClick={increment}>Clicked {count} times</button>;
The state variable in this example is count, and the function is setCount. used to update its value.
ReactJS Interview Questions for Intermediate
1. What are React Hooks and why were they introduced?
React hooks were introduced in React 16.8 to allow functional components to use state and lifecycle features without converting them into class components. Hooks like useState, useEffect, useReducer, and useContext provide a simpler and more reusable way of managing state, side effects, and context in functional components.
2. What is useEffect and how does it work?
useEffect is a React hook that lets you apply side effects to functional elements.It can be used for tasks like data fetching, subscriptions, or manually changing the DOM.
- Syntax: useEffect(callback, [dependencies])
- The callback function runs after the component renders.
- The second argument, [dependencies], is an array of values that the effect depends on. If any value in the array changes, the effect re-runs.
Example:
}, [someState]);
3. What is the difference between useEffect and componentDidMount?
- componentDidMount is a lifecycle method in class components that runs once after the component mounts.
- useEffect is the functional component equivalent. By providing an empty dependency array ([]), you can simulate the behavior of componentDidMount because it will run once after the initial render.
4. What is useCallback in React?
useCallback is a hook that returns a memoized version of the callback function. It is used to prevent unnecessary re-creations of functions, which can improve performance in some cases, especially when passing functions as props to child components.
Example:
}, [dependencies]);
5. What is useMemo in React?
useMemo is a hook that returns a memoized value. It helps optimize performance by preventing expensive calculations from being re-executed unless specific dependencies change. It’s useful when you need to optimize rendering performance, especially for calculations that depend on expensive logic.
Example:
6. What are Higher-Order Components (HOCs) in React?
A Higher-Order Component (HOC) is a function that takes a component and returns a new component with enhanced functionality. HOCs are used to share common logic between components without repeating code.
Example:
if (isLoading) {
return <div>Loading…</div>;
}
return <Component {…props} />;
};
}
7. In React, what are the controlled and uncontrolled components?
Controlled Components: In controlled components, the form element’s value is controlled by React state. The value is set via the state, and updates are handled by event handlers.
Example:
const handleChange = (event) => {
setValue(event.target.value);
};
return <input type=”text” value={value} onChange={handleChange} />;
}
Uncontrolled Components: In uncontrolled components, the form elements manage their own state, and you access their values using a ref instead of React state.
Example:
const handleSubmit = () => {
alert(inputRef.current.value);
};
return (
<div>
<input ref={inputRef} />
<button onClick={handleSubmit}>Submit</button>
</div>
);
}
8. What is React Router and how do you implement it in a React application?
React Router is a library used for handling routing in React applications. It allows for navigation between different views or pages within the app without reloading the entire page (client-side routing).
Basic implementation:
Install React Router: npm install react-router-dom
Define routes and navigation in your app:
return (
<Router>
<nav>
<Link to=”/”>Home</Link>
<Link to=”/about”>About</Link>
</nav>
<Switch>
<Route path=”/” exact component={Home} />
<Route path=”/about” component={About} />
</Switch>
</Router>
);
}
9. What is React Context API and how do you use it?
The React Context API is a way to exchange information throughout the component tree without requiring manual prop passing down at each level. It’s useful for global data like themes, user authentication, or language preferences.
Basic usage:
Create Context:
Provider:
<MyContext.Provider value=”someValue”>
<Child />
</MyContext.Provider>
);
}
Consumer:
return <h1>{value}</h1>;
}
10. What is the shouldComponentUpdate lifecycle method and how does it relate to performance?
The shouldComponentUpdate method in class components is used to optimize performance. It allows you to prevent unnecessary re-renders by returning false when React determines that the component does not need to be updated.
Example:
shouldComponentUpdate(nextProps, nextState) {
// Return false if props or state hasn’t changed
return nextProps.value !== this.props.value;
}
}
In functional components, React.memo is used to achieve similar functionality.
ReactJS Interview Questions for Advanced
1. What is (SSR) server-side rendering in React, and how is it different from client-side rendering (CSR)?
- Server-Side Rendering (SSR): React components are rendered to HTML on the server and can be sent to the client as a fully developed HTML page. The client then hydrates the application to make it interactive.
- Advantages:
- Faster initial load time, especially for SEO.
- Better performance on slower devices.
- Client-Side Rendering (CSR): The browser downloads a minimal HTML page and then loads the JavaScript bundle to render the application on the client-side.
Advantages:- After the initial load, subsequent interactions are faster.
- Less load on the server.
Example Tools: Next.js is a popular framework for SSR with React.
2. When would you use React Portals and what are they?
React Portals allow you to render components outside the main DOM hierarchy. This is helpful in situations such as popovers, tooltips, and modals, where the component should not be constrained by the parent component’s CSS or DOM structure.
Example:
<div>Modal Content</div>,
document.getElementById(‘modal-root’)
);
3. What is React’s Reconciliation process?
Reconciliation is React’s algorithm for updating the DOM. React:
- Generates a new Virtual DOM tree whenever the state or props of a component change.
- Compare the new Virtual DOM with the previous one.
- Calculates the minimal set of changes required.
- Updates the real DOM efficiently.
Key concepts:
- Keys: Help React identify which items in a list have changed.
- Diffing Algorithm: Optimized algorithm for comparing trees by breaking them into smaller pieces.
4. What is code-splitting in React, and how do you implement it?
Code-splitting allows you to split your application into more manageable packages that may be loaded whenever needed, reducing the initial load time.
Implementation:
function App() {
return (
<React.Suspense fallback={<div>Loading…</div>}>
<LazyComponent />
</React.Suspense>
);
}
Use a bundler like Webpack with tools like React’s React.lazy or libraries like loadable-components.
5. What are React Fiber and its goals?
React Fiber is the reimplementation of React’s reconciliation algorithm. It introduced incremental rendering, allowing React to split rendering work into chunks and prioritize updates, improving performance for complex applications.
Key Goals:
- Concurrency: Breaking rendering work into smaller units.
- Better responsiveness: Prioritizing updates like user interactions over less critical tasks.
- Pausing and resuming rendering: Allowing React to pause rendering for higher-priority updates.
6. How do you optimize performance in React applications?
Techniques:
- Memoization: Use React.memo, useMemo, and useCallback to prevent unnecessary re-renders or recomputations.
- Lazy Loading: Implement code-splitting and dynamic imports.
- Virtualization: Use libraries like react-window or react-virtualized for efficiently rendering large lists.
- Avoid Inline Functions: Define functions outside of JSX to avoid re-creation.
- Use Key Properly: Ensure unique and stable keys for list elements.
- State Management: Keep state local when possible and avoid unnecessary global state.
7. What is React Concurrent Mode?
Concurrent Mode is an experimental feature that allows React to work on multiple tasks at the same time. It improves the user experience by making React apps more responsive.
Key Features:
- Suspense: Pause rendering until data is loaded.
- Transitions: Mark updates as non-urgent to prioritize user interactions.
Example:
// Non-urgent state update
setFilter(value);
});
8. What is the difference between React.memo and useMemo?
React.memo: Prevents a component from re-rendering if its props have not changed. It’s a higher-order component.
Example:
useMemo: Memoizes a value to avoid re-calculating it unless its dependencies change. It’s a hook used inside functional components.
Example:
9. How do you handle state management in large React applications?
State management approaches:
- React Context API: Suitable for moderate state sharing.
- Third-party libraries:
- Redux: Centralized state management with strict state update rules.
- MobX: More flexible, uses observables for state management.
- Recoil: A React-specific library that offers fine-grained state management.
- Hooks: Local state management using useState and useReducer.
- Query Libraries: For server-state management, libraries like React Query or SWR are ideal.
10. What is hydration in React?
Attaching event listeners is the process of hydration. It makes a server-rendered HTML page interactive. It is used in SSR applications where the server sends pre-rendered HTML to the client, and React “hydrates” it by attaching the JavaScript logic.
Example:
Bonus Topics for Further Exploration:
- Custom Hooks: Writing reusable logic encapsulated in a function.
- Error Boundaries: Handling errors in components using componentDidCatch.
- React Suspense for Data Fetching: Simplifying asynchronous data fetching with Suspense.
- Dynamic Theming: Implementing themes using Context API or CSS-in-JS libraries.
- Testing in React: Unit testing with Jest and React Testing Library.
Conclusion
ReactJS is a very powerful library used to create dynamic and efficient user interfaces with ease. The only thing for beginners would be very important to know core concepts which are components, state, props, hooks, the Virtual DOM. So before going to interviews with such primary concepts, it would make a huge difference, better prepared for any question at all they throw at you.
From simple user interface development to complex single-page applications, React’s flexibility, performance optimization, and reusable component structure make it a go-to tool for developers. Get ready confidently for your next ReactJS interview with these fundamental concepts and show mastery of this popular library.
