Basic Level Questions
Q1: What is React?
React is a JavaScript library developed by Facebook for building fast and interactive user interfaces.It follows a component-based architecture, where the user interface is segmented into reusable components.React utilizes a virtual DOM to improve rendering efficiency by updating only the parts of the user interface that have changed.It focuses exclusively on the view layer of an application, making it flexible and easy to integrate with other libraries.React is widely used for creating single-page applications as it improves maintainability, scalability, and the overall user experience.
Q2: What are components in React?
Components serve as the fundamental building blocks of a React application. A component signifies a reusable segment of the user interface, such as a button, form, or an entire page. Components can be either functional or class-based, although functional components are more prevalent in contemporary React. Each component can receive inputs known as props and manage its own state. This modular design simplifies the maintenance, testing, and scaling of applications. By integrating multiple components, developers can efficiently construct complex user interfaces.
Q3: What is JSX?
JSX, which stands for JavaScript XML, is a syntax extension utilized in React. It enables developers to write HTML-like syntax within JavaScript files, enhancing the readability and expressiveness of UI code. Browsers do not directly interpret JSX, so it is transpiled into standard JavaScript using tools such as Babel. Internally, JSX is transformed into React.createElement calls. JSX facilitates the combination of logic and presentation in a single location, thereby boosting developer productivity and code clarity.
Q4: What is Virtual DOM?
The Virtual DOM is an efficient in-memory representation of the actual DOM utilized by React. When a component's state changes, React modifies the Virtual DOM rather than directly altering the real DOM. Subsequently, React employs a method known as reconciliation to compare the updated Virtual DOM with its previous iteration. Only the discrepancies are applied to the real DOM, enhancing performance. This technique reduces expensive DOM operations, leading to quicker and more fluid user interfaces.
Q5: What are props in React?
Props, which is short for properties, serve the purpose of transferring data from a parent component to a child component in React. They are immutable and cannot be altered by the component that receives them. Props facilitate the reusability and dynamism of components by enabling them to present varying data based on the provided values. The use of props guarantees a unidirectional flow of data, simplifying the understanding and debugging of the application. They are crucial in constructing adaptable and maintainable React applications.
Q6: What is state in React?
State is an inherent object in React that is utilized to store and manage data that may change over time. In contrast to props, state is confined to a component and can be modified through the setState function or the useState hook. When the state is updated, React automatically re-renders the component to display the new data. State is frequently employed to handle user input, form values, and UI behavior. Effective state management is vital for developing dynamic and interactive React applications.
Q7: What is the distinction between state and props?
Props serve the purpose of transferring data from parent components to child components, whereas state is utilized for managing data within a component. Props are immutable and cannot be altered by the child component, in contrast, state is mutable and can be modified. Changes in state lead to the re-rendering of the component, while modifications in props originate from updates in the parent. Props facilitate the reusability of components, while state aids in managing dynamic behaviors. Grasping the distinction is essential for constructing accurate React applications.
Q8: What does the useState hook do?
The useState hook is a built-in feature of React that enables functional components to handle state. It returns an array that includes the current state value along with a function to update it. The useState hook allows developers to incorporate stateful behavior without resorting to class components. Upon updating the state, React re-renders the component to reflect these changes. This hook streamlines state management and is extensively utilized in contemporary React development.
Q9: What is the purpose of the useEffect hook?
The useEffect hook is employed to execute side effects within functional components. Side effects encompass data fetching, subscriptions, DOM manipulation, and timers. The useEffect hook is executed after the component has rendered and can be managed through dependency arrays. It serves as a replacement for lifecycle methods such as componentDidMount and componentDidUpdate found in class components. The appropriate application of useEffect guarantees enhanced control over component behavior, thereby improving performance and maintainability.
Q10: What is conditional rendering in React?
In React, conditional rendering enables components to display different UI elements based on specific conditions. This is typically achieved through JavaScript constructs such as if statements, ternary operators, or logical operators. Conditional rendering facilitates the dynamic presentation of content in response to user actions, authentication status, or the state of the application. It enhances the interactivity of applications and their responsiveness to user behavior. The effective implementation of conditional rendering contributes to an improved user experience and greater clarity in the user interface.
Q11: What are controlled components?
Controlled components refer to form elements whose values are governed by the state in React. In this methodology, the value of the input is maintained in the component's state and modified through event handlers. This grants React complete authority over the form data, enabling validation, formatting, and conditional behavior. Controlled components render form management predictable and facilitate easier debugging. They are frequently utilized in contemporary React applications for handling user input.
Q12: What are uncontrolled components?
Uncontrolled components are form elements that handle their own state through the DOM instead of relying on React state. Rather than updating the state with every change, refs are employed to directly access the input values. This method is more straightforward but provides less control compared to controlled components. Uncontrolled components are advantageous for simple forms or when working with non-React libraries. Nevertheless, controlled components are typically favored for intricate form logic.
Q13: What does it mean to lift state up?
Lifting state up refers to a method employed when several components require access to the same data. Rather than maintaining state independently within each component, the state is elevated to their nearest common ancestor. The parent component subsequently provides the state and update functions as props to its child components. This approach guarantees a consistent data flow and synchronization. Lifting state up aids in preserving a single source of truth and enhances the predictability of the application.
Q14: What is the purpose of React Router?
React Router is a widely-used library designed for managing routing in React applications. It enables developers to construct single-page applications featuring multiple views without the need to reload the page. React Router associates URLs with components and facilitates navigation through links. It includes functionalities such as dynamic routes, nested routes, and route parameters. React Router enhances user experience by allowing seamless and rapid navigation within applications.
Q15: What is the Context API?
The Context API serves to share data among components without the necessity of manually passing props at every level. It is particularly beneficial for global data like authentication status, themes, or user preferences. Context offers a Provider and Consumer to oversee and access shared data. Utilizing Context minimizes prop drilling and enhances code readability. Nevertheless, it should be employed judiciously to prevent unnecessary re-renders.
Q16: What does prop drilling mean?
Prop drilling is the practice of transmitting data through several layers of components via props. This can complicate code maintenance and comprehension, particularly in extensive applications. Prop drilling takes place when intermediary components do not require the data but are still obligated to forward it. Utilizing the Context API or state management libraries such as Redux can assist in addressing this issue. Minimizing excessive prop drilling enhances code organization and clarity.
Q17: What is memoization in React?
In React, memoization is a technique for optimizing performance by preventing unnecessary re-renders. React.memo is employed to memoize functional components, ensuring they only re-render when their props change. Hooks like useMemo and useCallback are utilized to memoize values and functions. While memoization can enhance performance in intricate applications, it should be applied judiciously, as overuse may lead to increased code complexity without substantial advantages.
Q18: What is the useCallback hook?
The useCallback hook serves to memoize functions in React. It prevents the recreation of functions on every render unless the dependencies change. This is particularly beneficial when passing functions as props to child components that depend on reference equality. useCallback aids in optimizing performance and minimizing unnecessary re-renders. It is frequently used in conjunction with React.memo for components sensitive to performance.
Q19: What is useMemo hook?
The useMemo hook serves to memoize costly calculations within React. It provides a cached value that is recomputed solely when its dependencies change. This functionality enhances performance by preventing unnecessary recalculations during re-renders. useMemo is particularly beneficial when dealing with intricate computations or substantial data sets. Similar to other optimization hooks, it should be employed judiciously.
Q20: What are keys in React?
Keys are unique attributes utilized in React to distinguish elements within a list. They assist React in monitoring which items have been altered, added, or removed. For optimal rendering efficiency, keys must be unique and stable. The appropriate use of keys enhances performance and mitigates unexpected UI behavior. It is advisable to avoid using indexes as keys when the order of list items may change.
Advanced Level Questions
Q21: What is Redux?
Redux is a library for state management that is commonly used with React to oversee global application state. It adheres to a unidirectional data flow and employs a single store to maintain the entire state. Redux utilizes actions, reducers, and the store to modify state in a consistent manner. It proves advantageous for large applications that involve complex state logic. Despite its power, Redux introduces additional boilerplate and should be implemented only when necessary.
Q22: What does reconciliation mean in React?
Reconciliation refers to the method that React employs to efficiently update the DOM. When a component's state is modified, React generates a new Virtual DOM tree and contrasts it with the previous version. This analysis identifies the least number of alterations necessary to refresh the actual DOM. Reconciliation contributes to enhanced performance and guarantees seamless UI updates. It is a fundamental principle that underpins React’s efficiency.
Q23: What are Higher-Order Components?
Higher-Order Components, abbreviated as HOCs, are functions that accept a component and yield a new, improved component. They facilitate the reuse of component logic across various components. Typical applications include authentication, logging, and data retrieval. HOCs assist in adhering to the DRY principle and augment code reuse. Nevertheless, hooks have become more prevalent for sharing logic in contemporary React.
Q24: What does lazy loading signify in React?
Lazy loading is a strategy employed to load components solely when they are required. React offers React.lazy and Suspense to implement lazy loading. This approach diminishes the initial bundle size and enhances application performance. Lazy loading proves particularly beneficial for extensive applications with numerous routes. It aids in optimizing load times and improving user experience.
Q25: What is code splitting?
Code splitting refers to a performance enhancement strategy that divides a large JavaScript bundle into smaller segments. These segments are loaded as needed rather than all at once. React facilitates code splitting through dynamic imports and lazy loading. This technique enhances the initial loading time and the overall performance of the application. It is frequently utilized alongside React Router for route-based splitting.
Q26: What is server-side rendering?
Server-side rendering, abbreviated as SSR, is a method in which React components are rendered on the server prior to being delivered to the client. This approach enhances the initial page load speed and improves search engine optimization. Frameworks such as Next.js offer built-in support for SSR. SSR is particularly beneficial for applications rich in content and for public-facing websites. However, it introduces additional complexity when compared to client-side rendering.
Q27: What is hydration in React?
Hydration is the procedure of linking event listeners to HTML that has been rendered on the server, on the client side. Once the initial HTML is loaded, React takes control and makes the page interactive. Hydration enables React to utilize existing markup instead of re-rendering everything from scratch. This concept is crucial in applications that are server-side rendered. Effective hydration guarantees smooth transitions and optimal performance.
Q28: What are React fragments?
React fragments are a feature that enables developers to group several elements without introducing additional nodes into the DOM. They are particularly beneficial when a component is required to return multiple elements at the same hierarchical level. By utilizing fragments, the structure of the DOM is enhanced, and unnecessary wrapper elements are minimized. Fragments can be implemented using either the Fragment component or shorthand syntax. This approach aids in maintaining clean and efficient markup.
Q29: What is error boundary?
Error boundaries refer to React components designed to intercept JavaScript errors within their child component hierarchy. They serve to prevent the entire application from crashing by displaying a fallback user interface instead. Error boundaries are capable of catching errors that occur during rendering, within lifecycle methods, and in constructors. Their implementation enhances the stability of the application and improves the overall user experience. It is important to note that error boundaries must be created using class components.
Q30: What is strict mode in React?
Strict Mode is a tool utilized during development in React that assists in identifying potential issues within an application. It activates supplementary checks and warnings for deprecated APIs and unsafe lifecycle methods. It is important to highlight that Strict Mode does not influence production builds. This mode promotes best practices and prepares applications for forthcoming updates in React. The adoption of Strict Mode contributes to improved code quality and ensures long-term maintainability.
Q31: What is concurrent rendering?
Concurrent rendering refers to a feature in React that permits the interruption and resumption of rendering tasks. This capability enhances responsiveness by prioritizing critical updates, such as user interactions. Concurrent rendering facilitates smoother transitions in the user interface and optimizes performance. It serves as the underlying mechanism for functionalities like Suspense and automatic batching. This methodology enables React to manage complex applications with greater efficiency.
Q32: What is automatic batching?
Automatic batching is a React feature that consolidates multiple state updates into a single rendering process. This consolidation enhances performance by minimizing unnecessary re-renders. In the latest versions of React, batching is effective across event handlers, promises, and timeouts. Automatic batching streamlines code and boosts performance without requiring extra effort. It represents a notable advancement in the management of state updates.
Q33: What is forwardRef?
The forwardRef API in React is utilized to transfer refs from a parent component to a child component. This functionality allows for direct access to DOM elements or child components. forwardRef proves beneficial when creating reusable components, such as input fields or custom controls. It enhances flexibility and compatibility with third-party libraries. The appropriate application of forwardRef significantly improves the reusability of components.
Q34: What does the useRef hook do?
The useRef hook facilitates the creation of mutable references that remain consistent across renders. It is frequently utilized to access DOM elements or to retain mutable values without causing re-renders. Compared to state, useRef offers enhanced performance for non-UI data. It is advantageous for managing focus, animations, and previous values. useRef provides improved control over component behavior.
Q35: What is the purpose of the useReducer hook?
The useReducer hook serves as an alternative to useState for handling intricate state logic. It employs a reducer function to modify state in response to actions. useReducer is particularly beneficial when state transitions are complicated or involve several values. It enhances both readability and predictability. This hook is frequently used alongside the Context API.
Q36: What does tree shaking mean?
Tree shaking is a technique for optimizing builds by eliminating unused code from the final bundle. This process aids in decreasing bundle size and enhancing application performance. Tree shaking operates with ES module syntax and contemporary bundlers such as Webpack. React applications gain from tree shaking when libraries provide modular code. This method leads to quicker load times and improved efficiency.
Q37: What is the key role of reconciliation?
Keys are essential during reconciliation as they assist React in identifying which elements have undergone changes. Utilizing appropriate keys enables React to efficiently reuse components and prevent unnecessary re-renders. The use of incorrect keys may result in performance degradation and UI errors. It is important for keys to be unique and stable across renders. Employing the correct keys guarantees predictable behavior and optimized updates.
Q38: What constitutes a pure component?
A Pure Component in React performs a shallow comparison of props and state to ascertain the need for re-rendering. This mechanism aids in enhancing performance by avoiding unnecessary updates. Pure components can be utilized as React.PureComponent or through React.memo for functional components. They are particularly beneficial in performance-critical sections of applications. Nonetheless, they necessitate careful management of mutable data.
Q39: What is meant by synthetic event?
Synthetic events serve as React’s abstraction over native browser events. They offer a uniform interface across various browsers. By pooling event objects, synthetic events enhance compatibility and performance. Developers can interact with event properties in the same manner as with native events. This abstraction streamlines event handling and guarantees predictable behavior within React applications.
Q40: What is React Fiber?
React Fiber serves as the internal reconciliation engine that was introduced in React 16. It facilitates incremental rendering and enhances the prioritization of updates. Fiber significantly boosts performance and responsiveness, particularly for intricate user interfaces. It enables React to pause, resume, and abort rendering tasks. Fiber is the cornerstone for contemporary features such as concurrent rendering.
Q41: What is suspense in React?
Modern interfaces often depend on data that is not immediately available. React provides a mechanism that allows components to pause their display until required resources are ready. During this waiting period, a temporary interface can be shown to the user. This approach avoids manual loading checks scattered across components. It simplifies how asynchronous behavior is handled in complex interfaces.
Q42: What is portal in React?
Some interface elements do not fit well within the normal component hierarchy. React allows such elements to be placed elsewhere in the document while still behaving as part of the application. This is useful when visual layering or positioning becomes difficult. Interaction handling continues to work as expected. Designers gain greater freedom without breaking application structure.
Q43: What is the reconciliation algorithm?
Updating the entire browser interface on every change would be inefficient. React avoids this by comparing previous and current representations of the interface. Only the differences are applied to the actual screen. This comparison process follows internal rules designed for speed. Understanding this behavior helps developers avoid unnecessary rendering work.
Q44: What is the concept of functional programming in React?
React encourages building interfaces using predictable logic units that focus on input and output rather than hidden side effects. Components describe how the interface should look based on current data. Changes are handled by producing new values instead of altering existing ones. This approach improves reliability and reasoning. Hooks strengthen this pattern by organizing logic around behavior.
Q45: What does immutable state mean?
State in React is expected to change in a controlled manner. Instead of modifying existing values, updates are made by creating new versions. This makes changes easier to track and reason about. React can identify updates more reliably using this approach. Debugging becomes simpler as past values remain untouched.
Q46: What is hydration mismatch?
Applications that render content on the server and then activate it in the browser must produce consistent results. A mismatch occurs when the browser-generated output does not align with what was initially sent. This can lead to warnings or unstable visuals. Differences often arise from environment-specific logic. Keeping rendering behavior synchronized avoids such issues.
Q47: What is event delegation in React?
Handling individual event listeners for every element would be inefficient. React centralizes event handling by managing interactions at a higher level. This reduces memory usage and simplifies internal management. Developers interact with a unified event system. The result is consistent behavior across browsers and components.
Q48: What is one-way data binding?
In React, information flows in a clearly defined direction. Parent components pass data downward, while updates are requested through explicit actions. This structure avoids unexpected side effects. Tracing how data changes becomes straightforward. As applications grow, this clarity becomes increasingly valuable.
Q49: What is React DevTools?
Debugging complex interfaces requires insight into component behavior. React DevTools provide a way to inspect how components are structured and how data moves between them. Developers can observe updates and identify inefficiencies. This visibility speeds up troubleshooting. It becomes an essential aid during development.
Q50: Why is React popular?
React gained adoption by offering a practical balance between performance and flexibility. Its approach to building interfaces scales well as applications grow. Developers benefit from reusable structures and predictable updates. A strong ecosystem supports a wide range of use cases. These factors together make it a preferred choice for modern interface development.