100+ React Developer Interview Questions and Answers

To connect a component to Redux store, we use the connect function from the react-redux library.

• Import the connect function from react-redux.

• Use the connect function to wrap the component and connect it to the Redux store.

• The connect function takes two parameters: mapStateToProps and mapDispatchToProps.

• mapStateToProps is a function that maps the state from the Redux store to the component's props.

• mapDispatchToProps is an optional function that maps the dispatch function to th...read more

Answer to a React Developer interview question about class and functional components and passing props.

• Class component: class MyComponent extends React.Component {}

• Functional component: const MyComponent = (props) => {}

• Passing props from parent to child:

To avoid re-rendering, use shouldComponentUpdate or React.memo

• Use shouldComponentUpdate to compare current and next props/state

• Use React.memo to memoize functional components

• Use useMemo to memoize expensive computations

• Use useCallback to memoize event handlers

• Use PureComponent for class components

Functional components are simpler and easier to test, while class components have more features and lifecycle methods.

• Functional components are written as JavaScript functions, while class components are written as ES6 classes.

• Functional components are stateless and do not have their own internal state, while class components can have state.

• Functional components do not have lifecycle methods, while class components have lifecycle methods like componentDidMount and componentDi...read more

Higher Order Functions are functions that take other functions as arguments or return functions as their results.

• Higher Order Functions can be used to create reusable code by abstracting common functionality into a separate function.

• They can also be used to implement functional programming concepts like currying and composition.

• Example: Array.prototype.map() is a higher order function that takes a callback function as an argument and applies it to each element of an array, re...read more

Event Loop is a mechanism that allows JavaScript to handle asynchronous operations.

• Event Loop is a continuous process that checks the Event Queue and moves events to the Event Stack.

• Event Queue holds all the events that are waiting to be processed.

• Event Stack holds the events that are currently being processed.

• When the Event Stack is empty, the Event Loop checks the Event Queue for new events.

• JavaScript uses Event Loop to handle asynchronous operations like setTimeout(), setI...read more

Context API is a way to share data between components without passing props down manually.

• Context API provides a way to pass data through the component tree without having to pass props down manually at every level.

• It is useful for sharing data that needs to be accessed by many components at different levels of the tree.

• Initial state can be set in Context API using the createContext() function.

• There is no need to have an initial state in Context API, but it can be useful in s...read more

Error handling in React can be done using try-catch blocks, error boundaries, and handling asynchronous errors.

• Use try-catch blocks to handle synchronous errors

• Use error boundaries to catch errors in child components

• Handle asynchronous errors using promises or async/await

• Use third-party libraries like Sentry or Bugsnag for better error tracking

• Display user-friendly error messages to improve user experience

Minimum coverage for an app and code pushed to production/master branch while deployment.

• Minimum coverage for an app depends on the project requirements and complexity.

• Generally, a coverage of 80% or higher is considered good.

• Code pushed to production/master branch should be thoroughly tested and reviewed.

• Continuous integration and deployment can help automate this process.

• Examples of tools for code coverage include Jest, Istanbul, and Enzyme.

Export types in ReactJS allow components, functions, and variables to be accessed and used in other files.

• Exporting a component allows it to be imported and used in other files

• Exporting a function allows it to be imported and used in other files

• Exporting a variable allows it to be imported and used in other files

API URLs can be hidden by using environment variables and managing different URLs for different staging environments.

• Use environment variables to store API URLs

• Create separate environment files for different staging environments

• Set the appropriate API URL in the environment file for each staging environment

• Access the API URL from the environment variable in the code

Yes, it is mandatory for Reducer to have an initial state.

• Initial state defines the starting point of the state tree.

• It helps in defining the shape of the state tree.

• It is used to set default values for the state properties.

• It is also used to reset the state to its initial values.

• Example: const initialState = { count: 0 };

• Example: const initialState = { name: '', age: 0 };

The states of a promise object are pending, fulfilled, or rejected.

• A promise starts in the pending state.

• When a promise is resolved, it transitions to the fulfilled state.

• If a promise encounters an error, it transitions to the rejected state.

• Once a promise is settled (fulfilled or rejected), it cannot transition to any other state.

React is a JavaScript library used for building user interfaces.

• React allows for reusable components and efficient rendering.

• It uses a virtual DOM to update only the necessary parts of the UI.

• React is popular for its simplicity and flexibility.

• Examples of companies using React include Facebook, Instagram, and Airbnb.

Redux-Saga is a middleware for Redux that handles side effects. Actions can be dispatched from components using mapDispatchToProps.

• Redux-Saga is used to handle side effects like API calls and asynchronous actions.

• It uses generator functions to make asynchronous code look synchronous.

• Sagas listen for specific actions and perform tasks based on those actions.

• To dispatch actions from components, use mapDispatchToProps to connect the component to the Redux store.

• Then call the act...read more

React re-renders just the part of the page that has changed, thanks to its virtual DOM.

• React uses a virtual DOM to track changes in the UI.

• When a component's state or props change, React compares the virtual DOM with the real DOM and updates only the necessary parts.

• This approach improves performance by minimizing the number of DOM manipulations.

• React's diffing algorithm efficiently determines the minimal set of changes needed to update the UI.

No, const does not make the value immutable.

• const only makes the variable itself immutable, not the value it holds.

• For objects and arrays, const prevents reassignment but allows mutation of properties or elements.

• To make a value truly immutable, you can use Object.freeze() or other techniques.

Promises are a way to handle asynchronous operations in JavaScript. Promise.all is used to execute multiple promises concurrently.

• Promises represent a value that may not be available yet

• They have three states: pending, fulfilled, and rejected

• Promise.all takes an array of promises and returns a new promise that resolves when all promises in the array have resolved

• If any promise in the array is rejected, the returned promise is rejected with the reason of the first promise that...read more

Hooks are functions that allow you to use state and other React features without writing a class.

• useState is a hook that allows you to add state to functional components.

• useEffect is a hook that allows you to perform side effects in functional components.

• Hooks can only be used in functional components.

• Hooks must be called at the top level of a functional component.

• Hooks can be used to replace lifecycle methods in class components.

Let, Const, and Var are used to declare variables in JavaScript with different scoping and reassignment abilities.

• Var has function scope and can be redeclared and reassigned.

• Let has block scope and can be reassigned but not redeclared.

• Const has block scope and cannot be reassigned or redeclared.

Optimizing a React app involves reducing bundle size, using lazy loading, and optimizing rendering performance.

• Reduce bundle size by code splitting and using dynamic imports

• Use lazy loading to load components only when needed

• Optimize rendering performance by using shouldComponentUpdate and PureComponent

• Use React.memo to memoize functional components

• Avoid unnecessary re-renders by using useMemo and useCallback

• Use performance profiling tools like React DevTools and Chrome DevTo...read more

Hooks are functions that allow you to use state and other React features without writing a class.

• useState() - for managing state in functional components

• useEffect() - for performing side effects in functional components

• useContext() - for consuming context in functional components

• useReducer() - for managing complex state in functional components

• useCallback() - for memoizing functions in functional components

• useMemo() - for memoizing values in functional components

• useRef() - fo...read more

Lazy Loading and Suspense are techniques used to improve performance by loading components and data only when needed.

• Lazy Loading delays the loading of non-critical resources until they are needed, reducing initial load time.

• Suspense allows components to wait for data to load before rendering, improving user experience.

• Lazy Loading and Suspense can be used together to optimize performance and user experience.

• Example: A website with a large image gallery can use Lazy Loading t...read more

ES6 features: Spread Operator, Rest Parameter, Object Destructuring

• Spread Operator: allows an iterable to be expanded into individual elements

• Rest Parameter: allows a function to accept an indefinite number of arguments as an array

• Object Destructuring: allows extracting properties from an object and assigning them to variables

Responsiveness in app can be achieved using CSS media queries and responsive design principles.

• Use CSS media queries to apply different styles based on screen sizes

• Design the app layout to be flexible and adapt to different screen sizes

• Use responsive units like percentages and viewport units for sizing elements

• Test the app on different devices and screen sizes to ensure responsiveness

The 'this' keyword in JS refers to the object that is currently executing the code.

• The value of 'this' depends on how a function is called.

• In a method, 'this' refers to the object that the method belongs to.

• In a regular function, 'this' refers to the global object (window in a browser).

• In an event handler, 'this' refers to the element that triggered the event.

• The value of 'this' can be explicitly set using call(), apply(), or bind() methods.

Custom Hooks are reusable functions in React that allow you to extract logic from components.

• Custom Hooks are functions that start with the word 'use' and can call other Hooks if needed.

• They are used to share stateful logic between components without using higher-order components or render props.

• Custom Hooks can be used to handle common tasks like fetching data, managing form state, or subscribing to events.

• They promote code reusability and make it easier to separate concerns...read more

useState() is a React hook that allows functional components to manage state.

• useState() is used to declare a state variable in a functional component.

• It returns an array with two elements: the current state value and a function to update the state.

• The initial state value is passed as an argument to useState().

• The state can be updated by calling the update function returned by useState().

• useState() can be used multiple times in a component to manage multiple state variables.