100+ React Developer Interview Questions and Answers

Swap two numbers 'a' and 'b' and output the swapped values.

• Create a temporary variable to store one of the numbers

• Assign the value of 'a' to 'b' and the temporary variable to 'a'

• Output the swapped values as 'b' followed by 'a'

Maximize the number of vaccines administered on a specific day while adhering to certain rules.

• Given n days, maxVaccines available, and a specific dayNumber, distribute vaccines to maximize on dayNumber

• Administer positive number of vaccines each day with a difference of 1 between consecutive days

• Ensure sum of vaccines distributed does not exceed maxVaccines

• Output the maximum number of vaccines administered on dayNumber for each test case

The task is to find all distinct triplets in an array that sum up to a specified number.

• Iterate through the array and use nested loops to find all possible triplets.

• Keep track of the triplets that sum up to the target number.

• Handle cases where no triplet exists for a given target sum.

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

Given heights of buildings, bricks, and ladders, find the farthest building Ninja can reach using them optimally.

• Iterate through the buildings and calculate the difference in heights between consecutive buildings.

• Use bricks or ladders to cover the height difference when needed.

• Keep track of the farthest building Ninja can reach based on available bricks and ladders.

• Return the index of the farthest building Ninja can reach.

Hooks are a feature introduced in React 16.8 that allow developers to use state and other React features in functional components.

• useState() - for managing state in functional components

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

• useContext() - for accessing context in functional components

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

• useCallback() - for memoizing functions in functional components

• useMemo() - for memoizing...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

Promises in JavaScript represent the eventual completion (or failure) of an asynchronous operation.

• Promises are objects that represent the eventual completion (or failure) of an asynchronous operation.

• They have three states: pending, fulfilled, or rejected.

• A pending promise is one that hasn't been fulfilled or rejected yet.

• A fulfilled promise is one that has been successful.

• A rejected promise is one that has encountered an error.

• Promises help in handling asynchronous operatio...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

Refs are a way to access DOM nodes or React components directly. They can be used to get input values.

• Refs provide a way to access DOM nodes or React components directly.

• They are commonly used to get input values or trigger imperative animations.

• Refs can be created using the `createRef()` method or by using a callback function.

• To get the value of an input using refs, you can assign a ref to the input element and access its value property.

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 };

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

TRUNCATE is a DDL command that removes all records from a table, while DELETE is a DML command that removes specific records.

• TRUNCATE is faster than DELETE as it does not log individual row deletions.

• TRUNCATE resets the auto-increment value of the table, while DELETE does not.

• TRUNCATE cannot be rolled back, but DELETE can be rolled back using transactions.

• TRUNCATE does not trigger any triggers associated with the table, while DELETE does.

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.

Top-level directories in Redux should be structured based on functionality and feature modules.

• Separate directories for actions, reducers, and components

• Group related functionality together in separate directories

• Use feature modules to encapsulate related actions, reducers, and components

• Example: 'actions', 'reducers', 'components', 'utils', 'constants'

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

Class components are ES6 classes that extend from React.Component and have access to state and lifecycle methods, while functional components are simple functions that take props as arguments and return JSX.

• Class components are defined using ES6 classes and extend from React.Component

• Functional components are simple functions that take props as arguments and return JSX

• Class components have access to state and lifecycle methods, while functional components do not

• Functional com...read more

Stateless components do not have internal state, while stateful components have internal state.

• Stateless components are functional components that do not have internal state.

• Stateful components are class components that have internal state.

• Stateless components are simpler and easier to test.

• Stateful components are more complex and can hold and update internal state.

• Example: Stateless component - const Button = () => <button>Click me</button>

• Example: Stateful component - class...read more

mapStateToProps() is used to access the Redux state in a component, while mapDispatchToProps() is used to dispatch actions to update the state.

• mapStateToProps() is used to access the Redux state and return data as props for a component.

• mapDispatchToProps() is used to dispatch actions to update the Redux state.

• mapStateToProps() is a function that takes the current state as an argument and returns an object with props that will be passed to the component.

• mapDispatchToProps() is...read more

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.