CGI Group React Js Frontend Developer Interview Questions and Answers

Q1. What is prop drilling
Ans. 
Prop drilling is the process of passing props down multiple levels of nested components in React.
• Prop drilling occurs when a prop needs to be passed through multiple levels of components that do not need the prop themselves.

• It can lead to unnecessary passing of props through intermediate components, making the code harder to maintain.

• To avoid prop drilling, you can use Context API, Redux, or React's useContext and useR...read more
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Q2. What is hoisting
Ans. 
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during the compilation phase.
• Variable declarations are hoisted to the top of their scope, but not their assignments.

• Function declarations are fully hoisted, meaning they can be called before they are declared.

• Hoisting can lead to unexpected behavior if not understood properly.
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Q1. Explain useContext hook
Ans. 
useContext hook allows components to access data from a context without passing props down manually
• useContext hook is used to consume a context created by React.createContext

• It takes the context object as an argument and returns the current context value for that context

• It allows components to subscribe to context changes and re-render when the context value changes
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Online coding round which included question from React and js

Q1. Common questions from js
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Q2. Coding to do in react
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Q1. Managerial questions
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Q2. Questions based on resume
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Q1. Salary discussion
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Q1. Tell be about yourself
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Q2. Skills questions
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Q1. Online Quiz in My Skills
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Q1. Tell me about your self
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What is react , feature, axios, API

Redux Redux toolkit Redux thunk

Q1. What is Redux ? How we are using it in real time
Ans. 
Redux is a predictable state container for JavaScript apps.
• Redux is a state management library for JavaScript applications.

• It helps in managing the state of the application in a predictable way.

• Redux stores the entire state of the application in a single immutable object.

• Actions are dispatched to update the state, and reducers specify how the state changes in response to actions.

• Redux is commonly used with React to man...read more
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Q2. What is Virtual Dom ? and its work in reactjs
Ans. 
Virtual DOM is a lightweight copy of the actual DOM in memory, used by React to improve performance by minimizing actual DOM manipulations.
• Virtual DOM is a concept where a lightweight copy of the actual DOM is kept in memory.

• React uses Virtual DOM to improve performance by minimizing actual DOM manipulations.

• When state changes in a React component, a new Virtual DOM representation is created and compared with the previ...read more
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Q3. What is difference in usememo and react.memo
Ans. 
useMemo is used for memoizing expensive calculations, while React.memo is used for memoizing functional components.
• useMemo is a hook used to memoize expensive calculations and only recompute the value when the dependencies change.

• React.memo is a higher order component used to memoize functional components and prevent unnecessary re-renders.

• useMemo is typically used for optimizing performance by caching values, while Re...read more
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Q4. Difference between map and forEach ?
Ans. 
map returns a new array with modified elements, forEach does not return anything and just iterates over the array.
• map returns a new array with the results of calling a provided function on every element in the array.

• forEach executes a provided function once for each array element without returning anything.

• Example: const numbers = [1, 2, 3]; const doubled = numbers.map(num => num * 2); // doubled will be [2, 4, 6]; num...read more
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Q5. Find Duplicacy Array
Ans. 
Find duplicates in an array of strings
• Iterate through the array and use a hash map to keep track of the frequency of each element

• If an element is already in the hash map, it is a duplicate
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Q6. What is Currying function ?
Ans. 
Currying is a technique in functional programming where a function with multiple arguments is transformed into a sequence of functions, each taking a single argument.
• Currying helps in creating reusable functions and improving code readability.

• It allows partial application of functions, where some arguments are fixed and others are left to be provided later.

• Example: const add = (a) => (b) => a + b; add(2)(3) will return
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Q1. What is React Query, how is it different from Redux
Ans. 
React Query is a library for managing server state in React applications, while Redux is a state management library for managing client-side state.
• React Query is specifically designed for managing server state, making it easier to fetch, cache, and update data from APIs.

• Redux is a more general-purpose state management library that can be used for managing client-side state in a predictable way.

• React Query provides buil...read more
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Q2. Difference between interface and type
Ans. 
Interface is for defining object shapes and types, while type is for creating aliases for existing types.
• Interface is used for defining the structure of an object in TypeScript.

• Type is used to create aliases for existing types, making code more readable and reusable.

• Interfaces can be extended or implemented, while types can be used to create union types or intersection types.

• Interfaces are open for extension, while typ
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Q1. Write HOC Component ?
Ans. 
A Higher Order Component (HOC) is a function that takes a component and returns a new component with additional functionality.
• HOCs are used to share code between components, add additional features, or modify behavior.

• They are commonly used for tasks like authentication, logging, and data fetching.

• Example: const withAuth = (WrappedComponent) => { return class extends React.Component { render() { return
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Q2. Write custom hook ?
Ans. 
A custom hook is a reusable function that allows you to extract component logic into a separate function.
• Custom hooks start with 'use' keyword.

• Custom hooks can call other hooks if needed.

• Custom hooks can be shared and reused across multiple components.
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Q3. Write closure function
Ans. 
A closure function is a function that has access to its own scope, as well as the outer scope in which it was defined.
• A closure function can access variables from its outer scope even after the outer function has finished executing.

• Closures are commonly used to create private variables in JavaScript.

• Example: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerVar);...read more
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Q1. What is the mechanism of virtual DOM?
Ans. 
Virtual DOM is a lightweight copy of the actual DOM that React uses to improve performance by minimizing direct manipulation of the actual DOM.
• Virtual DOM is a representation of the actual DOM in the form of a lightweight copy.

• When changes are made to the virtual DOM, React compares it with the actual DOM to identify the minimal number of changes needed.

• This process helps in optimizing performance by reducing direct ma...read more
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Q2. What is reconciliation in react?
Ans. 
Reconciliation in React is the process of updating the DOM to match the virtual DOM.
• Reconciliation is the algorithm that React uses to update the UI efficiently.

• It compares the virtual DOM with the previous virtual DOM and only updates the necessary components.

• Reconciliation helps in improving performance by minimizing DOM manipulations.

• Key prop is important for reconciliation to identify which items have changed, adde...read more
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Q3. What are closures?
Ans. 
Closures are functions that have access to variables from their outer scope even after the outer function has finished executing.
• Closures allow functions to access variables from their parent function's scope

• They maintain a reference to the variables they need, even after the parent function has finished executing

• Closures are commonly used in event handlers and callbacks
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Q4. What is hoisting?
Ans. 
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during compilation.
• Hoisting applies to both variable and function declarations.

• Variable declarations are hoisted to the top of their scope but not their assignments.

• Function declarations are fully hoisted, meaning they can be called before they are declared.
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Q5. What are es6 features ? and code output
Ans. 
ES6 features are modern JavaScript syntax enhancements that make code more concise and readable.
• Arrow functions for more concise syntax: const add = (a, b) => a + b;

• Let and const for block-scoped variables: let x = 5; const y = 10;

• Template literals for string interpolation: const name = 'Alice'; console.log(`Hello, ${name}!`);

• Destructuring assignment for easily extracting values from objects or arrays: const { firstNam...read more
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Q6. What is react fiber?
Ans. 
React Fiber is a complete rewrite of React's core algorithm for rendering UI components.
• React Fiber is a reimplementation of the React core algorithm.

• It allows for incremental rendering, prioritizing updates based on their priority level.

• Fiber reconciles the virtual DOM tree in a more efficient and flexible way.

• It enables features like error boundaries, async rendering, and more.

• Fiber was introduced in React version 16
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Q7. How do you implement dark mode in an application?
Ans. 
Implementing dark mode in an application involves changing the color scheme to darker tones for better visibility in low light environments.
• Use CSS variables to define color schemes for light and dark modes

• Toggle between light and dark mode using a state variable in React

• Apply the appropriate color scheme based on the selected mode

• Allow users to switch between light and dark mode using a toggle button
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Q1. What is agile ? What ceremonies does your project follow?
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Q2. What are some bottlenecks you faced in your project?
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Q1. Salary discussion and basic negotiation questions
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