20+ Offtronix Interview Questions and Answers

I have used libraries like React, Redux, Axios, and Moment.js for frontend development.

• React

• Redux

• Axios

• Moment.js

Yes, Redux does not have built-in persistence for state.

• Redux itself does not have built-in persistence for state

• Persisting state in Redux typically involves using middleware like redux-persist or saving state to local storage

• Examples of persisting state in Redux include using redux-persist to save state to local storage or using cookies to store state

We are currently using React version 17.0.2.

• React version 17.0.2 is the latest stable version at the time of this interview.

• It was released on March 22, 2021, and includes improvements like React Hooks, Concurrent Mode, and more.

• We regularly update our dependencies to stay up-to-date with the latest features and improvements.

I have used React versions 16 and 17 in my projects.

• Used React 16 for a project that required class components and lifecycle methods

• Currently using React 17 for a project that focuses on functional components and hooks

States in React are passed down from parent components to child components using props.

• States are passed down as props from parent components to child components

• Parent components can pass states to child components using props

• States can be updated in parent components and passed down to child components for re-rendering

Using useContext to implement increment and decrement feature in React

• Create a context using createContext()

• Define a provider component to wrap the components that need access to the context

• Use useContext() hook in the components to access the context and update the state

• Implement functions for increment and decrement within the context provider

Redux passes states to components through the connect function provided by react-redux library.

• Redux passes states to components by connecting the component to the Redux store using the connect function.

• The connect function takes mapStateToProps as an argument, which maps the state from the Redux store to props of the component.

• The connected component can then access the state from the Redux store as props.

Browsers use various storage options like cookies, local storage, session storage, and IndexedDB.

• Cookies are small pieces of data stored in the browser that can be accessed by both the server and the client.

• Local storage allows storing data without expiration date, and it remains even after the browser is closed.

• Session storage is similar to local storage but data is cleared when the session ends.

• IndexedDB is a low-level API for client-side storage of significant amounts of s...read more

Yes, multiple stores can be created in Redux for managing different parts of the application state.

• Redux allows creating multiple stores, each with its own reducers and actions

• This can be useful for managing different parts of the application state separately

• Example: creating one store for user authentication and another for managing shopping cart

Redux uses the browser's localStorage as its default storage mechanism.

• Redux uses the browser's localStorage API to persist state across sessions.

• The data stored in localStorage is serialized to a string before being saved.

• Developers can also implement custom storage solutions by creating middleware in Redux.

Browsers use various storage options for state management, including cookies, local storage, session storage, and IndexedDB.

• Cookies: small pieces of data stored in the browser that can be accessed by both the server and client

• Local Storage: stores data with no expiration date, and remains after the browser is closed

• Session Storage: stores data for one session only, and is cleared when the browser is closed

• IndexedDB: a low-level API for client-side storage of significant amoun...read more

useSelector and useDispatch hooks are used in React Redux for accessing state and dispatching actions respectively.

• useSelector hook is used to extract data from the Redux store state in a functional component.

• useDispatch hook is used to dispatch actions to the Redux store in a functional component.

• Example: const data = useSelector(state => state.data); dispatch(action());

Redux is a predictable state container for JavaScript apps.

• Single source of truth: The state of your whole application is stored in an object tree within a single store.

• State is read-only: The only way to change the state is to emit an action, an object describing what happened.

• Changes are made with pure functions: To specify how the state tree is transformed by actions, you write pure reducers.

• State changes are made with pure functions: To specify how the state tree is trans...read more

The critical rendering path is the sequence of steps browsers take to convert HTML, CSS, and JavaScript into pixels on the screen.

• Includes steps like parsing HTML, constructing the DOM tree, rendering the CSSOM, combining them into a render tree, layout, and painting.

• Optimizing the critical rendering path involves minimizing render-blocking resources, reducing the number of critical resources, and optimizing the order of resource loading.

• Examples of optimization techniques in...read more

TypeScript is used to add static typing to JavaScript, catching errors at compile time and improving code quality.

• Provides static typing, catching errors at compile time

• Improves code quality and maintainability

• Supports modern JavaScript features like ES6+

• Enhances code editor tooling and IntelliSense

• Facilitates easier refactoring and code navigation

useContext is used for accessing context values in functional components, while useReducer is used for managing state in a more complex way.

• useContext is used for accessing context values without prop drilling

• useReducer is used for managing state in a more complex way by using a reducer function

• useContext is simpler and more straightforward, while useReducer provides more control and flexibility

• Example: useContext can be used to access a theme context value in a component, wh...read more

Infinite scrolling is a design pattern that continuously loads content as the user scrolls down a webpage.

• Infinite scrolling helps to provide a seamless user experience by eliminating the need for pagination.

• It is commonly used in social media feeds, image galleries, and news websites.

• Infinite scrolling can be implemented using JavaScript to fetch and append new content dynamically.

Specificity in CSS determines which style rule is applied to an element when multiple rules have conflicting selectors.

• Specificity is calculated based on the type of selector used in a CSS rule.

• Inline styles have the highest specificity, followed by IDs, classes, and elements.

• Specificity is represented by a four-part value, with each part corresponding to a different type of selector.

• For example, the selector 'div p' has a lower specificity than 'p.class'.

Virtualization is the process of creating a virtual version of a device or resource, such as a server, storage device, network or operating system.

• Virtualization allows multiple virtual instances to run on a single physical machine, maximizing resource utilization.

• Examples of virtualization technologies include VMware, VirtualBox, and Docker.

• Virtualization can improve scalability, flexibility, and efficiency in IT environments.

useContext is a React hook for managing global state within a component tree, while Redux is a standalone state management library for React applications.

• useContext is built into React and is used for managing local state within a component or passing data down through the component tree.

• Redux is a separate library that allows for managing global state across the entire application, with actions and reducers to update and access the state.

• useContext is simpler and more lightw...read more

The code will output 'undefined' and then '10' in the console.

• The variable 'a' is declared after the first console.log statement, so it will be undefined at that point.

• The value of 'a' is assigned as 10 before the second console.log statement, so it will output 10.

The CSS DOM (Document Object Model) is a representation of the structure of a webpage created by CSS styles.

• CSS DOM is a tree-like structure that represents the HTML elements of a webpage along with their CSS styles.

• It allows developers to access and manipulate the styles of HTML elements using CSS properties and selectors.

• Changes made to the CSS DOM dynamically update the visual appearance of the webpage.

• Example: Selecting an element with a specific class and changing its ba...read more

Debouncing and throttling are techniques used in web development to limit the number of times a function is called.

• Debouncing delays the execution of a function until after a specified time has elapsed without any further calls. It is typically used for events like scroll or resize that can fire multiple times in a short period.

• Throttling limits the rate at which a function is called. It ensures that the function is not called more than once in a specified time interval. It i...read more

The output will be false because arrays are reference types and the comparison is based on reference, not values.

• Arrays are reference types in JavaScript, so when comparing two arrays using ===, it checks if they refer to the same object in memory.

• Since the two empty arrays [] are separate instances in memory, the comparison will result in false.

A program in React to display a countdown timer using state with user control and timer setting

• Create a React component with state variables for countdown time and user input

• Implement functions to start, pause, reset, and set the countdown timer

• Use setInterval to update the countdown timer every second

• Allow user input to set the countdown timer duration

State and props are managed in React by using setState and passing data through props. Data can be transferred from child to parent using callback functions.

• Manage state using setState method in React

• Pass data from parent to child components using props

• Transfer data from child to parent components by using callback functions

Closures and higher order functions are important concepts in JavaScript for creating modular and reusable code.

• Closures are functions that have access to variables from their outer scope even after the outer function has finished executing.

• Higher order functions are functions that can take other functions as arguments or return functions as results.

• Example of closure: function outerFunction() { let outerVar = 'I am outer'; return function innerFunction() { console.log(outerV...read more

React class components have lifecycle methods that allow developers to run code at specific points in a component's lifecycle.

• ComponentDidMount() is called after the component is rendered for the first time.

• ComponentDidUpdate() is called after the component's state or props change.

• ComponentWillUnmount() is called before the component is removed from the DOM.