Title: Javascript / Interview / SRIQ - Reactjs interview questions - sudheer jonna / Part I - Fundamentals / Chapter 1 - Core React

Front: What is state in React?
Back: State of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.
Let's take an example of User component with message state. Here, useState hook has been used to add state to the User component and it returns an array with current state and function to update it.
import { useState } from 'react';
function User() {
    const [message, setMessage] = useState('Welcome to React world');
    return (
        <div>
            <h1>{message}</h1>
        </div>
    );
}
Whenever React calls your component or access useState hook, it gives you a snapshot of the state for that particular render.
See Class
import React from 'react';
class User extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            message: 'Welcome to React world',
        };
    }
    render() {
        return (
            <div>
                <h1>{this.state.message}</h1>
            </div>
        );
    }
}
State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.

Front: What are Pure Components?
Back: Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized React.memo() API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.
But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.
The syntactic representation of memoized components looks like below,
const MemoizedComponent = memo(SomeComponent, arePropsEqual?);
Below is the example of how child component(i.e., EmployeeProfile) prevents re-renders for the same props passed by parent component(i.e.,EmployeeRegForm).
import { memo, useState } from 'react';
const EmployeeProfile = memo(function EmployeeProfile({ name, email }) {
    return (
        <>
            <p>Name:{name}</p>
            <p>Email: {email}</p>
        </>
    );
});
export default function EmployeeRegForm() {
    const [name, setName] = useState('');
    const [email, setEmail] = useState('');
    return (
        <>
            <label>
                Name:{' '}
                <input value={name} onChange={(e) => setName(e.target.value)} />
            </label>
            <label>
                Email:{' '}
                <input
                    value={email}
                    onChange={(e) => setEmail(e.target.value)}
                />
            </label>
            <hr />
            <EmployeeProfile name={name} />
        </>
    );
}
In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.
In class components, the components extending React.PureComponent instead of React.Component become the pure components. When props or state changes, PureComponent will do a shallow comparison on both props and state by invoking shouldComponentUpdate() lifecycle method.
Note: React.memo() is a higher-order component.

Front: What are the popular React-specific linters?
Back: ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
Another popular plugin is eslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text and tabindex, for example, will not be picked up by regular plugins.

Front: What is the benefit of styles modules?
Back: It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
For example, these styles could be extracted into a separate component:
export const colors = {
    white,
    black,
    blue,
};
export const space = [0, 8, 16, 32, 64];
And then imported individually in other components:
import { space, colors } from './styles';

Front: What are the popular packages for animation?
Back: React Transition Group and React Motion are popular animation packages in React ecosystem.

Front: What are the common folder structures for React?
Back: There are two common practices for React project file structure.
1. Grouping by features or routes:
One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
common/
├─ Avatar.js
├─ Avatar.css
├─ APIUtils.js
└─ APIUtils.test.js
feed/
├─ index.js
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
└─ FeedAPI.js
profile/
├─ index.js
├─ Profile.js
├─ ProfileHeader.js
├─ ProfileHeader.css
└─ ProfileAPI.js
2. Grouping by file type:
Another popular way to structure projects is to group similar files together.
api/
├─ APIUtils.js
├─ APIUtils.test.js
├─ ProfileAPI.js
└─ UserAPI.js
components/
├─ Avatar.js
├─ Avatar.css
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
├─ Profile.js
├─ ProfileHeader.js
└─ ProfileHeader.css

Front: Is it possible to use async/await in plain React?
Back: If you want to use async/await in React, you will need Babel and transform-async-to-generator plugin. React Native ships with Babel and a set of transforms.

Front: What are the exceptions on React component naming?
Back: The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
For example, the below tag can be compiled to a valid component,
     render() {
          return (
            <obj.component/> // `React.createElement(obj.component)`
          )
    }

Front: How to import and export components using React and ES6?
Back: You should use default for exporting the components
import User from "user";
export default function MyProfile {
    return <User type="customer">//...</User>;
}
See Class
import React from 'react';
import User from 'user';
export default class MyProfile extends React.Component {
    render() {
        return <User type='customer'>//...</User>;
    }
}
With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.

Front: How do you apply vendor prefixes to inline styles in React?
Back: React does not apply vendor prefixes automatically. You need to add vendor prefixes manually.
<div
    style={{
        transform: 'rotate(90deg)',
        WebkitTransform: 'rotate(90deg)', // note the capital 'W' here
        msTransform: 'rotate(90deg)', // 'ms' is the only lowercase vendor prefix
    }}
/>

Front: How to add Google Analytics for React Router?
Back: Add a listener on the history object to record each page view:
history.listen(function (location) {
    window.ga('set', 'page', location.pathname + location.search);
    window.ga('send', 'pageview', location.pathname + location.search);
});

Front: When to use a Class Component over a Function Component?
Back: After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.
But even there are two reasons to use Class components over Function components.
1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
2. In older versions, If the component needs state or lifecycle methods then you need to use class component.
So the summary to this question is as follows:
Use Function Components:
- If you don't need state or lifecycle methods, and your component is purely presentational.
- For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.
Use Class Components:
- If you need to manage state or use lifecycle methods.
- In scenarios where backward compatibility or integration with older code is necessary.
Note: You can also use reusable react error boundary third-party component without writing any class. i.e, No need to use class components for Error boundaries.
The usage of Error boundaries from the above library is quite straight forward.
Note when using react-error-boundary: ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a "use client"; directive.
'use client';
import { ErrorBoundary } from 'react-error-boundary';
<ErrorBoundary fallback={<div>Something went wrong</div>}>
    <ExampleApplication />
</ErrorBoundary>;

Front: How can we find the version of React at runtime in the browser?
Back: You can use React.version to get the version.
const REACT_VERSION = React.version;
ReactDOM.render(
    <div>{`React version: ${REACT_VERSION}`}</div>,
    document.getElementById('app'),
);

Front: How to focus an input element on page load?
Back: You need to use useEffect hook to set focus on input field during page load time for functional component.
import React, { useEffect, useRef } from 'react';
const App = () => {
    const inputElRef = useRef(null);
    useEffect(() => {
        inputElRef.current.focus();
    }, []);
    return (
        <div>
            <input defaultValue={"Won't focus"} />
            <input ref={inputElRef} defaultValue={'Will focus'} />
        </div>
    );
};
ReactDOM.render(<App />, document.getElementById('app'));
See Class
You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:
class App extends React.Component {
    componentDidMount() {
        this.nameInput.focus();
    }
    render() {
        return (
            <div>
                <input defaultValue={"Won't focus"} />
                <input
                    ref={(input) => (this.nameInput = input)}
                    defaultValue={'Will focus'}
                />
            </div>
        );
    }
}
ReactDOM.render(<App />, document.getElementById('app'));

Front: Why you can't update props in React?
Back: The React philosophy is that props should be immutable(read only) and top-down. This means that a parent can send any prop values to a child, but the child can't modify received props.

Front: How to pretty print JSON with React?
Back: We can use <pre> tag so that the formatting of the JSON.stringify() is retained:
const data = { name: "John", age: 42 };
function User {
    return <pre>{JSON.stringify(data, null, 2)}</pre>;
}
const container = createRoot(document.getElementById("container"));
container.render(<User />);
See Class
const data = { name: 'John', age: 42 };
class User extends React.Component {
    render() {
        return <pre>{JSON.stringify(data, null, 2)}</pre>;
    }
}
React.render(<User />, document.getElementById('container'));

Front: How to re-render the view when the browser is resized?
Back: You can use the useState hook to manage the width and height state variables, and the useEffect hook to add and remove the resize event listener. The [] dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.
import React, { useState, useEffect } from 'react';
function WindowDimensions() {
    const [dimensions, setDimensions] = useState({
        width: window.innerWidth,
        height: window.innerHeight,
    });
    useEffect(() => {
        function handleResize() {
            setDimensions({
                width: window.innerWidth,
                height: window.innerHeight,
            });
        }
        window.addEventListener('resize', handleResize);
        return () => window.removeEventListener('resize', handleResize);
    }, []);
    return (
        <span>
            {dimensions.width} x {dimensions.height}
        </span>
    );
}
Using Class Component
You can listen to the resize event in componentDidMount() and then update the dimensions (width and height). You should remove the listener in componentWillUnmount() method.
class WindowDimensions extends React.Component {
    constructor(props) {
        super(props);
        this.updateDimensions = this.updateDimensions.bind(this);
    }
    componentWillMount() {
        this.updateDimensions();
    }
    componentDidMount() {
        window.addEventListener('resize', this.updateDimensions);
    }
    componentWillUnmount() {
        window.removeEventListener('resize', this.updateDimensions);
    }
    updateDimensions() {
        this.setState({
            width: window.innerWidth,
            height: window.innerHeight,
        });
    }
    render() {
        return (
            <span>
                {this.state.width} x {this.state.height}
            </span>
        );
    }
}

Front: How to combine multiple inline style objects?
Back: You can use spread operator in regular React:
<button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
    {'Submit'}
</button>
If you're using React Native then you can use the array notation:
<button style={[styles.panel.button, styles.panel.submitButton]}>
    {'Submit'}
</button>

Front: How to use React label element?
Back: If you try to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that attribute and prints a warning to the console.
<label for={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />
Since for is a reserved keyword in JavaScript, use htmlFor instead.
<label htmlFor={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />

Front: Why ReactDOM is separated from React?
Back: The React team worked on extracting all DOM-related features into a separate library called ReactDOM. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, react-native, react-art, react-canvas, and react-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
To build more environments that React can render to, React team planned to split the main React package into two: react and react-dom. This paves the way to writing components that can be shared between the web version of React and React Native.

Front: What is the difference between React and ReactDOM?
Back: The react package contains React.createElement(), React.Component, React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The react-dom package contains ReactDOM.render(), and in react-dom/server we have server-side rendering support with ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().

Front: How to conditionally apply class attributes?
Back: You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
<div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
Instead you need to move curly braces outside (don't forget to include spaces between class names):
<div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
Template strings will also work:
<div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>

Front: How to create components in React?
Back: Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.
1. Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the one and only one parameter and return React elements to render the output:
function Greeting({ message }) {
    return <h1>{`Hello, ${message}`}</h1>;
}
2. Class Components: You can also use ES6 class to define a component. The above function component can be written as a class component:
class Greeting extends React.Component {
    render() {
        return <h1>{`Hello, ${this.props.message}`}</h1>;
    }
}

Front: What is React proptype array with shape?
Back: If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape() as an argument to React.PropTypes.arrayOf().
ReactComponent.propTypes = {
    arrayWithShape: React.PropTypes.arrayOf(
        React.PropTypes.shape({
            color: React.PropTypes.string.isRequired,
            fontSize: React.PropTypes.number.isRequired,
        }),
    ).isRequired,
};

Front: How do you access props in attribute quotes?
Back: React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
<img className='image' src='images/{this.props.image}' />
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
<img className='image' src={'images/' + this.props.image} />
Using template strings will also work:
<img className='image' src={`images/${this.props.image}`} />

Front: How to loop inside JSX?
Back: You can simply use Array.prototype.map with ES6 arrow function syntax.
For example, the items array of objects is mapped into an array of components:
<tbody>
    {items.map((item) => (
        <SomeComponent key={item.id} name={item.name} />
    ))}
</tbody>
But you can't iterate using for loop:
<tbody>
  for (let i = 0; i < items.length; i++) {
    <SomeComponent key={items[i].id} name={items[i].name} />
  }
</tbody>
This is because JSX tags are transpiled into function calls, and you can't use statements inside expressions. This may change thanks to do expressions which are stage 1 proposal.

Front: Are custom DOM attributes supported in React v16?
Back: Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
For example, let's take a look at the below attribute:
<div mycustomattribute={'something'} />
Would render an empty div to the DOM with React v15:
<div />
In React v16 any unknown attributes will end up in the DOM:
<div mycustomattribute="something" />
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.

Front: Why should component names start with capital letter?
Back: If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
function SomeComponent {
  // Code goes here
}
You can define function component whose name starts with lowercase letter, but when it's imported it should have a capital letter. Here lowercase is fine:
function myComponent {
  render() {
    return <div />;
  }
}
export default myComponent;
While when imported in another file it should start with capital letter:
import MyComponent from './myComponent';

Front: What are the Pointer Events supported in React?
Back: Pointer Events provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.
The following event types are now available in React DOM:
1. onPointerDown
2. onPointerMove
3. onPointerUp
4. onPointerCancel
5. onGotPointerCapture
6. onLostPointerCapture
7. onPointerEnter
8. onPointerLeave
9. onPointerOver
10. onPointerOut

Front: What are React Mixins?
Back: Mixins are a way to totally separate components to have a common functionality. Mixins should not be used and can be replaced with higher-order components or decorators.
One of the most commonly used mixins is PureRenderMixin. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:
const PureRenderMixin = require('react-addons-pure-render-mixin');
const Button = React.createClass({
    mixins: [PureRenderMixin],
    // ...
});

Front: What is a switching component?
Back: A switching component is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on page prop:
import HomePage from './HomePage';
import AboutPage from './AboutPage';
import ServicesPage from './ServicesPage';
import ContactPage from './ContactPage';
const PAGES = {
    home: HomePage,
    about: AboutPage,
    services: ServicesPage,
    contact: ContactPage,
};
const Page = (props) => {
    const Handler = PAGES[props.page] || ContactPage;
    return <Handler {...props} />;
};
// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
    page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
};

Front: Do Hooks replace render props and higher order components?
Back: Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.

Front: How to enable production mode in React?
Back: You should use Webpack's DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.

Front: What is the difference between Element and Component?
Back: An Element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other Elements in their props. Creating a React element is cheap. Once an element is created, it cannot be mutated.
The JavaScript representation(Without JSX) of React Element would be as follows:
const element = React.createElement('div', { id: 'login-btn' }, 'Login');
and this element can be simiplified using JSX
<div id="login-btn">Login</div>
The above React.createElement() function returns an object as below:
{
  type: 'div',
  props: {
    children: 'Login',
    id: 'login-btn'
  }
}
Finally, this element renders to the DOM using ReactDOM.render().
Whereas a component can be declared in several different ways. It can be a class with a render() method or it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
const Button = ({ handleLogin }) => (
    <div id={'login-btn'} onClick={handleLogin}>
        Login
    </div>
);
Then JSX gets transpiled to a React.createElement() function tree:
const Button = ({ handleLogin }) =>
    React.createElement(
        'div',
        { id: 'login-btn', onClick: handleLogin },
        'Login',
    );

Front: How you implement Server Side Rendering or SSR?
Back: React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
import ReactDOMServer from 'react-dom/server';
import App from './App';
ReactDOMServer.renderToString(<App />);
This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.

Front: How do you memoize a component?
Back: There are memoize libraries available which can be used on function components.
For example moize library can memoize the component in another component.
import moize from 'moize';
import Component from './components/Component'; // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component);
const Consumer = () => {
    <div>
        {'I will memoize the following entry:'}
        <MemoizedFoo />
    </div>;
};
Update: Since React v16.6.0, we have a React.memo. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.
const MemoComponent = React.memo(function MemoComponent(props) {
    /* render using props */
});
OR;
export default React.memo(MyFunctionComponent);

Front: Why we need to be careful when spreading props on DOM elements?
Back: When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with ...rest operator, so it will add only required props.
For example,
const ComponentA = () => (
    <ComponentB isDisplay={true} className={'componentStyle'} />
);
const ComponentB = ({ isDisplay, ...domProps }) => (
    <div {...domProps}>{'ComponentB'}</div>
);

Front: How do you conditionally render components?
Back: In some cases you want to render different components depending on some state. JSX does not render false or undefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.
const MyComponent = ({ name, address }) => (
    <div>
        <h2>{name}</h2>
        {address && <p>{address}</p>}
    </div>
);
If you need an if-else condition then use ternary operator.
const MyComponent = ({ name, address }) => (
    <div>
        <h2>{name}</h2>
        {address ?
            <p>{address}</p>
        :   <p>{'Address is not available'}</p>}
    </div>
);

Front: What is the impact of indexes as keys?
Back: Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.
{
    todos.map((todo, index) => <Todo {...todo} key={index} />);
}
If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
{
    todos.map((todo) => <Todo {...todo} key={todo.id} />);
}
Note: If you don't specify key prop at all, React will use index as a key's value while iterating over an array of data.

Front: How events are different in React?
Back: Handling events in React elements has some syntactic differences:
1. React event handlers are named using camelCase, rather than lowercase.
2. With JSX you pass a function as the event handler, rather than a string.

Front: How to use styles in React?
Back: The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.
const divStyle = {
    color: 'blue',
    backgroundImage: 'url(' + imgUrl + ')',
};
function HelloWorldComponent() {
    return <div style={divStyle}>Hello World!</div>;
}
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).

Front: How to use innerHTML in React?
Back: The dangerouslySetInnerHTML attribute is React's replacement for using innerHTML in the browser DOM. Just like innerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html object as key and HTML text as value.
In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:
function createMarkup() {
    return { __html: 'First &middot; Second' };
}
function MyComponent() {
    return <div dangerouslySetInnerHTML={createMarkup()} />;
}

Front: What is ReactDOMServer?
Back: The ReactDOMServer object enables you to render components to static markup (typically used on node server). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:
1. renderToString()
2. renderToStaticMarkup()
For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call renderToString to render your root component to a string, which you then send as response.
// using Express
import { renderToString } from 'react-dom/server';
import MyPage from './MyPage';
app.get('/', (req, res) => {
    res.write('<!DOCTYPE html><html><head><title>My Page</title></head><body>');
    res.write('<div id="content">');
    res.write(renderToString(<MyPage />));
    res.write('</div></body></html>');
    res.end();
});

Front: What is the use of react-dom package?
Back: The react-dom package provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
1. render()
2. hydrate()
3. unmountComponentAtNode()
4. findDOMNode()
5. createPortal()

Front: What is JSX?
Back: JSX stands for JavaScript XML and it is an XML-like syntax extension to ECMAScript. Basically it just provides the syntactic sugar for the React.createElement(type, props, ...children) function, giving us expressiveness of JavaScript along with HTML like template syntax.
In the example below, the text inside <h1> tag is returned as JavaScript function to the render function.
export default function App() {
    return <h1 className='greeting'>{'Hello, this is a JSX Code!'}</h1>;
}
If you don't use JSX syntax then the respective JavaScript code should be written as below,
import { createElement } from 'react';
export default function App() {
    return createElement(
        'h1',
        { className: 'greeting' },
        'Hello, this is a JSX Code!',
    );
}
See Class
class App extends React.Component {
    render() {
        return <h1 className='greeting'>{'Hello, this is a JSX Code!'}</h1>;
    }
}
Note: JSX is stricter than HTML

Front: What are the recommended ways for static type checking?
Back: Normally we use PropTypes library (React.PropTypes moved to a prop-types package since React v15.5) for type checking in the React applications. For large code bases, it is recommended to use static type checkers such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.

Front: What are the limitations of React?
Back: Apart from the advantages, there are few limitations of React too,
1. React is just a view library, not a full framework.
2. There is a learning curve for beginners who are new to web development.
3. Integrating React into a traditional MVC framework requires some additional configuration.
4. The code complexity increases with inline templating and JSX.
5. Too many smaller components leading to over engineering or boilerplate.

Front: What are the advantages of React?
Back: Below are the list of main advantages of React,
1. Increases the application's performance with Virtual DOM.
2. JSX makes code easy to read and write.
3. It renders both on client and server side (SSR).
4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
5. Easy to write unit and integration tests with tools such as Jest.

Front: How to apply validation on props in React?
Back: When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It's disabled in production mode due to performance impact. The mandatory props are defined with isRequired.
The set of predefined prop types:
1. PropTypes.number
2. PropTypes.string
3. PropTypes.array
4. PropTypes.object
5. PropTypes.func
6. PropTypes.node
7. PropTypes.element
8. PropTypes.bool
9. PropTypes.symbol
10. PropTypes.any
We can define propTypes for User component as below:
import React from 'react';
import PropTypes from 'prop-types';
class User extends React.Component {
    static propTypes = {
        name: PropTypes.string.isRequired,
        age: PropTypes.number.isRequired,
    };
    render() {
        return (
            <>
                <h1>{`Welcome, ${this.props.name}`}</h1>
                <h2>{`Age, ${this.props.age}`}</h2>
            </>
        );
    }
}
Note: In React v15.5 PropTypes were moved from React.PropTypes to prop-types library.
The Equivalent Functional Component
import React from 'react';
import PropTypes from 'prop-types';
function User({ name, age }) {
    return (
        <>
            <h1>{`Welcome, ${name}`}</h1>
            <h2>{`Age, ${age}`}</h2>
        </>
    );
}
User.propTypes = {
    name: PropTypes.string.isRequired,
    age: PropTypes.number.isRequired,
};

Front: What are stateful components?
Back: If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are either function components with hooks or class components.
Let's take an example of function stateful component which update the state based on click event,
import React, {useState} from 'react';
const App = (props) => {
const [count, setCount] = useState(0);
handleIncrement() {
  setCount(count+1);
}
return (
  <>
    <button onClick={handleIncrement}>Increment</button>
    <span>Counter: {count}</span>
  </>
  )
}
See Class
The equivalent class stateful component with a state that gets initialized in the `constructor`.
class App extends Component {
    constructor(props) {
        super(props);
        this.state = { count: 0 };
    }
    handleIncrement() {
        setState({ count: this.state.count + 1 });
    }
    render() {
        <>
            <button onClick={() => this.handleIncrement}>Increment</button>
            <span>Count: {count}</span>
        </>;
    }
}

Front: What are stateless components?
Back: If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.

Front: What are portals in React?
Back: Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component. When using
CSS transform in a component, its descendant elements should not use fixed positioning, otherwise the layout will blow up.
ReactDOM.createPortal(child, container);
The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.

Front: Why fragments are better than container divs?
Back: Below are the list of reasons to prefer fragments over container DOM elements,
1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
2. Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
3. The DOM Inspector is less cluttered.

Front: What are fragments?
Back: It's a common pattern or practice in React for a component to return multiple elements. Fragments let you group a list of children without adding extra nodes to the DOM.
You need to use either <Fragment> or a shorter syntax having empty tag (<></>).
Below is the example of how to use fragment inside Story component.
function Story({ title, description, date }) {
    return (
        <Fragment>
            <h2>{title}</h2>
            <p>{description}</p>
            <p>{date}</p>
        </Fragment>
    );
}
It is also possible to render list of fragments inside a loop with the mandatory key attribute supplied.
function StoryBook() {
    return stories.map((story) => (
        <Fragment key={story.id}>
            <h2>{story.title}</h2>
            <p>{story.description}</p>
            <p>{story.date}</p>
        </Fragment>
    ));
}
Usually, you don't need to use <Fragment> until there is a need of key attribute. The usage of shorter syntax looks like below.
function Story({ title, description, date }) {
    return (
        <>
            <h2>{title}</h2>
            <p>{description}</p>
            <p>{date}</p>
        </>
    );
}

Front: Why React uses className over class attribute?
Back: The attribute names written in JSX turned into keys of JavaScript objects and the JavaScript names cannot contain dashes or reversed words, it is recommended to use camelCase wherever applicable in JSX code. The attribute class is a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principle reason why React uses className instead of class. Pass a string as the className prop.
render() {
  return <span className="menu navigation-menu">{'Menu'}</span>
}

Front: What are the major features of React?
Back: The major features of React are:
- Uses JSX syntax, a syntax extension of JS that allows developers to write HTML in their JS code.
- It uses Virtual DOM instead of Real DOM considering that Real DOM manipulations are expensive.
- Supports server-side rendering which is useful for Search Engine Optimizations(SEO).
- Follows Unidirectional or one-way data flow or data binding.
- Uses reusable/composable UI components to develop the view.

Front: Does the lazy function support named exports?
Back: No, currently React.lazy function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
Let's take a component file which exports multiple named components,
// MoreComponents.js
export const SomeComponent = /* ... */;
export const UnusedComponent = /* ... */;
and reexport MoreComponents.js components in an intermediate file IntermediateComponent.js
// IntermediateComponent.js
export { SomeComponent as default } from './MoreComponents.js';
Now you can import the module using lazy function as below,
import React, { lazy } from 'react';
const SomeComponent = lazy(() => import('./IntermediateComponent.js'));

Front: What is reconciliation?
Back: Reconciliation is the process through which React updates the Browser DOM and makes React work faster. React use a diffing algorithm so that component updates are predictable and faster. React would first calculate the difference between the real DOM and the copy of DOM (Virtual DOM) when there's an update of components.
React stores a copy of Browser DOM which is called Virtual DOM. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by Diffing Algorithm.
Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called Reconciliation.

Front: How to write comments in React?
Back: The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
Single-line comments:
<div>
    {/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
    {`Welcome ${user}, let's play React`}
</div>
Multi-line comments:
<div>
    {/* Multi-line comments for more than
   one line */}
    {`Welcome ${user}, let's play React`}
</div>

Front: What is children prop?
Back: Children is a prop that allows you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component as children prop.
A simple usage of children prop looks as below,
function MyDiv({ children }){
    return (
      <div>
        {children}
      </div>;
    );
}
export default function Greeting() {
  return (
    <MyDiv>
      <span>{"Hello"}</span>
      <span>{"World"}</span>
    </MyDiv>
  );
}
See Class
const MyDiv = React.createClass({
    render: function () {
        return <div>{this.props.children}</div>;
    },
});
ReactDOM.render(
    <MyDiv>
        <span>{'Hello'}</span>
        <span>{'World'}</span>
    </MyDiv>,
    node,
);
Note: There are several methods available in the legacy React API to work with this prop. These include React.Children.map, React.Children.forEach, React.Children.count, React.Children.only, React.Children.toArray.

Front: What are Higher-Order Components?
Back: A higher-order component (HOC) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.
We call them pure components because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.
const EnhancedComponent = higherOrderComponent(WrappedComponent);
HOC can be used for many use cases:
1. Code reuse, logic and bootstrap abstraction.
2. Render hijacking.
3. State abstraction and manipulation.
4. Props manipulation.

Front: What is Lifting State Up in React?
Back: When several components need to share the same changing data then it is recommended to lift the shared state up to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.

Front: What is the difference between createElement and cloneElement?
Back: JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the object representation of UI. Whereas cloneElement is used to clone an element and pass it new props.

Front: What are uncontrolled components?
Back: The Uncontrolled Components are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
The uncontrolled components will be implemented using the below steps,
1. Create a ref using useRef react hook in function component or React.createRef() in class based component.
2. Attach this ref to the form element.
3. The form element value can be accessed directly through ref in event handlers or componentDidMount for class components
In the below UserProfile component, the username input is accessed using ref.
import React, { useRef } from 'react';
function UserProfile() {
    const usernameRef = useRef(null);
    const handleSubmit = (event) => {
        event.preventDefault();
        console.log('The submitted username is: ' + usernameRef.current.value);
    };
    return (
        <form onSubmit={handleSubmit}>
            <label>
                Username:
                <input type='text' ref={usernameRef} />
            </label>
            <button type='submit'>Submit</button>
        </form>
    );
}
In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.
See Class
class UserProfile extends React.Component {
    constructor(props) {
        super(props);
        this.handleSubmit = this.handleSubmit.bind(this);
        this.input = React.createRef();
    }
    handleSubmit(event) {
        alert('A name was submitted: ' + this.input.current.value);
        event.preventDefault();
    }
    render() {
        return (
            <form onSubmit={this.handleSubmit}>
                <label>
                    {'Name:'}
                    <input type='text' ref={this.input} />
                </label>
                <input type='submit' value='Submit' />
            </form>
        );
    }
}

Front: What are controlled components?
Back: A component that controls the input elements within the forms on subsequent user input is called Controlled Component, i.e, every state mutation will have an associated handler function. That means, the displayed data is always in sync with the state of the component.
The controlled components will be implemented using the below steps,
1. Initialize the state using use state hooks in function components or inside constructor for class components.
2. Set the value of the form element to the respective state variable.
3. Create an event handler to handle the user input changes through useState updater function or setState from class component.
4. Attach the above event handler to form elements change or click events
For example, the name input field updates the user name using handleChange event handler as below,
import React, { useState } from 'react';
function UserProfile() {
    const [username, setUsername] = useState('');
    const handleChange = (e) => {
        setUsername(e.target.value);
    };
    return (
        <form>
            <label>
                Name:
                <input type='text' value={username} onChange={handleChange} />
            </label>
        </form>
    );
}

Front: What is the main goal of React Fiber?
Back: The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.
from documentation
Its main goals are:
1. Ability to split interruptible work in chunks.
2. Ability to prioritize, rebase and reuse work in progress.
3. Ability to yield back and forth between parents and children to support layout in React.
4. Ability to return multiple elements from render().
5. Better support for error boundaries.

Front: What is the history behind React evolution?
Back: The history of ReactJS started in 2010 with the creation of XHP. XHP is a PHP extension which improved the syntax of the language such that XML document fragments become valid PHP expressions and the primary purpose was used to create custom and reusable HTML elements.
The main principle of this extension was to make front-end code easier to understand and to help avoid cross-site scripting attacks. The project was successful to prevent the malicious content submitted by the scrubbing user.
But there was a different problem with XHP in which dynamic web applications require many roundtrips to the server, and XHP did not solve this problem. Also, the whole UI was re-rendered for small change in the application. Later, the initial prototype of React is created with the name FaxJ by Jordan inspired from XHP. Finally after sometime React has been introduced as a new library into JavaScript world.
Note: JSX comes from the idea of XHP

Front: What is React Fiber?
Back: Fiber is the new reconciliation engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.

Front: What is the difference between Shadow DOM and Virtual DOM?
Back: The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.

Front: How Virtual DOM works?
Back: The Virtual DOM works in three simple steps.
1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
2. Then the difference between the previous DOM representation and the new one is calculated.
3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed.

Front: What is Virtual DOM?
Back: The Virtual DOM (VDOM) is an in-memory representation of Real DOM. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called reconciliation.

Front: What is "key" prop and what is the benefit of using it in arrays of elements?
Back: A key is a special attribute you should include when mapping over arrays to render data. Key prop helps React identify which items have changed, are added, or are removed.
Keys should be unique among its siblings. Most often we use ID from our data as key:
const todoItems = todos.map((todo) => <li key={todo.id}>{todo.text}</li>);
When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) => <li key={index}>{todo.text}</li>);
Note:
1. Using indexes for keys is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
2. If you extract list item as separate component then apply keys on list component instead of li tag.
3. There will be a warning message in the console if the key prop is not present on list items.
4. The key attribute accepts either string or number and internally convert it as string type.
5. Don't generate the key on the fly something like key={Math.random()}. Because the keys will never match up between re-renders and DOM created everytime.

Front: What are inline conditional expressions?
Back: You can use either if statements or ternary expressions which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator &&.
<h1>Hello!</h1>;
{
    messages.length > 0 && !isLogin ?
        <h2>You have {messages.length} unread messages.</h2>
    :   <h2>You don't have unread messages.</h2>;
}

Front: What are synthetic events in React?
Back: SyntheticEvent is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including stopPropagation() and preventDefault(), except the events work identically across all browsers. The native events can be accessed directly from synthetic events using nativeEvent attribute.
Let's take an example of BookStore title search component with the ability to get all native event properties
function BookStore() {
    function handleTitleChange(e) {
        console.log('The new title is:', e.target.value);
        // 'e' represents synthetic event
        const nativeEvent = e.nativeEvent;
        console.log(nativeEvent);
        e.stopPropagation();
        e.preventDefault();
    }
    return <input name='title' onChange={handleTitleChange} />;
}

Front: What is the difference between HTML and React event handling?
Back: Below are some of the main differences between HTML and React event handling,
1. In HTML, the event name usually represents in lowercase as a convention:
<button onclick="activateLasers()"></button>
Whereas in React it follows camelCase convention:
<button onClick={activateLasers}>
2. In HTML, you can return false to prevent default behavior:
<a href="#" onclick='console.log("The link was clicked."); return false;' />
Whereas in React you must call preventDefault() explicitly:
function handleClick(event) {
    event.preventDefault();
    console.log('The link was clicked.');
}
3. In HTML, you need to invoke the function by appending ()
Whereas in react you should not append () with the function name. (refer "activateLasers" function in the first point for example)

Front: What is the difference between state and props?
Back: In React, both state and props are plain JavaScript objects and used to manage the data of a component, but they are used in different ways and have different characteristics.
The state entity is managed by the component itself and can be updated using the setter(setState() for class components) function. Unlike props, state can be modified by the component and is used to manage the internal state of the component. i.e, state acts as a component's memory. Moreover, changes in the state trigger a re-render of the component and its children. The components cannot become reusable with the usage of state alone.
On the otherhand, props (short for "properties") are passed to a component by its parent component and are read-only, meaning that they cannot be modified by the own component itself. i.e, props acts as arguments for a function. Also, props can be used to configure the behavior of a component and to pass data between components. The components become reusable with the usage of props.

Front: What are props in React?
Back: Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.
The primary purpose of props in React is to provide following component functionality:
1. Pass custom data to your component.
2. Trigger state changes.
3. Use via this.props.reactProp inside component's render() method.
For example, let us create an element with reactProp property:
<Element reactProp={'1'} />
This reactProp (or whatever you came up with) attribute name then becomes a property attached to React's native props object which originally already exists on all components created using React library.
props.reactProp;
For example, the usage of props in function component looks like below:
import React from 'react';
import ReactDOM from 'react-dom';
const ChildComponent = (props) => {
    return (
        <div>
            <p>{props.name}</p>
            <p>{props.age}</p>
            <p>{props.gender}</p>
        </div>
    );
};
const ParentComponent = () => {
    return (
        <div>
            <ChildComponent name='John' age='30' gender='male' />
            <ChildComponent name='Mary' age='25' geneder='female' />
        </div>
    );
};
The properties from props object can be accessed directly using destructing feature from ES6 (ECMAScript 2015). It is also possible to fallback to default value when the prop value is not specified. The above child component can be simplified like below.
const ChildComponent = ({ name, age, gender = 'male' }) => {
    return (
        <div>
            <p>{name}</p>
            <p>{age}</p>
            <p>{gender}</p>
        </div>
    );
};
Note: The default value won't be used if you pass null or 0 value. i.e, default value is only used if the prop value is missed or undefined value has been passed.
See Class
The Props accessed in Class Based Component as below
import React from 'react';
import ReactDOM from 'react-dom';
class ChildComponent extends React.Component {
    render() {
        return (
            <div>
                <p>{this.props.name}</p>
                <p>{this.props.age}</p>
                <p>{this.props.gender}</p>
            </div>
        );
    }
}
class ParentComponent extends React.Component {
    render() {
        return (
            <div>
                <ChildComponent name='John' age='30' gender='male' />
                <ChildComponent name='Mary' age='25' gender='female' />
            </div>
        );
    }
}

Front: What is React?
Back: React (aka React.js or ReactJS) is an open-source front-end JavaScript library that is used for building composable user interfaces, especially for single-page applications. It is used for handling view layer for web and mobile apps based on components in a declarative approach.
React was created by Jordan Walke, a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.