Interview question and answer – Part 5

How to debug forwardRefs in DevTools?
React.forwardRef accepts a render function as parameter and DevTools uses this function to determine what to display for the ref forwarding component.

For example, If you don’t name the render function or not using displayName property then it will appear as ”ForwardRef” in the DevTools,

const WrappedComponent = React.forwardRef((props, ref) => { return <LogProps {…props} forwardedRef={ref} />;});

But If you name the render function then it will appear as ”ForwardRef(myFunction)”
const WrappedComponent = React.forwardRef(function myFunction(props, ref) { return <LogProps {...props} forwardedRef={ref} />;});

As an alternative, You can also set displayName property for forwardRef function,

When component props defaults to true?
If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.

For example, below expressions are equivalent,

Note: It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, {name} which is short for {name: name})

What is NextJS and major features of it?

js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,

1. Server-rendered by default
2. Automatic code splitting for faster page loads
3. Simple client-side routing (page based)
4. Webpack-based dev environment which supports (HMR)
5. Able to implement with Express or any other Node.js HTTP server
6. Customizable with your own Babel and Webpack configurations

How do you pass an event handler to a component?
You can pass event handlers and other functions as props to child components. It can be used in child component as below,

Is it good to use arrow functions in render methods?
Yes, You can use. It is often the easiest way to pass parameters to callback functions. But you need to optimize the performance while using it.
class Foo extends Component { handleClick() { console.log("Click happened"); } render() { return <button onClick={() => this.handleClick()}>Click Me</button>; }}

Note: Using an arrow function in render method creates a new function each time the component renders, which may have performance implications

How to prevent a function from being called multiple times?
If you use an event handler such as onClick or onScroll and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,

1. Throttling: Changes based on a time based frequency. For example, it can be used using _.throttle lodash function
2. Debouncing: Publish changes after a period of inactivity. For example, it can be used using _.debounce lodash function
3. RequestAnimationFrame throttling: Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function

How JSX prevents Injection Attacks?
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.

For example, you can embed user input as below,

const name = response.potentiallyMaliciousInput;const element = <h1>{name}</h1>;

This way you can prevent XSS(Cross-site-scripting) attacks in the application.

How do you update rendered elements?
You can update UI(represented by rendered element) by passing the newly created element to ReactDOM’s render method.

For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
function tick() { const element = ( <div> <h1>Hello, world!</h1> <h2>It is {new Date().toLocaleTimeString()}.</h2> </div> ); ReactDOM.render(element, document.getElementById("root"));} setInterval(tick, 1000);

How do you say that props are readonly?strong
When you declare a component as a function or a class, it must never modify its own props.

Let us take a below capital function,

function capital(amount, interest) { return amount + interest;}

The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying “All React components must act like pure functions with respect to their props.”

How do you say that state updates are merged?
When you call setState() in the component, React merges the object you provide into the current state.

For example, let us take a facebook user with posts and comments details as state variables,

constructor(props) { super(props); this.state = { posts: [], comments: [] }; }

Now you can update them independently with separate setState() calls as below,
componentDidMount() { fetchPosts().then(response => { this.setState({ posts: response.posts }); }); fetchComments().then(response => { this.setState({ comments: response.comments }); }); }
As mentioned in the above code snippets, this.setState({comments}) updates only comments variable without modifying or replacing posts variable.

How do you pass arguments to an event handler?
During iterations or loops, it is common to pass an extra parameter to an event handler. This can be achieved through arrow functions or bind method.

Let us take an example of user details updated in a grid,

<button onClick={(e) => this.updateUser(userId, e)}>Update User details</button><button onClick={this.updateUser.bind(this, userId)}>Update User details</button>

In the both approaches, the synthetic argument e is passed as a second argument. You need to pass it explicitly for arrow functions and it will be passed automatically for bind method.

How to prevent component from rendering?
You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.

In the above example, the greeting component skips its rendering section by applying condition and returning null value.

What are the conditions to safely use the index as a key?
There are three conditions to make sure, it is safe use the index as a key.

1. The list and items are static– they are not computed and do not change
2. The items in the list have no ids
3. The list is never reordered or filtered.

Should keys be globally unique?
The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.

For example, the below Book component uses two arrays with different arrays,

What is the popular choice for form handling?
Formik is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.

In detail, You can categorize them as follows,

1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission

It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.

What are the advantages of formik over redux form library?
Below are the main reasons to recommend formik over redux form library,

1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB

Why are you not required to use inheritance?
In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely. Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.

Can I use web components in react application?
Yes, you can use web components in a react application. Even though many developers won’t use this combination, it may require especially if you are using third-party UI components that are written using Web Components.

For example, let us use Vaadin date picker web component as below,

import React, { Component } from “react”;import “./App.css”;import “@vaadin/vaadin-date-picker”;class App extends Component { render() { return ( <div className=”App”> <vaadin-date-picker label=”When were you born?”></vaadin-date-picker> </div> ); }}export default App;

What is dynamic import?
You can achieve code-splitting in your app using dynamic import.

Let’s take an example of addition,

Normal Import

import { add } from “./math”;console.log(add(10, 20));

Dynamic Import

import(“./math”).then((math) => { console.log(math.add(10, 20));});

What are loadable components?
If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components because React.lazy and Suspense is not yet available for server-side rendering. Loadable lets you render a dynamic import as a regular component.

Lets take an example,

import loadable from “@loadable/component”; const OtherComponent = loadable(() => import(“./OtherComponent”)); function MyComponent() { return ( <div> <OtherComponent /> </div> );}

Now OtherComponent will be loaded in a separated bundle

What is suspense component?
If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using Suspense component.

For example, the below code uses suspense component,

const OtherComponent = React.lazy(() => import(“./OtherComponent”)); function MyComponent() { return ( <div> <Suspense fallback={<div>Loading…</div>}> <OtherComponent /> </Suspense> </div> );}

As mentioned in the above code, Suspense is wrapped above the lazy component.

What is route based code splitting?
One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won’t be disturbed.

Let us take an example of route based website using libraries like React Router with React.lazy,

import { BrowserRouter as Router, Route, Switch } from “react-router-dom”;import React, { Suspense, lazy } from “react”; const Home = lazy(() => import(“./routes/Home”));const About = lazy(() => import(“./routes/About”)); const App = () => ( <Router> <Suspense fallback={<div>Loading…</div>}> <Switch> <Route exact path=”/” component={Home} /> <Route path=”/about” component={About} /> </Switch> </Suspense> </Router>);

In the above code, the code splitting will happen at each route level.

Give an example on How to use context?
Context is designed to share data that can be considered global for a tree of React components.

For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.

//Lets create a context with a default theme value “luna”const ThemeContext = React.createContext(“luna”);// Create App component where it uses provider to pass theme value in the treeclass App extends React.Component { render() { return ( <ThemeContext.Provider value=”nova”> <Toolbar /> </ThemeContext.Provider> ); }}// A middle component where you don’t need to pass theme prop anymorefunction Toolbar(props) { return ( <div> <ThemedButton /> </div> );}// Lets read theme value in the button component to useclass ThemedButton extends React.Component { static contextType = ThemeContext; render() { return <Button theme={this.context} />; }}

What is the purpose of default value in context?
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.

Below code snippet provides default theme value as Luna.

const MyContext = React.createContext(defaultValue);

How do you use contextType?
ContextType is used to consume the context object. The contextType property can be used in two ways,

contextType as property of class: The contextType property on a class can be assigned a Context object created by React.createContext(). After that, you can consume the nearest current value of that Context type using this.context in any of the lifecycle methods and render function.

Lets assign contextType property on MyClass as below,

class MyClass extends React.Component { componentDidMount() { let value = this.context; /* perform a side-effect at mount using the value of MyContext */ } componentDidUpdate() { let value = this.context; /* … */ } componentWillUnmount() { let value = this.context; /* … */ } render() { let value = this.context; /* render something based on the value of MyContext */ }}MyClass.contextType = MyContext;

Static field You can use a static class field to initialize your contextType using public class field syntax.

class MyClass extends React.Component { static contextType = MyContext; render() { let value = this.context; /* render something based on the value */ }}

What is a consumer?
A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree.

Lets take a simple example,

<MyContext.Consumer> {value => /* render something based on the context value */}</MyContext.Consumer>

How do you solve performance corner cases while using context?
The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders.

For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.

class App extends React.Component { render() { return ( <Provider value={{ something: “something” }}> <Toolbar /> </Provider> ); }}

This can be solved by lifting up the value to parent state,

class App extends React.Component { constructor(props) { super(props); this.state = { value: { something: “something” }, }; } render() { return ( <Provider value={this.state.value}> <Toolbar /> </Provider> ); }}

What is the purpose of forward ref in HOCs?
Refs will not get passed through because ref is not a prop. It is handled differently by React just like key. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API.

The below HOC logs all props,

Let’s use this HOC to log all props that get passed to our “fancy button” component,

class FancyButton extends React.Component { focus() { // … } // …}export default logProps(FancyButton);

Now let’s create a ref and pass it to FancyButton component. In this case, you can set focus to button element.

import FancyButton from “./FancyButton”; const ref = React.createRef();ref.current.focus();<FancyButton label=”Click Me” handleClick={handleClick} ref={ref} />;

Is ref argument available for all functions or class components?
Regular function or class components don’t receive the ref argument, and ref is not available in props either. The second ref argument only exists when you define a component with React.forwardRef call.

Why do you need additional care for component libraries while using forward refs?
When you start using forwardRef in a component library, you should treat it as a breaking change and release a new major version of your library. This is because your library likely has a different behavior such as what refs get assigned to, and what types are exported. These changes can break apps and other libraries that depend on the old behavior.

How to create react class components without ES6?
If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.

Note: If you use createReactClass then auto binding is available for all methods. i.e, You don’t need to use .bind(this) with in constructor for event handlers.

Is it possible to use react without JSX?
Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling React.createElement(component, props, …children).

For example, let us take a greeting example with JSX,

class Greeting extends React.Component { render() { return <div>Hello {this.props.message}</div>; }} ReactDOM.render( <Greeting message=”World” />, document.getElementById(“root”));

You can write the same code without JSX as below,

class Greeting extends React.Component { render() { return React.createElement(“div”, null, `Hello ${this.props.message}`); }} ReactDOM.render( React.createElement(Greeting, { message: “World” }, null), document.getElementById(“root”));

What is diffing algorithm?
React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.

In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:

1. Two elements of different types will produce different trees.
2. The developer can hint at which child elements may be stable across different renders with a key prop.

What are the rules covered by diffing algorithm?
When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,

Elements Of Different Types: Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements to , or from

to of different types lead a full rebuild.

DOM Elements Of The Same Type: When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,

3. <div className=”show” title=”ReactJS” />4. <div className=”hide” title=”ReactJS” />

Component Elements Of The Same Type: When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
Recursing On Children: when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.

<ul> <li>first</li> <li>second</li></ul> <ul> <li>first</li> <li>second</li> <li>third</li></ul>

Handling keys: React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,

<ul> <li key=”2015″>Duke</li> <li key=”2016″>Villanova</li></ul> <ul> <li key=”2014″>Connecticut</li> <li key=”2015″>Duke</li> <li key=”2016″>Villanova</li></ul>

When do you need to use refs?
There are few use cases to go for refs,

1. Managing focus, text selection, or media playback.
2. Triggering imperative animations.
3. Integrating with third-party DOM libraries.

Must prop be named as render for render props?
Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,

<Mouse children={(mouse) => ( <p> The mouse position is {mouse.x}, {mouse.y} </p> )}/>

Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,

<Mouse> {(mouse) => ( <p> The mouse position is {mouse.x}, {mouse.y} </p> )}</Mouse>

While using this above technique(without any name), explicitly state that children should be a function in your propTypes.

Mouse.propTypes = { children: PropTypes.func.isRequired,};

What are the problems of using render props with pure components?
If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.

How do you create HOC using render props?
You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a component, you could easily create one using a regular with a render prop.

function withMouse(Component) { return class extends React.Component { render() { return ( <Mouse render={(mouse) => <Component {…this.props} mouse={mouse} />} /> ); } };}

This way render props gives the flexibility of using either pattern.

What is windowing technique?
Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.

How do you print falsy values in JSX?
The falsy values such as false, null, undefined, and true are valid children but they don’t render anything. If you still want to display them then you need to convert it to string. Let’s take an example on how to convert to a string,

<div>My JavaScript variable is {String(myVariable)}.</div>

What is the typical use case of portals?
React portals are very useful when a parent component has overflow: hidden or has properties that affect the stacking context (e.g. z-index, position, opacity) and you need to visually “break out” of its container.

For example, dialogs, global message notifications, hovercards, and tooltips.

How do you set default value for uncontrolled component?
In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a defaultValue attribute instead of value.

render() { return ( <form onSubmit={this.handleSubmit}> <label> User Name: <input defaultValue=”John” type=”text” ref={this.input} /> </label> <input type=”submit” value=”Submit” /> </form> );}

The same applies for select and textArea inputs. But you need to use defaultChecked for checkbox and radio inputs.

What is your favorite React stack?
Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.

What is the difference between Real DOM and Virtual DOM?
Below are the main differences between Real DOM and Virtual DOM,

Real DOM	Virtual DOM
Updates are slow	Updates are fast
DOM manipulation is very expensive.	DOM manipulation is very easy
You can update HTML directly.	You Can’t directly update HTML
It causes too much of memory wastage	There is no memory wastage
Creates a new DOM if element updates	It updates the JSX if element update

How to add Bootstrap to a react application?
Bootstrap can be added to your React app in a three possible ways,

Using the Bootstrap CDN: This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
Bootstrap as Dependency: If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application

npm install bootstrap

1. React Bootstrap Package: In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
  1. react-bootstrap
  2. reactstrap

Can you list down top websites or applications using react as front end framework?
Below are the top 10 websites using React as their front-end framework,

1. Facebook
2. Uber
3. Instagram
4. WhatsApp
5. Khan Academy
6. Airbnb
7. Dropbox
8. Flipboard
9. Netflix
10. PayPal

Is it recommended to use CSS In JS technique in React?
React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate *.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.

Do I need to rewrite all my class components with hooks?
No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.

How to fetch data with React Hooks?
The effect hook called useEffect can be used to fetch data from an API and to set the data in the local state of the component with the useState hook’s update function.

Here is an example of fetching a list of react articles from an API using fetch.

import React from “react”; function App() { const [data, setData] = React.useState({ hits: [] }); React.useEffect(() => { fetch(“http://hn.algolia.com/api/v1/search?query=react”) .then(response => response.json()) .then(data => setData(data)) }, []); return ( <ul> {data.hits.map((item) => ( <li key={item.objectID}> <a href={item.url}>{item.title}</a> </li> ))} </ul> );} export default App;

A popular way to simplify this is by using the library axios.

We provided an empty array as second argument to the useEffect hook to avoid activating it on component updates. This way, it only fetches on component mount.

Is Hooks cover all use cases for classes?
Hooks doesn’t cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon getSnapshotBeforeUpdate and componentDidCatch lifecycles yet.

What is the stable release for hooks support?
React includes a stable implementation of React Hooks in 16.8 release for below packages

1. React DOM
2. React DOM Server
3. React Test Renderer
4. React Shallow Renderer

Why do we use array destructuring (square brackets notation) in useState?
When we declare a state variable with useState, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.

For example, the array index access would look as follows:

var userStateVariable = useState(“userProfile”); // Returns an array pairvar user = userStateVariable[0]; // Access first itemvar setUser = userStateVariable[1]; // Access second item

Whereas with array destructuring the variables can be accessed as follows:

const [user, setUser] = useState(“userProfile”);

What are the sources used for introducing hooks?
Hooks got the ideas from several different sources. Below are some of them,

1. Previous experiments with functional APIs in the react-future repository
2. Community experiments with render prop APIs such as Reactions Component
3. State variables and state cells in DisplayScript.
4. Subscriptions in Rx.
5. Reducer components in ReasonReact.

How do you access imperative API of web components?
Web Components often expose an imperative API to implement its functions. You will need to use a ref to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a wrapper for your Web Component.

What is formik?
Formik is a small react form library that helps you with the three major problems,

1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission

What are typical middleware choices for handling asynchronous calls in Redux?
Some of the popular middleware choices for handling asynchronous calls in Redux eco system are Redux Thunk, Redux Promise, Redux Saga.

Do browsers understand JSX code?
No, browsers can’t understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.

Describe about data flow in react?
React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.

What is react scripts?
The react-scripts package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The react-scripts start command sets up the development environment and starts a server, as well as hot module reloading.

What are the features of create react app?
Below are the list of some of the features provided by create react app.

1. React, JSX, ES6, Typescript and Flow syntax support.
2. Autoprefixed CSS
3. CSS Reset/Normalize
4. A live development server
5. A fast interactive unit test runner with built-in support for coverage reporting
6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
7. An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.

What is the purpose of renderToNodeStream method?
The ReactDOMServer#renderToNodeStream method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes. Note: Remember this method is not available in the browser but only server.

What is MobX?
MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For reactJs application, you need to install below packages,

npm install mobx –savenpm install mobx-react –save

What are the differences between Redux and MobX?
Below are the main differences between Redux and MobX,

Topic	Redux	MobX
Definition	It is a javascript library for managing the application state	It is a library for reactively managing the state of your applications
Programming	It is mainly written in ES6	It is written in JavaScript(ES5)
Data Store	There is only one large store exist for data storage	There is more than one store for storage
Usage	Mainly used for large and complex applications	Used for simple applications
Performance	Need to be improved	Provides better performance
How it stores	Uses JS Object to store	Uses observable to store the data

Should I learn ES6 before learning ReactJS?
No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let’s see some of the frequently used ES6 features,

Destructuring: To get props and use them in a component

2. // in es 53. var someData = this.props.someData;4. var dispatch = this.props.dispatch;5. 6. // in es6const { someData, dispatch } = this.props;

Spread operator: Helps in passing props down into a component

// in es 5<SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} /> // in es6<SomeComponent {…this.props} />

Arrow functions: Makes compact syntax

// es 5var users = usersList.map(function (user) { return <li>{user.name}</li>;});// es 6const users = usersList.map((user) => <li>{user.name}</li>);

What is Concurrent Rendering?
The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there’s something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,

// 1. Part of an app by wrapping with ConcurrentMode<React.unstable_ConcurrentMode> <Something /></React.unstable_ConcurrentMode>; // 2. Whole app using createRootReactDOM.unstable_createRoot(domNode).render(<App />);

What is the difference between async mode and concurrent mode?
Both refers the same thing. Previously concurrent Mode being referred to as “Async Mode” by React team. The name has been changed to highlight React’s ability to perform work on different priority levels. So it avoids the confusion from other approaches to Async Rendering.

Can I use javascript urls in react16.9?
Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like <a href> and create a security hole.

const companyProfile = { website: “javascript: alert(‘Your website is hacked’)”,};// It will log a warning<a href={companyProfile.website}>More details</a>;

Remember that the future versions will throw an error for javascript URLs.

What is the purpose of eslint plugin for hooks?
The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,

1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
2. Hooks are called in the same order on every render.

What is the difference between Imperative and Declarative in React?
Imagine a simple UI component, such as a “Like” button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.

The imperative way of doing this would be:

if (user.likes()) { if (hasBlue()) { removeBlue(); addGrey(); } else { removeGrey(); addBlue(); }}

Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.

In contrast, the declarative approach would be:

if (this.state.liked) { return <blueLike />;} else { return <greyLike />;}

Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a sepecific state, and is therefore much simpler to understand.

What are the benefits of using typescript with reactjs?
Below are some of the benefits of using typescript with Reactjs,

It is possible to use latest JavaScript features
Use of interfaces for complex type definitions
IDEs such as VS Code was made for TypeScript
Avoid bugs with the ease of readability and Validation

<>How do you make sure that user remains authenticated on page refresh while using Context API State Management?
When a user logs in and reload, to persist the state generally we add the load user action in the useEffect hooks in the main App.js. While using Redux, loadUser action can be easily accessed.

App.js

import { loadUser } from “../actions/auth”;store.dispatch(loadUser());

But while using Context API, to access context in App.js, wrap the AuthState in index.js so that App.js can access the auth context. Now whenever the page reloads, no matter what route you are on, the user will be authenticated as loadUser action will be triggered on each re-render.

index.js

import React from “react”;import ReactDOM from “react-dom”;import App from “./App”;import AuthState from “./context/auth/AuthState”; ReactDOM.render( <React.StrictMode> <AuthState> <App /> </AuthState> </React.StrictMode>, document.getElementById(“root”));

App.js

const authContext = useContext(AuthContext); const { loadUser } = authContext; useEffect(() => { loadUser();}, []);

loadUser

What are the benefits of new JSX transform?
There are three major benefits of new JSX transform,

1. It is possible to use JSX without importing React packages
2. The compiled output might improve the bundle size in a small amount
3. The future improvements provides the flexibility to reduce the number of concepts to learn React.

How is the new JSX transform different from old transform??
The new JSX transform doesn’t require React to be in scope. i.e, You don’t need to import React package for simple scenarios.

Let’s take an example to look at the main differences between the old and the new transform,

Old Transform:

import React from “react”; function App() { return <h1>Good morning!!</h1>;}

Now JSX transform convert the above code into regular JavaScript as below,

import React from “react”; function App() { return React.createElement(“h1”, null, “Good morning!!”);}

New Transform:

The new JSX transform doesn’t require any React imports

function App() { return <h1>Good morning!!</h1>;}

Under the hood JSX transform compiles to below code

import { jsx as _jsx } from “react/jsx-runtime”; function App() { return _jsx(“h1”, { children: “Good morning!!” });}

Note: You still need to import React to use Hooks.

How do you get redux scaffolding using create-react-app?
Redux team has provided official redux+js or redux+typescript templates for create-react-app project. The generated project setup includes,

Redux Toolkit and React-Redux dependencies
Create and configure Redux store
React-Redux <Provider> passing the store to React components
Small “counter” example to demo how to add redux logic and React-Redux hooks API to interact with the store from components The below commands need to be executed along with template option as below,
Javascript template:

npx create-react-app my-app –template redux

Typescript template:

npx create-react-app my-app –template redux-typescript

What are React Server components?
React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend.

Note: React Server Components is still under development and not recommended for production yet.

What is prop drilling?
Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.

What is state mutation and how to prevent it?
State mutation happens when you try to update the state of a component without actually using setState function. This can happen when you are trying to do some computations using a state variable and unknowingly save the result in the same state variable. This is the main reason why it is advised to return new instances of state variables from the reducers by using Object.assign({}, …) or spread syntax.

This can cause unknown issues in the UI as the value of the state variable got updated without telling React to check what all components were being affected from this update and it can cause UI bugs.

Ex:

class A extends React.component { constructor(props) { super(props); this.state = { loading: false } } componentDidMount() { let { loading } = this.state; loading = (() => true)(); // Trying to perform an operation and directly saving in a state variable}

How to prevent it: Make sure your state variables are immutable by either enforcing immutability by using plugins like Immutable.js, always using setState to make updates, and returning new instances in reducers when sending updated state values.

What is the difference between useState and useRef hook?

1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes. Essentially, useRef is like a “box” that can hold a mutable value in its (.current) property.
2. useState allows us to update the state inside components. While useRef allows referencing DOM elements.

What is a wrapper component?
A wrapper in React is a component that wraps or surrounds another component or group of components. It can be used for a variety of purposes such as adding additional functionality, styling, or layout to the wrapped components.

For example, consider a simple component that displays a message:

const Message = ({ text }) => { return <p>{text}</p>;};

We can create a wrapper component that will add a border to the message component:

const MessageWrapper = (props) => { return ( <div style={{ border: “1px solid black” }}> <Message {…props} /> </div> );};

Now we can use the MessageWrapper component instead of the Message component and the message will be displayed with a border:

Wrapper component can also accept its own props and pass them down to the wrapped component, for example, we can create a wrapper component that will add a title to the message component:

const MessageWrapperWithTitle = ({title, …props}) => { return ( <div> <h3>{title}</h3> <Message {…props} /> </div> );};

Now we can use the MessageWrapperWithTitle component and pass title props:

This way, the wrapper component can add additional functionality, styling, or layout to the wrapped component while keeping the wrapped component simple and reusable.

What are the differences between useEffect and useLayoutEffect hooks?
useEffect and useLayoutEffect are both React hooks that can be used to synchronize a component with an external system, such as a browser API or a third-party library. However, there are some key differences between the two:

Timing: useEffect runs after the browser has finished painting, while useLayoutEffect runs synchronously before the browser paints. This means that useLayoutEffect can be used to measure and update layout in a way that feels more synchronous to the user.
Browser Paint: useEffect allows browser to paint the changes before running the effect, hence it may cause some visual flicker. useLayoutEffect synchronously runs the effect before browser paints and hence it will avoid visual flicker.
Execution Order: The order in which multiple useEffect hooks are executed is determined by React and may not be predictable. However, the order in which multiple useLayoutEffect hooks are executed is determined by the order in which they were called.
Error handling: useEffect has a built-in mechanism for handling errors that occur during the execution of the effect, so that it does not crash the entire application. useLayoutEffect does not have this mechanism, and errors that occur during the execution of the effect will crash the entire application.

In general, it’s recommended to use useEffect as much as possible, because it is more performant and less prone to errors. useLayoutEffect should only be used when you need to measure or update layout, and you can’t achieve the same result using useEffect.

What are the differences between Functional and Class Components?
There are two different ways to create components in ReactJS. The main differences are listed down as below,
1. Syntax:
The classs components uses ES6 classes to create the components. It uses render function to display the HTML content in the webpage.

The syntax for class component looks like as below.

class App extends Reacts.Component { render(){ return <h1>This is a class component</h1>} }

Note: The Pascal Case is the recommended approach to provide naming to a component.

Functional component has been improved over the years with some added features like Hooks. Here is a syntax for functional component.

function App(){ return <div className=”App”> <h1>Hello, I’m a function component</h1> </div>}
2. State:
State contains information or data about a component which may change over time.

In class component, you can update the state when a user interacts with it or server updates the data using the setState() method. The initial state is going to be assigned in the Constructor( ) method using the the this.state object and it is possible to different data types in the this.state object such as string, boolean, numbers, etc. A simple example showing how we use the setState() and constructor()

class App extends Component { constructor() { super(); this.state = { message: “This is a class component”, }; } updateMessage() { this.setState({t message: “Updating the class component”, }); } render() { return ( <> <h1>{this.state.message}</h1> <button onClick={() => { this.updateMessage(); }}> Click!! </button> </> ); }}

You not use state in functional components because it was only supported in class components. But over the years hooks have been implemented in functional component which enable to use state in functional component too.

The useState() hook can used to implement state in funcitonal component. It returns an array with two items: the first item is current state and the next one is a function (setState) that updates the value of the current state.

Let’s see an example to demonstrate the state in functional components,

4. Props:
Props are referred to as “properties”. The props are passed into react component just like arguments passed to a function. In otherwords, they are similar to HTML attributes.

The props are accessible in child class component using this.props as shown in below example,

class Child extends React.Component { render() { return <h1> This is a functional component and component name is {this.props.name} </h1>; }} class Parent extends React.Component { render() { return ( <div className=”Parent”> <Child name=”First child component” /> <Child name=”Second child component” /> </div> ); }}

Props in functional components are similar to that of the class components but the difference is the absence of ‘this’ keyword.

What is the benefit of strict mode?
The will be helpful in the below cases,

Whenever the component
Identifying components with unsafe lifecycle methods.
Warning about legacy string ref API usage.
Detecting unexpected side effects.
Detecting legacy context
Warning about deprecated findDOMNode usage

Why does strict mode render twice in React?
StrictMode renders components twice in development mode(not production) in order to detect any problems with your code and warn you about those problems. This is used to detect accidental side effects in the render phase. If you used create-react-app development tool then it automatically enables StrictMode by default.

ReactDOM.render( <React.StrictMode> {App} </React.StrictMode>, document.getElementById(‘root’));

If you want to disable this behavior then you can remove strict mode.

ReactDOM.render( {App}, document.getElementById(‘root’));

To detect side effects the following functions are invoked twice:

Class component constructor, render, and shouldComponentUpdate methods
Class component static getDerivedStateFromProps method
Function component bodies
State updater functions
Functions passed to useState, useMemo, or useReducer (any Hook)

React JS: Interview question and answer – Part 5