Interview question and answer – Part 4

How to add multiple middlewares to Redux?
You can use applyMiddleware().

For example, you can add redux-thunk and logger passing them as arguments to applyMiddleware():

import { createStore, applyMiddleware } from “redux”;
const createStoreWithMiddleware = applyMiddleware( ReduxThunk, logger)(createStore);

How to set initial state in Redux?
You need to pass initial state as second argument to createStore:

const rootReducer = combineReducers({
todos: todos,
visibilityFilter: visibilityFilter,
});

const initialState = {
todos: [{ id: 123, name: “example”, completed: false }],
};
const store = createStore(rootReducer, initialState);

How Relay is different from Redux?
Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via GraphQL queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.

What is an action in Redux?
Actions are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.

For example, let’s take an action which represents adding a new todo item:

{ type: ADD_TODO, text: ‘Add todo item’}

React Native
What is the difference between React Native and React?
React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.

React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.

How to test React Native apps?
React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.

How to do logging in React Native?
You can use console.log, console.warn, etc. As of React Native v0.29 you can simply run the following to see logs in the console:

$ react-native log-ios$ react-native log-android

How to debug your React Native?
Follow the below steps to debug React Native app:

1. Run your application in the iOS simulator.
2. Press Command + D and a webpage should open up at http://localhost:8081/debugger-ui.
3. Enable Pause On Caught Exceptions for a better debugging experience.
4. Press Command + Option + I to open the Chrome Developer tools, or open it via View -> Developer -> Developer Tools.
5. You should now be able to debug as you normally would.

React supported libraries & Integration
What is reselect and how it works?
Reselect is a selector library (for Redux) which uses memoization concept. It was originally written to compute derived data from Redux-like applications state, but it can’t be tied to any architecture or library.

Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It’s memoization and cache are fully customizable.

What is Flow?
Flow is a static type checker designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving null, unlike most type systems.

What is the difference between Flow and PropTypes?
Flow is a static analysis tool (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.

PropTypes is a basic type checker (runtime checker) which has been patched onto React. It can’t check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.

How to use Font Awesome icons in React?
The below steps followed to include Font Awesome in React:

Install font-awesome:

$ npm install –save font-awesome

Import font-awesome in your index.js file:

import “font-awesome/css/font-awesome.min.css”;

Add Font Awesome classes in className:

render() { return <div><i className={‘fa fa-spinner’} /></div>}

What is React Dev Tools?
React Developer Tools let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).

The official extensions available for different browsers or environments.

1. Chrome extension
2. Firefox extension
3. Standalone app (Safari, React Native, etc)

Why is DevTools not loading in Chrome for local files?
If you opened a local HTML file in your browser (file://…) then you must first open Chrome Extensions and check Allow access to file URLs.

How to use Polymer in React?
You need to follow below steps to use Polymer in React,

Create a Polymer element:

2. <link3. rel=”import”4. href=”../../bower_components/polymer/polymer.html”5. />;6. Polymer({7. is: “calendar-element”,8. ready: function () {9. this.textContent = “I am a calendar”;10. },});

Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the index.html of your React application:

Use that element in the JSX file:

import React from “react”; class MyComponent extends React.Component { render() { return <calendar-element />; }} export default MyComponent;

What are the advantages of React over Vue.js?
React has the following advantages over Vue.js:

1. Gives more flexibility in large apps developing.
2. Easier to test.
3. Suitable for mobile apps creating.
4. More information and solutions available.

Note: The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

What is the difference between React and Angular?
Let’s see the difference between React and Angular in a table format.

React	Angular
React is a library and has only the View layer	Angular is a framework and has complete MVC functionality
React handles rendering on the server side	AngularJS renders only on the client side but Angular 2 and above renders on the server side
React uses JSX that looks like HTML in JS which can be confusing	Angular follows the template approach for HTML, which makes code shorter and easy to understand
React Native, which is a React type to build mobile applications are faster and more stable	Ionic, Angular’s mobile native app is relatively less stable and slower
In React, data flows only in one way and hence debugging is easy	In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult

Note: The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.

Why React tab is not showing up in DevTools?
When the page loads, React DevTools sets a global named __REACT_DEVTOOLS_GLOBAL_HOOK__, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won’t show up the tab.

What are Styled Components?
styled-components is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.

Give an example of Styled Components?
Lets create <Title> and <Wrapper> components with specific styles for each.

import React from “react”;
import styled from “styled-components”;
// Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5emconst Title = styled.h1` font-size: 1.5em; text-align: center; color: palevioletred;`; // Create a <Wrapper> component that renders a <section> with some padding and a papayawhip backgroundconst Wrapper = styled.section` padding: 4em; background: papayawhip;`;

These two variables, Title and Wrapper, are now components that you can render just like any other react component.

<Wrapper> <Title>{“Lets start first styled component!”}</Title></Wrapper>

What is Relay?
Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.

How to use TypeScript in create-react-app application?
Starting from react-scripts@2.1.0 or higher, there is a built-in support for typescript. i.e, create-react-app now supports typescript natively. You can just pass –typescript option as below

npx create-react-app my-app –typescript # or yarn create react-app my-app –typescript

But for lower versions of react scripts, just supply –scripts-version option as react-scripts-ts while you create a new project. react-scripts-ts is a set of adjustments to take the standard create-react-app project pipeline and bring TypeScript into the mix.

Now the project layout should look like the following:

my-app/├─ .gitignore├─ images.d.ts├─ node_modules/├─ public/├─ src/│ └─ …├─ package.json├─ tsconfig.json├─ tsconfig.prod.json├─ tsconfig.test.json└─ tslint.json

Miscellaneous
What are the main features of Reselect library?
Let’s see the main features of Reselect library,

Selectors can compute derived data, allowing Redux to store the minimal possible state.
Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
Selectors are composable. They can be used as input to other selectors.

Give an example of Reselect usage?
Let’s take calculations and different amounts of a shipment order with the simplified usage of Reselect:

import { createSelector } from “reselect”;
const shopItemsSelector = (state) => state.shop.items;const taxPercentSelector = (state) => state.shop.taxPercent;
const subtotalSelector = createSelector(shopItemsSelector,
(items) => items.reduce((acc, item) => acc + item.value, 0));

Does the statics object work with ES6 classes in React?
No, statics only works with React.createClass():

someComponent = React.createClass({ statics: { someMethod: function () { // .. }, },});

But you can write statics inside ES6+ classes as below,

class Component extends React.Component { static propTypes = { // … }; static someMethod() { // … }}

or writing them outside class as below,

class Component extends React.Component { ….} Component.propTypes = {…}Component.someMethod = function(){….}

Can Redux only be used with React?
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.

Do you need to have a particular build tool to use Redux?
Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.

How Redux Form initialValues get updated from state?
You need to add enableReinitialize : true setting.

const InitializeFromStateForm = reduxForm({ form: “initializeFromState”, enableReinitialize: true,})(UserEdit);

If your initialValues prop gets updated, your form will update too.

How React PropTypes allow different types for one prop?
You can use oneOfType() method of PropTypes.

For example, the height property can be defined with either string or number type as below:

Component.propTypes = { size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),};

Can I import an SVG file as react component?
You can import SVG directly as component instead of loading it as a file. This feature is available with react-scripts@2.0.0 and higher.

import { ReactComponent as Logo } from “./logo.svg”;
const App = () => ( <div> {/* Logo is an actual react component */} <Logo /> </div>);

Note: Don’t forget about the curly braces in the import.

Why are inline ref callbacks or functions not recommended?
If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.

class UserForm extends Component {
handleSubmit = () => {
console.log(“Input Value is: “,
this.input.value);
};

render() {
return (
<form onSubmit={this.handleSubmit}>
<input type=”text” ref={(input) => (this.input = input)} />
//        Access DOM input in handle submit
<button type=”submit”>Submit</button>
</form>    ); }}

But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function

class UserForm extends Component {
handleSubmit = () => {
console.log(“Input Value is: “, this.input.value);
};

setSearchInput = (input) => {
this.input = input;
};
render() {
return (
<form onSubmit={this.handleSubmit}>
<input type=”text” ref={this.setSearchInput} />
// Access DOM input        in handle submit
<button type=”submit”>Submit</button>      </form>
);
}}

What is render hijacking in react?
The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.

What are HOC factory implementations?
There are two main ways of implementing HOCs in React.

Props Proxy (PP) and
Inheritance Inversion (II).

But they follow different approaches for manipulating the WrappedComponent.

Props Proxy

In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name Props Proxy.

function ppHOC(WrappedComponent) { return class PP extends React.Component { render() { return <WrappedComponent {…this.props} />; } };}

Inheritance Inversion

In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems inverse.

function iiHOC(WrappedComponent) { return class Enhancer extends WrappedComponent { render() { return super.render(); } };}

How to pass numbers to React component?
You should be passing the numbers via curly braces({}) where as strings in quotes

React.render( <User age={30} department={“IT”} />, document.getElementById(“container”));

Do I need to keep all my state into Redux? Should I ever use react internal state?
It is up to the developer’s decision, i.e., it is developer’s job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component’s internal state.

Below are the thumb rules to determine what kind of data should be put into Redux

1. Do other parts of the application care about this data?
2. Do you need to be able to create further derived data based on this original data?
3. Is the same data being used to drive multiple components?
4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
5. Do you want to cache the data (i.e, use what’s in state if it’s already there instead of re-requesting it)?

What is the purpose of registerServiceWorker in React?
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that’s what you should know about service worker for now. It’s all about adding offline capabilities to your site.

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

What is React memo function?
Class components can be restricted from re-rendering when their input props are the same using PureComponent or shouldComponentUpdate. Now you can do the same with function components by wrapping them in React.memo.

const MyComponent = React.memo(function MyComponent(props) { /* only rerenders if props change */});

What is React lazy function?
The React.lazy function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the OtherComponent when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.

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

Note: React.lazy and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.

How to prevent unnecessary updates using setState?
You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return null to stop re-rendering otherwise return the latest state value.

For example, the user profile information is conditionally rendered as follows,

getUserProfile = (user) => { const latestAddress = user.address; this.setState((state) => { if (state.address === latestAddress) { return null; } else { return { title: latestAddress }; } });};

How do you render Array, Strings and Numbers in React 16 Version?
Arrays: Unlike older releases, you don’t need to make sure render method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.

For example, let us take the below list of developers,

const ReactJSDevs = () => {
return [
<li key=”1″>John</li>,
<li key=”2″>Jackie</li>,
<li key=”3″>Jordan</li>,
];
};

You can also merge this array of items in another array component.

const JSDevs = () => {
return (
<ul>
<li>Brad</li>
<li>Brodge</li>
<ReactJSDevs />
<li>Brandon</li>
</ul>
);
};

Strings and Numbers: You can also return string and number type from the render method.

render() { return ‘Welcome to ReactJS questions’;}// Numberrender() { return 2018;}

How to use class field declarations syntax in React classes?
React Class Components can be made much more concise using the class field declarations. You can initialize the local state without using the constructor and declare class methods by using arrow functions without the extra need to bind them.

Let’s take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,

class Counter extends Component {
state = { value: 0 };
handleIncrement = () => {
this.setState((prevState) => ({
value: prevState.value + 1,
}));
};

handleDecrement = () => {
this.setState((prevState) => ({
value: prevState.value – 1,
})); };   render() {
return (
<div>
{this.state.value}
<button onClick={this.handleIncrement}>+</button>
<button onClick={this.handleDecrement}>-</button>
</div>    ); }}

What are hooks?
Hooks is a special JavaScript function that allows you use state and other React features without writing a class. This pattern has been introduced as a new feature in React 16.8 and helped to isolate the stateful logic from the components.

Let’s see an example of useState hook:

import { useState } from “react”;
function Example() {
// Declare a new state variable, which we’ll call “count”
const [count, setCount] = useState(0);
return (
<>
<p>You clicked {count} times</p>
<button onClick={() =>
setCount(count + 1)}>Click me</button>    </>
);}

Note: Hooks can be used inside an existing function component without rewriting the component.

What rules need to be followed for hooks?
You need to follow two rules in order to use hooks,

1. Call Hooks only at the top level of your react functions: You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple useState and useEffect calls.
2. Call Hooks from React Functions only: You shouldn’t call Hooks from regular JavaScript functions. Instead, you should call them from either function components or custom hooks.

The eslint plugin named eslint-plugin-react-hooks can be used to enforce these two rules.

How to ensure hooks followed the rules in your project?
React team released an ESLint plugin called eslint-plugin-react-hooks that enforces these two rules. You can add this plugin to your project using the below command,

npm install eslint-plugin-react-hooks@next

And apply the below config in your ESLint config file,

// Your ESLint configuration{ “plugins”: [ // … “react-hooks” ], “rules”: { // … “react-hooks/rules-of-hooks”: “error” }}

For example, the linter enforce proper naming convention for hooks. If you rename your custom hooks which as prefix “use” to something else then linter won’t allow you to call built-in hooks such as useState, useEffect etc inside of your custom hook anymore.

Note: This plugin is intended to use in Create React App by default.

What are the differences between Flux and Redux?
Below are the major differences between Flux and Redux

Flux	Redux
State is mutable	State is immutable
The Store contains both state and change logic	The Store and change logic are separate
There are multiple stores exist	There is only one store exist
All the stores are disconnected and flat	Single store with hierarchical reducers
It has a singleton dispatcher	There is no concept of dispatcher
React components subscribe to the store	Container components uses connect function

What are the benefits of React Router V4?
Below are the main benefits of React Router V4 module,

1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(<BrowserRouter>) which wraps specific child router components(<Route>).
2. You don’t need to manually set history. The router module will take care history by wrapping routes with <BrowserRouter> component.
3. The application size is reduced by adding only the specific router module(Web, core, or native)

Can you describe about componentDidCatch lifecycle method signature?
The componentDidCatch lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,

error: – The error object which was thrown
info: – An object with a componentStack key contains the information about which component threw the error.

The method structure would be as follows

componentDidCatch(error, info);

In which scenarios error boundaries do not catch errors?
Below are the cases in which error boundaries doesn’t work,

1. Inside Event handlers
2. Asynchronous code using setTimeout or requestAnimationFrame callbacks
3. During Server side rendering
4. When errors thrown in the error boundary code itself

Why do you not need error boundaries for event handlers?
Error boundaries do not catch errors inside event handlers.

React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.

If you need to catch an error inside an event handler, use the regular JavaScript try / catch statement:

class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { error: null };
this.handleClick = this.handleClick.bind(this);
}

handleClick() {
try {
// Do something that could throw
} catch (error) {
this.setState({ error });
}
}
render() {
if (this.state.error) {
return <h1>Caught an error.</h1>;
}
return <button onClick={this.handleClick}>Click Me</button>;
}}

Note that the above example is demonstrating regular JavaScript behavior and doesn’t use error boundaries.

What is the difference between try catch block and error boundaries?
Try catch block works with imperative code whereas error boundaries are meant for declarative code to render on the screen.

For example, the try catch block used for below imperative code

try { showButton();} catch (error) { // …}

Whereas error boundaries wrap declarative code as below,

So if an error occurs in a componentDidUpdate method caused by a setState somewhere deep in the tree, it will still correctly propagate to the closest error boundary.

What is the behavior of uncaught errors in react 16?
In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.

What is the proper placement for error boundaries?
The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,

1. You can wrap top-level route components to display a generic error message for the entire application.
2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.

What is the benefit of component stack trace from error boundary?
Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.

What is the required method to be defined for a class component?
The render() method is the only required method in a class component. i.e, All methods other than render method are optional for a class component.

What are the possible return types of render method?
Below are the list of following types used and return from render method,

1. React elements: Elements that instruct React to render a DOM node. It includes html elements such as <div/> and user defined elements.
2. Arrays and fragments: Return multiple elements to render as Arrays and Fragments to wrap multiple elements
3. Portals: Render children into a different DOM subtree.
4. String and numbers: Render both Strings and Numbers as text nodes in the DOM
5. Booleans or null: Doesn’t render anything but these types are used to conditionally render content.

What is the main purpose of constructor?
The constructor is mainly used for two purposes,

To initialize local state by assigning object to this.state
For binding event handler methods to the instance For example, the below code covers both the above cases,

constructor(props) { super(props); // Don’t call this.setState() here! this.state = { counter: 0 }; this.handleClick = this.handleClick.bind(this);}

Is it mandatory to define constructor for React component?
No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.

What are default props?
The defaultProps can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for null props. That means, If you provide null value then it remains null value.

For example, let us create color default prop for the button component,

function MyButton { // …} MyButton.defaultProps = { color: “red”,};

If props.color is not provided then it will set the default value to ‘red’. i.e, Whenever you try to access the color prop it uses the default value

render() { return <MyButton /> ; // props.color will contain red value }

Why should not call setState in componentWillUnmount?
You should not call setState() in componentWillUnmount() because once a component instance is unmounted, it will never be mounted again.

What is the purpose of getDerivedStateFromError?
This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value to update state.

The signature of the lifecycle method is as follows,

static getDerivedStateFromError(error)

Let us take error boundary use case with the above lifecycle method for demonstration purpose,

class ErrorBoundary extends React.Component { constructor(props) { super(props); this.state = { hasError: false }; } static getDerivedStateFromError(error) { // Update state so the next render will show the fallback UI. return { hasError: true }; } render() { if (this.state.hasError) { // You can render any custom fallback UI return <h1>Something went wrong.</h1>; } return this.props.children; }}

What is the methods order when component re-rendered?
An update can be caused by changes to props or state. The below methods are called in the following order when a component is being re-rendered.

1. static getDerivedStateFromProps()
2. shouldComponentUpdate()
3. render()
4. getSnapshotBeforeUpdate()
5. componentDidUpdate()

What are the methods invoked during error handling?
Below methods are called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.

1. static getDerivedStateFromError()
2. componentDidCatch()

What is the purpose of displayName class property?
The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.

For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.

function withSubscription(WrappedComponent) {
class WithSubscription extends React.Component { /* … */ }
WithSubscription.displayName = `WithSubscription(${getDisplayName(
WrappedComponent
)})`;

return WithSubscription;}
function getDisplayName(WrappedComponent) {
return (
WrappedComponent.displayName || WrappedComponent.name || “Component”
);
}

What is the browser support for react applications?
React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use es5-shim and es5-sham polyfill then it even support old browsers that doesn’t support ES5 methods.

What is the purpose of unmountComponentAtNode method?
This method is available from react-dom package and it removes a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns true if a component was unmounted and false if there was no component to unmount.

The method signature would be as follows,

ReactDOM.unmountComponentAtNode(container);

What is code-splitting?
Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.

For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the ‘Load’ button. moduleA.js

const moduleA = “Hello”; export { moduleA };

App.js

import React, { Component } from “react”;
class App extends Component {
handleClick = () => {
import(“./moduleA”)      .then(({ moduleA }) => {        // Use moduleA      })      .catch((err) => {        // Handle failure
});
};

render() {
return (
<div>        <button onClick={this.handleClick}>Load</button>      </div>    ); }} export default App;

What are Keyed Fragments?
The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,

function Glossary(props) {
return (
<dl>      {props.items.map((item) => (
// Without the `key`, React will fire a key warning
<React.Fragment key={item.id}>
<dt>{item.term}</dt>
<dd>{item.description}</dd>
</React.Fragment>
))}
</dl> );}

Note: key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.

Does React support all HTML attributes?
As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.

Let us take few props with respect to standard HTML attributes,

<div tabIndex=”-1″ /> // Just like node.tabIndex DOM API<div className=”Button” /> // Just like node.className DOM API<input readOnly={true} /> // Just like node.readOnly DOM API

These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.

What are the limitations with HOCs?
Higher-order components come with a few caveats apart from its benefits. Below are the few listed in an order,

Don’t use HOCs inside the render method: It is not recommended to apply a HOC to a component within the render method of a component.

2. render() {3. // A new version of EnhancedComponent is created on every render4. // EnhancedComponent1 !== EnhancedComponent25. const EnhancedComponent = enhance(MyComponent);6. // That causes the entire subtree to unmount/remount each time!7. return <EnhancedComponent />;}

The above code impacts on performance by remounting a component that causes the state of that component and all of its children to be lost. Instead, apply HOCs outside the component definition so that the resulting component is created only once.
Static methods must be copied over: When you apply a HOC to a component the new component does not have any of the static methods of the original component

// Define a static methodWrappedComponent.staticMethod = function () { /*…*/};// Now apply a HOCconst EnhancedComponent = enhance(WrappedComponent); // The enhanced component has no static methodtypeof EnhancedComponent.staticMethod === “undefined”; // true

You can overcome this by copying the methods onto the container before returning it,

function enhance(WrappedComponent) { class Enhance extends React.Component { /*…*/ } // Must know exactly which method(s) to copy 🙁 Enhance.staticMethod = WrappedComponent.staticMethod; return Enhance;}

React JS: Interview question and answer – Part 4