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A list of top frequently asked React Interview Questions and Answers are given below.
React is a declarative, efficient, flexible open source front-end JavaScript library developed by Facebook in 2011. It follows the component-based approach for building reusable UI components, especially for single page application. It is used for developing interactive view layer of web and mobile apps. It was created by Jordan Walke, a software engineer at Facebook. It was initially deployed on Facebook's News Feed section in 2011 and later used in its products like WhatsApp & Instagram.
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React framework gaining quick popularity as the best framework among web developers. The main features of React are:
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Following is a list of the most crucial advantages of using React:
React is easy to learn and use
React comes with good availability of documentation, tutorials, and training resources. It is easy for any developer to switch from JavaScript background to React and easily understand and start creating web apps using React. Anyone with little knowledge of JavaScript can start building web applications using React.
React follows the MVC architecture.
React is the V (view part) in the MVC (Model-View-Controller) architecture model and is referred to as "one of the JavaScript frameworks." It is not fully featured but has many advantages of the open-source JavaScript User Interface (UI) library, which helps execute the task in a better manner.
React uses Virtual DOM to improve efficiency.
React uses virtual DOM to render the view. The virtual DOM is a virtual representation of the real DOM. Each time the data changes in a react app, a new virtual DOM gets created. Creating a virtual DOM is much faster than rendering the UI inside the browser. Therefore, with the use of virtual DOM, the efficiency of the app improves. That's why React provides great efficiency.
Creating dynamic web applications is easy.
In React, creating a dynamic web application is much easier. It requires less coding and gives more functionality. It uses JSX (JavaScript Extension), which is a particular syntax letting HTML quotes and HTML tag syntax to render particular subcomponents.
React is SEO-friendly.
React facilitates a developer to develop an engaging user interface that can be easily navigated in various search engines. It also allows server-side rendering, which is also helpful to boost the SEO of your app.
React allows reusable components.
React web applications are made up of multiple components where each component has its logic and controls. These components provide a small, reusable piece of HTML code as an output that can be reused wherever you need them. The code reusability helps developers to make their apps easier to develop and maintain. It also makes the nesting of the components easy and allows developers to build complex applications of simple building blocks. The reuse of components also increases the pace of development.
Support of handy tools
React provides a lot of handy tools that can make the task of the developers understandable and easier. Use these tools in Chrome and Firefox dev extension, allowing us to inspect the React component hierarchies in the virtual DOM. It also allows us to select the particular components and examine and edit their current props and state.
React has a rich set of libraries.
React has a huge ecosystem of libraries and provides you the freedom to choose the tools, libraries, and architecture for developing the best application based on your requirement.
Scope for testing the codes
React web applications are easy to test. These applications provide a scope where the developer can test and debug their codes with the help of native tools.
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Following is the list of the biggest limitations of React:
JSX stands for JavaScript XML. It is a React extension which allows writing JavaScript code that looks similar to HTML. It makes HTML file easy to understand. The JSX file makes the React application robust and boosts its performance. JSX provides you to write XML-like syntax in the same file where you write JavaScript code, and then preprocessor (i.e., transpilers like Babel) transform these expressions into actual JavaScript code. Just like XML/HTML, JSX tags have a tag name, attributes, and children.
Example
In the above example, text inside
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Browsers cannot read JSX directly because they can only understand JavaScript objects, and JSX is not a regular JavaScript object. Thus, we need to transform the JSX file into a JavaScript object using transpilers like Babel and then pass it to the browser.
A Virtual DOM is a lightweight JavaScript object which is an in-memory representation of real DOM. It is an intermediary step between the render function being called and the displaying of elements on the screen. It is similar to a node tree which lists the elements, their attributes, and content as objects and their properties. The render function creates a node tree of the React components and then updates this node tree in response to the mutations in the data model caused by various actions done by the user or by the system.
Virtual DOM works in three steps:
1. Whenever any data changes in the React App, the entire UI is re-rendered in Virtual DOM representation.
2. Now, the difference between the previous DOM representation and the new DOM is calculated.
3. Once the calculations are completed, the real DOM updated with only those things which are changed.
The React is different from Angular in the following ways.
Angular | React | |
---|---|---|
Author | Facebook Community | |
Developer | Misko Hevery | Jordan Walke |
Initial Release | October 2010 | March 2013 |
Language | JavaScript, HTML | JSX |
Type | Open Source MVC Framework | Open Source JS Framework |
Rendering | Client-Side | Server-Side |
Data-Binding | Bi-directional | Uni-directional |
DOM | Regular DOM | Virtual DOM |
Testing | Unit and Integration Testing | Unit Testing |
App Architecture | MVC | Flux |
Performance | Slow | Fast, due to virtual DOM. |
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The React's ES6 syntax has changed from ES5 syntax in the following aspects.
require vs. Import
exports vs. export
component and function
props
state
The main differences between ReactJS and React Native are given below.
SN | ReactJS | React Native |
---|---|---|
1. | Initial release in 2013. | Initial release in 2015. |
2. | It is used for developing web applications. | It is used for developing mobile applications. |
3. | It can be executed on all platforms. | It is not platform independent. It takes more effort to be executed on all platforms. |
4. | It uses a JavaScript library and CSS for animations. | It comes with built-in animation libraries. |
5. | It uses React-router for navigating web pages. | It has built-in Navigator library for navigating mobile applications. |
6. | It uses HTML tags. | It does not use HTML tags. |
7. | In this, the Virtual DOM renders the browser code. | In this, Native uses its API to render code for mobile applications. |
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The following table specifies the key differences between the Real DOM and Virtual DOM:
The real DOM creates a new DOM if the element updates.Real DOM | Virtual DOM |
---|---|
The real DOM updates slower. | The virtual DOM updates faster. |
The real DOM can directly update HTML. | The virtual DOM cannot directly update HTML. |
The virtual DOM updates the JSX if the element updates. | |
In real DOM, DOM manipulation is very expensive. | In virtual DOM, DOM manipulation is very easy. |
There is a lot of memory wastage in The real DOM. | There is no memory wastage in the virtual DOM. |
In React, components are the building blocks of React applications. These components divide the entire React application's UI into small, independent, and reusable pieces of code. React renders each of these components independently without affecting the rest of the application UI. Hence, we can say that, in React, everything is a component.
It is mandatory for each React component to have a render() function. Render function is used to return the HTML which you want to display in a component. If you need to rendered more than one HTML element, you need to grouped together inside single enclosing tag (parent tag) such as
Example: If you need to display a heading, you can do this as below.
Points to Note:
You can embed two or more components into the following way:
Props stand for "Properties" in React. They are read-only inputs to components. Props are an object which stores the value of attributes of a tag and work similar to the HTML attributes. It gives a way to pass data from the parent to the child components throughout the application.
It is similar to function arguments and passed to the component in the same way as arguments passed in a function.
Props are immutable so we cannot modify the props from inside the component. Inside the components, we can add attributes called props. These attributes are available in the component as this.props and can be used to render dynamic data in our render method.
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The State is an updatable structure which holds the data and information about the component. It may be changed over the lifetime of the component in response to user action or system event. It is the heart of the react component which determines the behavior of the component and how it will render. It must be kept as simple as possible.
Let's create a "User" component with "message state."
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The major differences between States and Props are given below.
SN | Props | State |
---|---|---|
1. | Props are read-only. | State changes can be asynchronous. |
2. | Props are immutable. | State is mutable. |
3. | Props allow you to pass data from one component to other components as an argument. | State holds information about the components. |
4. | Props can be accessed by the child component. | State cannot be accessed by child components. |
5. | Props are used to communicate between components. | States can be used for rendering dynamic changes with the component. |
6. | The stateless component can have Props. | The stateless components cannot have State. |
7. | Props make components reusable. | The State cannot make components reusable. |
8. | Props are external and controlled by whatever renders the component. | The State is internal and controlled by the component itself. |
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We can update the State of a component using this.setState() method. This method does not always replace the State immediately. Instead, it only adds changes to the original State. It is a primary method which is used to update the user interface(UI) in response to event handlers and server responses.
Example
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The difference between stateless and stateful components are:
SN | Stateless Component | Stateful Component |
---|---|---|
1. | The stateless components do not hold or manage state. | The stateful components can hold or manage state. |
2. | It does not contain the knowledge of past, current, and possible future state changes. | It can contain the knowledge of past, current, and possible future changes in state. |
3. | It is also known as a functional component. | It is also known as a class component. |
4. | It is simple and easy to understand. | It is complex as compared to the stateless component. |
5. | It does not work with any lifecycle method of React. | It can work with all lifecycle method of React. |
6. | The stateless components cannot be reused. | The stateful components can be reused. |
The Arrow function is the new feature of the ES6 standard. If you need to use arrow functions, it is not necessary to bind any event to 'this.' Here, the scope of 'this' is global and not limited to any calling function. So If you are using Arrow Function, there is no need to bind 'this' inside the constructor. It is also called 'fat arrow '(=>) functions.
An event is an action which triggers as a result of the user action or system generated event like a mouse click, loading of a web page, pressing a key, window resizes, etc. In React, the event handling system is very similar to handling events in DOM elements. The React event handling system is known as Synthetic Event, which is a cross-browser wrapper of the browser's native event.
Handling events with React have some syntactical differences, which are:
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We can create an event as follows.
Example
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A synthetic event is an object which acts as a cross-browser wrapper around the browser's native event. It combines the behavior of different browser's native event into one API, including stopPropagation() and preventDefault().
In the given example, e is a Synthetic event.
The difference between controlled and uncontrolled components are:
SN | Controlled | Uncontrolled |
---|---|---|
1. | It does not maintain its internal state. | It maintains its internal states. |
2. | Here, data is controlled by the parent component. | Here, data is controlled by the DOM itself. |
3. | It accepts its current value as a prop. | It uses a ref for their current values. |
4. | It allows validation control. | It does not allow validation control. |
5. | It has better control over the form elements and data. | It has limited control over the form elements and data. |
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Lists are used to display data in an ordered format. In React, Lists can be created in a similar way as we create it in JavaScript. We can traverse the elements of the list using the map() function.
Example
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A key is a unique identifier. In React, it is used to identify which items have changed, updated, or deleted from the Lists. It is useful when we dynamically created components or when the users alter the lists. It also helps to determine which components in a collection needs to be re-rendered instead of re-rendering the entire set of components every time. It increases application performance.
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Forms allow the users to interact with the application as well as gather information from the users. Forms can perform many tasks such as user authentication, adding user, searching, filtering, etc. A form can contain text fields, buttons, checkbox, radio button, etc.
React offers a stateful, reactive approach to build a form. The forms in React are similar to HTML forms. But in React, the state property of the component is only updated via setState(), and a JavaScript function handles their submission. This function has full access to the data which is entered by the user into a form.
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The different phases of React component's lifecycle are:
Initial Phase: It is the birth phase of the React lifecycle when the component starts its journey on a way to the DOM. In this phase, a component contains the default Props and initial State. These default properties are done in the constructor of a component.
Mounting Phase: In this phase, the instance of a component is created and added into the DOM.
Updating Phase: It is the next phase of the React lifecycle. In this phase, we get new Props and change State. This phase can potentially update and re-render only when a prop or state change occurs. The main aim of this phase is to ensure that the component is displaying the latest version of itself. This phase repeats again and again.
Unmounting Phase: It is the final phase of the React lifecycle, where the component instance is destroyed and unmounted(removed) from the DOM.
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The important React lifecycle methods are:
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Pure components introduced in React 15.3 version. The React.Component and React.PureComponent differ in the shouldComponentUpdate() React lifecycle method. This method decides the re-rendering of the component by returning a boolean value (true or false). In React.Component, shouldComponentUpdate() method returns true by default. But in React.PureComponent, it compares the changes in state or props to re-render the component. The pure component enhances the simplicity of the code and performance of the application.
In React, Higher Order Component is an advanced technique for reusing component logic. It is a function that takes a component and returns a new component. In other words, it is a function which accepts another function as an argument. According to the official website, it is not the feature(part) in React API, but a pattern that emerges from React's compositional nature.
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You can do many tasks with HOC, some of them are given below:
The main differences between Elements and Components are:
SN | Element | Component |
---|---|---|
1. | An element is a plain JavaScript object which describes the component state and DOM node, and its desired properties. | A component is the core building block of React application. It is a class or function which accepts an input and returns a React element. |
2. | It only holds information about the component type, its properties, and any child elements inside it. | It can contain state and props and has access to the React lifecycle methods. |
3. | It is immutable. | It is mutable. |
4. | We cannot apply any methods on elements. | We can apply methods on components. |
5. | Example: const element = React.createElement( 'div', {id: 'login-btn'}, 'Login' ) | Example: function Button ({ onLogin }) { return React.createElement( 'div', {id: 'login-btn', onClick: onLogin}, 'Login' ) } |
In React, we can write comments as we write comments in JavaScript. It can be in two ways:
1. Single Line Comments: We can write comments as /* Block Comments */ with curly braces:
2. Multiline Comments: If we want to comment more that one line, we can do this as
In React, it is necessary to start component names with a capital letter. If we start the component name with lower case, it will throw an error as an unrecognized tag. It is because, in JSX, lower case tag names are considered as HTML tags.
In was introduced in React 16.2 version. In React, Fragments are used for components to return multiple elements. It allows you to group a list of multiple children without adding an extra node to the DOM.
Example
There is also a shorthand syntax exists for declaring Fragments, but it's not supported in many tools:
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Props validation is a tool which helps the developers to avoid future bugs and problems. It makes your code more readable. React components used special property PropTypes that help you to catch bugs by validating data types of values passed through props, although it is not necessary to define components with propTypes.
We can apply validation on props using App.propTypes in React component. When some of the props are passed with an invalid type, you will get the warnings on JavaScript console. After specifying the validation patterns, you need to set the App.defaultProps.
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Create React App is a tool introduced by Facebook to build React applications. It provides you to create single-page React applications. The create-react-app are preconfigured, which saves you from time-consuming setup and configuration like Webpack or Babel. You need to run a single command to start the React project, which is given below.
This command includes everything which we need to build a React app. Some of them are given below:
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There are two possible ways to create a component in React:
Function Components: This is the simplest way to create a component in React. These are the pure JavaScript functions that accept props object as the first parameter and return React elements:
Class Components: The class components method facilitates you to use ES6 class to define a component. The above function component can be written as:
If a component needs state or lifecycle methods, we should use the class component; otherwise, use the function component. However, after React 16.8, with the addition of Hooks, you could use state, lifecycle methods, and other features that were only available in the class component right in your function component.
Web browsers can't read JSX directly. This is because the web browsers are built to read the regular JS objects only, and JSX is not a regular JavaScript object.
If you want a web browser to read a JSX file, you must transform the files into a regular JavaScript object. For this purpose, Babel is used.
In react, the state of a component is an object that holds some information that may change over the component's lifetime. It would be best to try to make your state as simple as possible and minimize the number of stateful components.
Let's see how to create a user component with message state:
The state is very 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.
Following are the most visible syntax we can see while comparing ES6 and ES5:
Syntax in ES5:
Syntax in ES6:
Syntax in ES5:
Syntax in ES6:
Syntax in ES5:
Syntax in ES6:
Syntax in ES5:
Syntax in ES6:
Syntax in ES5:
Syntax in ES6:
In React, the props are inputs to components. They are single values or objects containing a set of values passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.
The main purpose of props in React is to provide the following component functionality:
For example, let us create an element with reactProp property:
This reactProp name becomes a property attached to React's native props object, which already exists on all React library components.
Refs is the shorthand used for references in React. It is an attribute which helps to store a reference to particular DOM nodes or React elements. It provides a way to access React DOM nodes or React elements and how to interact with it. It is used when we want to change the value of a child component, without making the use of props.
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Refs can be created by using React.createRef() and attached to React elements via the ref attribute. It is commonly assigned to an instance property when a component is created, and then can be referenced throughout the component.
Ref forwarding is a feature which is used for passing a ref through a component to one of its child components. It can be performed by making use of the React.forwardRef() method. It is particularly useful with higher-order components and specially used in reusable component libraries.
Example
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The preferred option is to use callback refs over findDOMNode() API. Because callback refs give better control when the refs are set and unset whereas findDOMNode() prevents certain improvements in React in the future.
The recommended approach is:
The Ref in React is used in the following cases:
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React Router is a standard routing library system built on top of the React. It is used to create Routing in the React application using React Router Package. It helps you to define multiple routes in the app. It provides the synchronous URL on the browser with data that will be displayed on the web page. It maintains the standard structure and behavior of the application and mainly used for developing single page web applications.
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React Router plays an important role to display multiple views in a single page application. It is used to define multiple routes in the app. When a user types a specific URL into the browser, and if this URL path matches any 'route' inside the router file, the user will be redirected to that particular Route. So, we need to add a Router library to the React app, which allows creating multiple routes with each leading to us a unique view.
The important advantages of React Router are given below:
The difference between React Routing and Conventional Routing are:
SN | Conventional Routing | React Routing |
---|---|---|
1. | In Conventional Routing, each view contains a new file. | In React Routing, there is only a single HTML page involved. |
2. | The HTTP request is sent to a server to receive the corresponding HTML page. | Only the History attribute |
3. | In this, the user navigates across different pages for each view. | In this, the user is thinking he is navigating across different pages, but its an illusion only. |
It is because you have not to wrap your Route's in a
Example
should be
The 'switch' keyword is used to display only a single Route to rendered amongst the several defined Routes. The
We can use style attribute for styling in React applications, which adds dynamically-computed styles at render time. It accepts a JavaScript object in camelCased properties rather than a CSS string. The style attribute is consistent with accessing the properties on DOM nodes in JavaScript.
Example
We can style React Component in mainly four ways, which are given below:
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CSS Module is a CSS file where all class names and animation names are scoped locally by default. It is available only for the component which imports it, and without your permission, it cannot be applied to any other Components. You can create CSS Module file with the .module.css extension.
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Styled-Components is a library for React. It is the successor of CSS Modules. It uses enhance CSS for styling React component systems in your application, which is written with a mixture of JavaScript and CSS. It is scoped to a single component and cannot leak to any other element in the page.
The styled-components provides:
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Hooks are the new feature introduced in React 16.8 version that facilitates us to use state and other React features without writing a class.
See the following example of useState hook:
We have to follow the following two rules to use hooks in React:
In React, forms are used to enable users to interact with web applications. Following is a list of the most common usage of forms in React:
An error boundary is a concept introduced in version 16 of React. Error boundaries provide a way to find out the errors that occur in the render phase. Any component which uses one of the following lifecycle methods is considered an error boundary. Let's see the places where an error boundary can detect an error:
Let's see an example to understand it better:
Without using error boundaries:
In the above code, you can see that when the counterValue equals 2, it throws an error inside the render method. We know that any error inside the render method leads to unmounting of the component so, to display an error that occurs inside the render method, we use error boundaries. When we are not using the error boundary, we see a blank page instead of seeing an error.
With error boundaries:
We have specified earlier that error boundary is a component using one or both of the following methods:
See the following code where we create an error boundary to handle errors in render phase:
You can see in the above code the getDerivedStateFromError function renders the fallback UI interface when the render method has an error.
The componentDidCatch logs the error information to an error tracking service.
Now with error boundary, we can render the CounterComponent in the following way:
Following are some cases in which error boundaries don't catch errors:
The major problems with the MVC framework are:
Flux is an application architecture that Facebook uses internally for building the client-side web application with React. It is neither a library nor a framework. It is a kind of architecture that complements React as view and follows the concept of Unidirectional Data Flow model. It is useful when the project has dynamic data, and we need to keep the data updated in an effective manner.
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Redux is an open-source JavaScript library used to manage application state. React uses Redux for building the user interface. The Redux application is easy to test and can run in different environments showing consistent behavior. It was first introduced by Dan Abramov and Andrew Clark in 2015.
React Redux is the official React binding for Redux. It allows React components to read data from a Redux Store, and dispatch Actions to the Store to update data. Redux helps apps to scale by providing a sensible way to manage state through a unidirectional data flow model. React Redux is conceptually simple. It subscribes to the Redux store, checks to see if the data which your component wants have changed, and re-renders your component.
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The three principles that redux follows are:
The components of Redux are given below.
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Reducers read the payloads from the actions and then updates the Store via the State accordingly. It is a pure function which returns a new state from the initial State. It returns the previous State as it is if no work needs to be done.
A Store is an object which holds the application's State and provides methods to access the State, dispatch Actions and register listeners via subscribe(listener). The entire State tree of an application is saved in a single Store which makes the Redux simple and predictable. We can pass middleware to the Store which handles the processing of data as well as keep a log of various actions that change the Store's State. All the Actions return a new state via reducers.
The Redux is different from Flux in the following manner.
SN | Redux | Flux |
---|---|---|
1. | Redux is an open-source JavaScript library used to manage application State. | Flux is neither a library nor a framework. It is a kind of architecture that complements React as view and follows the concept of Unidirectional Data Flow model. |
2. | Store's State is immutable. | Store's State is mutable. |
3. | In this, Store and change logic are separate. | In this, the Store contains State and change logic. |
4. | It has only a single Store. | It can have multiple Store. |
5. | Redux does not have Dispatcher concept. | It has single Dispatcher, and all actions pass through that Dispatcher. |
The main advantages of React Redux are:
You need to export the Store from the module where it created with createStore() method. Also, you need to assure that it will not pollute the global window space.
1) What is Babel in React?
2) What do you understand by the Reconciliation process in React?
3) Which of the following is used to pass data to a component from outside React applications?
4) Which of the following function allows you to render React content on an HTML page?
5) Which of the following shows the correct phases of the component lifecycle?
6) In MVC (Model, View, Controller) model, how can you specify the role of the React?
7) Which of the following is the most precise difference between Controlled Component and Uncontrolled Component?
8) What do the arbitrary inputs of components in React are called?
9) What do you understand by the "key" prop in React?
10) Which of the following is the correct data flow sequence of flux concept in React?
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