Top 100 React Js Interview Question

1.What is React JS?

React JS is an Open Source Javascript Library Primarily used for forming user interfaces precisely for Singular Page Applications. Facebook Developers Created the React JS in 2011 and up to now it was widely used and garnered a large community of learners and developers.


2.What are error boundaries?

Error Boundaries are primarily used for acquiring Javascript errors anywhere in the Child Components. Logging the errors and Projecting the Fallback UI was its major Usage.


3.Can you tell me uses of the ‘key’ in list?

List Elements would be assigned with stable identity by the help of Keys. These keys are unique. Mostly the ID of data would be used as Key.


4.What is conditional rendering?

Based on logic rendering will happen. If particular condition is met then we shall render the page.


5.What is an event?

User Actions like Mouse Clicks, Keypress would be considered as an Event. Event Contains a set of different properties.


6.Can we create one list without using ‘key’ in react?

There is no way to create a list without key for element, if it is bound to happen there would be errors in the rendering. The browser console too throw back an error.


7.Explain Lifting states up in the React?

In Case of Multiple States sharing the Common data, there is a possibility to lift up the state to Close Common Ancestor. Parents hold the Common state where the Child do not hold its any state. Child and Parent Components would use the same state.


8.What are Fragments?

Grouping of Child Component without the use of Additional Node is called Fragment.


9.What are higher-Order components?

The Higher Order Component is the function that intake a component and return a new component. For Reusing the Component logic, the higher order component function is used.


10.What is StrictMode?

The Mode that is used for running additional Checks and Warnings for React Components. It do not affect production build since it runs only on development build. It would highlight the application problem in User Interface.


11.What are empty tags <> used for?

Empty tags are primarily used to declare the fragments in React.


12.What is react Context?

Passing of Data through a tree of react components is called react context. This Context is used to share data globally between two react components.


13.What are Context provider and Context consumer?

Provision of Values to the context is called Context provider. These values can be used by any other components in the application. Context Consumers are primarily used for reading the values.


14.What are the Benefits of using ReactJS?

Learning curve: Students with strong Javascript, Cascading Styling Sheet Understanding can be able to learn react with ease. Code Readability: The JSX Usage marginally enhances the code readability. Performance: The Virtual DOM presence accelerates the performance since Updating UI do not require the entire website to all single changes. Reusable components: Single Component can be used at multiple places at no need of code rewriting. SSR: The SSR was Supported by React and it also enhances the loading time of webpages.


15.What is meant by SSR?

Rendering of Web page in the server whereas usually it will be applied on browser, this process is called as Service Side Rendering. It improves the page loading speed and also performance, SEO, User Experience to Web Pages.


16.What is meant by JSX?

The JSX is the syntax extension of JavaScript and used to create the React elements. These elements are then rendered to the React DOM.


17.What is meant by virtual DOM?

Real DOM Manipulation would make the application much slower, there would be a Virtual DOM specially for every assigned Real DOM. Virtual DOM offers similar features and it can be Manipulate much faster than Real DOM.


18.Is there possibility for reading the JSX Files?

JSX is dissimilar to the Javascript, Since browsers only read Javascript files. Babel is primarily used to transform JSX code into the Javascript Objects.


19.What is meant by React component?

The Building Blocks of React Application are Called React Components. It is basically a function that takes the inputs which are called as props would return back the elements of react that should need to be projected on the screen. Components let you to split the UI into Independent and Reusable Pieces.


20.Mention types of the React?

The major two types of Components in the React are The first one is called as Function Component The Second one is known as Class Component


21.What do you mean by Function & Class Components?

The Javascript Functions where react element would be returned with the input properties are called Function Component. Class Component The ES6 Javascript class that extend to React. Component are called Class Component.


22.Mention the cases that React needs Class Component?

When dealing with state wise or life cycle methods, the Class components in the react would be used.


23.What is meant by React State?

The Object that defines the rendering and behaviour of component is React ‘State’. Component private party is the state and also stores information that changes over time of the component. Regarding the state value, UI would be updated by react components.


24.What do you mean by React props?

For passing the data to component the props are used. Single / Multiple values can be passed to the component. It is primarily used for passing component data and make any changes in the state.


25.Explain between the React State as well as Props?

The Duo State and the Props holds the information about the react component. State objects are available inside the component. In other hand props are utilized to send data from component to other component.


26.How to update the state in the react?

State can be updated Directly / Indirectly. If there is a case to update directly, UI Element that was linked to state would not be Updated. The best way to do that is use setState method, this involves the Updation of State Object & Automatic re rendering of the Component.


27.What is a stateless component?

The Component that do not holds any state is Called as Stateless Component. It would take Props as argument and returns a react element on the values of ‘props’. There is no existence of Lifecycle for these Components


28.What is a stateful component?

Component that hold only one state which is used for UI rendering are called Stateful Component. They are generally referred as Class Components.


29.What are createElement and cloneElement?

React createElement is primarily used to build one react element that can be used as a User interface component. CloneElement is mostly used for cloning the element.


30.How does ‘setState’ function work?

‘setState’ function was primarily used for updating the state of a react component. In the case of UI component dependency regarding the value we are going to update, the UI will get re-rendered. The Middleware that allows the coder to add the creator action that revert the functions rather than the actions is Redux. The other purpose to use this was as a delay function like delaying the action dispatch only if some condition is met. getState() and dispatch() are inner function parameters. To Activate the Redux, Middleware() method was applied.


31.What is meant by Flux?

Flux is generally an illustration which is useful in managing the single directional data stream. These are meant to be controlling the data fragments and make them to interfere with the data without issues. Flux is not specified to any react application or collection of react application


32.What do you understand by the term polling?

Server needs frequent monitoring for the updates with the time. The general case would be to know whether novel comments are present or not, this process is commonly known as pooling. It would check for updates for every 5 seconds and we can also change the time period with ease. It can create issues that is related to the things that are happening, thus pooling is considered.


33.What are the reasons behind the success of ReactJS?

Complex tasks can be easily handled by the React JS. No need to care of the bus while performing the tasks, there by error free outcomes and developing scalable apps. It is growing technology with learners and developers, we can rely on its quality outcomes.


34.Mention the key benefits of Flux?

Applications which are developed using flux have the components that can be tested easily. We can manage and test any react component by updating the store thereby decreasing the risk of data affection. These applications are scalable and holds no compatibility issues.


35.What is meant by Synthetic Events?

ReactJS would transform the browser events thereby monitoring their behaviour. The primary purpose to do this is events should hold the logical properties even the browsers are different, in layman words ReactJS is like logical event system cross-browser.


36.What is meant by DOM diffing?

The case components that are rendering two times, Virtual DOM would check the modification elements, the elements that needs to change was represented, there are multiple elements that don’t go through the changes. DOM doffing is used to cut DOM changes, it generally improves the browser performance, this pave the browser a way to perform all tasks at a quicker pace.


37.How to nest JSX elements to other JSX Elements?

There is a case of possibility, it is like HTML Elements Nestation. The limitation would be there are things that are different in it, need to have good knowledge towards the elements like source and destination to complete the task quickly.


38.What are createElement and cloneElement?

There is a need of one JSX Element which helps the task to be completed easily, if there is the existence of multiple expressions the process would get slow, people who are new to the technology would get confused if there are multiple expressions present in it.


39.How React Actually Works?

The below mentioned steps gives an idea on how react actually works,

  • Diffing algorithm would be run by React to identify changes that need to be made in Virtual DOM.
  • The next step would be used to update the DOM as per new features which is Reconciliation.
  • Virtual DOM that is too light in nature would be detached from specific implementations of the browser.
  • Virtual DOM Elements would be used to build basic nodes
  • After the Component Change in State, Diffing algorithm would run quicker and identify changes. After the changes has been identified, it would automatically update the DOM with the Change difference.


40.Does ReactJS use HTML?

No, It only uses JSX which is similar to HTML.


41.What are the limitations faced by React JS?

ReactJS too holds lot of limitations like other platforms, the major drawback would be the library size, being complex would create a lot of uncertainty among the code developers. The inline templating with JSX also adds voice, most importantly the one only one layer of app is covered. In order to develop the app, developers rely on multiple technologies that take time.


42.Explain controlled as well as uncontrolled components in React JS?

A Controlled Component would take current values via props as well as notify changes with callbacks just like the onChange. Parent components “controls” by handling all the callback as well as managing own state & passing the newer values as props to controlled component. This is also called as “dumb component”. An Uncontrolled Component stores its state internally and need to query DOM using reference to find the current value when you need to find it, it is similar to the HTML.


43.Tell me about React Router?

React router decides the components that needs to be rendered and also components that not needs to be rendered. Dictation is also performed by Router for various activities.


44.What are the stateless components?

Applications which are developed using flux have the components that can be tested easily. We can manage and test any react component by updating the store thereby decreasing the risk of data affection. These applications are scalable and holds no compatibility issues.


45.What is meant by Synthetic Events?

ReactJS would transform the browser events thereby monitoring their behaviour. The primary purpose to do this is events should hold the logical properties even the browsers are different, in layman words ReactJS is like logical event system cross-browser.


46.Explain why we should provide the reflection()?

It is one of the opposite single elements. It represents own components of DOM. If we have two or more HTML element? They we should link with the fuzzy index. It is just like . It is pure element. This means we must provide the reflection whenever implementation is done.


47.Can you tell me few benefits of the reflecting?

Following are few major benefits of the reflection. They are,

  • We can increase the performance of the apps with this reflection.
  • We can use this on server as well as server side.
  • We can increase all the code reading with JSX.
  • We can be able to integrate easily with any other structures just like angle, meter, etc.


48.Why we should use switch for Rounder v4?

We use

for connecting many paths of router. In case if we want to display way for multiple routes. Then we should use this ‘switch’ key. If we type URL in the defined ways specially in tag line? At the time of first match, this offers certain way. In such way it avoids all the remaining paths.


49.State some Fluoxes benefits which you know?

It is very simple to test if we develop the apps on flux components. Simply when we update the shop, developers can be able to test all discrimination. Using this we can eliminate all the risk of the data fondness. Every app is superior. Also, they don’t have any of the issues which are compatible.

50.Tell me about rounder?

In Reactjs organizing elements plays a vital role. We use this rounder for determining the pay type is not done. Its dictum in various activities.


51.Explain the artificial events?

In ReactJS, we can easily modify original events for the browsers. It is possible just by monitor the behavior of the browser. It is mainly for ensuring whether there are logical properties of the events. If it is beyond various browsers? It operates in the envelope of the cross browser around logical event.


52.Can you able to find the props in the parenting elements?

Yes, we can find it with the help of spread operator. We can easily list all the properties. Still, it is very complex process.


53.Explain why should we capitalize all the elements?

Element differs from the DOM element. Still, the developers need it. If we don’t use this? Then it causes many problems which confuse the developers. Specially with different components. Also, there is a problem in integrating all components as well as commands.


54.Explain about the Doom variation?

If we provide same components two times? Then we can find changes in the virtual Tom. Simply it represents modification of element. We have many more components which will not pass the changes. The DOM tuning will be considered as the best. It reduces all the changes of DOM based on the user actions. Generally, we use browser for increasing the efficiency. This is based on the ability of doing tasks quickly.


55.Tell me when we should use class elements in function element?

In case, if our element is state / lifetime cycle. Then we can go for the class elements.


56.Can you tell me when to avoid setState() after we unmounted a component?

Mounted() is used for avoiding the call of the setState().


57.Tell me in parsing, how to share between the elements?

We can use the State as well as Prop to share the data.


58.Can you list out the cases which uses HOC?

Following are some of the uses of the HOC. They are,

  • Props manipulation.
  • Render hijacking.
  • Code reusing, logic as well as bootstrap abstraction.
  • State abstraction as well as manipulation.


59. Explain in the constructor why to use the super()?

Child component will not use the reference until super() is called. This is the main reason for calling this super().


60.Can you state the uses for this.prop.children?

It allows us for passing components as the data to various other components.


61.Can you tell me about the reconciliation?

Whenever a state or the props of the component get change. At that time react compares rendered element with the previous rendered. Then it updates actual DOM in case if it requires. This is known as reconciliation.


62.Can you list predefined proptypes?

Following are some predefined proptypes list with prefix proptypes. For example: PropTypes.string. They are,

  • string
  • number
  • object
  • array
  • node
  • func
  • bool
  • element
  • any
  • symbol


63.List some methods in the package of react-dom?

Following are some methods in the react-dom package. They are,

  • hydrate()
  • createPortal()
  • render()
  • findDOMNode()
  • unmountComponentAtNode()


64.How to call AJAX in react?

class workers extend React.Component { constructor(prop) { super(prop) this.state = { workers: [], error: nulls } } componentDidMounts() { fetch(‘ .then(res => res.jsons()) .then( (output) => { this.setState({ workers: output.workers }) }, (errors) => { this.setState({ errors }) } ) } renders() { consts { errors, workers} = this.states if (errors) { return

Errors: {errors.message}; }else { return (
    { => (
  • {}
  • ))}
) } } }


65.Can you tell me the implementation data binding in two-way?

class DataBindings extends React.Components{ states = { student:”reacts” } handleChanges = (e) =>{ this.setStates({ student: e.targets.values }) } renders(){ returns(


) } }


66.Explain in which way you will trigger the click event?

We need to create ref. Specially in the render method just like given below. this.inputElement = input} /> Then we need to apply the click event for our event handler. It is just like given below.


67.List out pointer events which you know in react?

Following are some of pointer events. They are,

  • onPointerMove
  • onPointerDown
  • onPointerCancel
  • onPointerUp
  • onLostPointerCaptur
  • onGotPointerCapture
  • onPointerLeave
  • onPointerEnter
  • onPointerOut
  • onPointerOver


68.Explain how will you render the children into the DOM node which exists outside of DOM hierarchy?

We must use the Portal to the render the children into the DOM node which exists outside of DOM hierarchy. ReactDOM.createPortal(child, container)


69.Can you tell me the advantages in using the redux?

Following are few of the advantages in using the redux. They are,

  • Truth of single source: We can store the state apps in object tree. Especially within single store. This is because it is very easy for the apps debugging if we use single source.
  • Read only state: If we want to update state? Then we must want to emit action as well as state updating. You didn’t update all the state directly.


70.Explain about the {…this.props} uses?

Here 3 dots represent the spread operator. In props {…this.props} will spread out “own” properties as a discrete property.


71.Can you compare pure component with the component?

We can say that both are same. But the pure component handles shouldComponentUpdate. If the props or state changes? Then the Pure Component perform the shadow comparison. Specially for both the state as well as props. But the component will not do this comparison for any state / props.


72.Can you tell me about the shallow rendering?

It allows us to render the component deep as well as assert the facts about return the method of render.


73.Can you tell me in react about the Predefined keywords alternate to the html attributes?

We can replace class with the className. Also, we can replace for with the htmlFor.


74.Can you tell me how to access the redux store specially outside the components?

We can access the store especially outside components just by doing the ‘export’.


75.Tell me in react how will you update the props?

In React we can’t update the props. This is because its read-only. So, we cannot modify props which we receive from the parent to the child.


76.Can you tell me the method of life which listen to the state changes?

componentdidupdate componentwillupdate


77.Can you tell me about the Super Props?

We can declare this super Props in the constructor. To get parent DOM properties, we use this.


78.Explain about Destructuring?

This is simple process to extract objects of the array. We get every object separately after destructuring in the separate variable.


79.Tell me if it is possible for updating props values?

No, we can’t be able to update the values of the props. Because it is immutable, so we can’t edit props values. Specially in the render functions.


80.Explain about the Arrow Function?

Generally, we call it as a Fat Arrow Fn. The main difference between the normal and fat fn is, this is very short. Also, it differs from the binding way.


81.Explain about the Mounting as well as Demounting?

Mounting: This is a process for attaching all elements to DOM. Demounting: This is a process to detach all elements from DOM.


82.Tell me about the refs?

In DOM elements, Refs is an attribute. We can easily find all DOM elements just by using refs.


83.Explain me in react about the render( )?

For creating and updating the elements in DOM we use this render( ). Value renders in the render fn is immutable. We can’t edit it once we render it.


84.Can you tell me about the Stateless function?

We can also call it as a Functional Component. This is very simple and old functions in the JavaScript.


85.Can you tell me the syntax for creating the state?

creating the state. this.state = {‘state name’: “ ”}


86.Can you tell me the syntax for changing the state value?

We need to use the below syntax for changing the state value. this.setState({‘state name’:”new value”})


87.Explain about the Event handling?

It is a process to do few functions which are based on actions just like drag, click, double click. Specially in DOM elements.


88.Can you tell me about the ComponentWillMount()?

We used to call it after we remove the DOM elements from the DOM. Also, it swipes the memory just for increasing access space.


89.Can you list out the lifecycle events?

Following are the lifecycle event list. They are,

  • Destruction.
  • Construction
  • Updates


90.Name the function which we use for listing the values in React?

In React, we use Map ( ) for listing the components. It returns all the array elements in list item.


91.Can you tell me about react with the architecture flow of redux?

We can be able to pass data for between the components through reducer. These reducers the data passing in parent component with the help of storage.


92.Can you tell How will you split reducers?

We can split the reducers on the basics of event actions. We can split these actions in separate modules.


93.Can you tell me the purpose to use the bindActionsCreators?

For binding all the events in the event handler on the basics of action dispatcher. This is to HTML elements.


94.Can you tell me how will you dispatch data in the store?

Dispatching data between the component on the basis of action creators. These data stores in parent component.


95.Tell me how will you create the action creators with the redux?

We can be able to create action creators which pass data flow. Specially for the previous to current state. This is done in the unidirectional ways.


96.Explain how will you set data flow with the redux?

React is unidirectional data flow. So, we can pass the data from parent component to child with the help of store.


97.Can you tell me about benchmark in the react compares with other frameworks in javascript?

Following are some of the comparison with other frameworks in Javascript.

  • In react we use virtual DOM which is based on diff algorithm.
  • It is very fast.
  • We can quickly update the DOM.


98.Can you tell me about the Virtual DOM?

Object representation for the DOM object just like light weighted copy. Node tree which list all the attributes & elements & content as an object as well as properties. It keeps in the memory as well as synced with “Real DOM”. It displays all the element on screen. This complete process is known as reconciliation.


99.Can you tell me about className over the class attribute?

In the JavaScript, class is keyword & JSX is extension of the javascript. So, we can’t use the class keyword inside the JSX. This is the reason for using className as replacement of the class.


100.Can you list few features of the ReactJS?

Following are the features of the ReactJS. They are,

  • Open Source
  • Adaptability
  • Asynchronous Functions as well as Generators
  • Server based communication
  • Flux Library
  • Usefulness of the JSX
  • Destructuring assignments
  • Decorators from the ES7


January 24, 2021
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