Top 30+ ReactJS Interview Questions And Answers


ReactJS Interview Questions And Answers

Reactjs interview questions and answers

We provide ReactJS Interview Questions and Answers for work placements and purposes. ReactJS is the most basic course in the current situation because there are more work opportunities and higher pay for ReactJS and other related jobs. These are the best ReactJS Interview Questions and Answers provided by experienced people. The most frequently asked ReactJS Interview Questions are listed below.

These questions and answers are suitable for all levels of beginners and experienced professionals. The questions are aimed to be for intermediate to advanced ReactJS developers, but even beginners can understand the answers provided. Top ReactJS Interview Questions is for you if you’re an ambitious front-end developer preparing for interviews. Let’s begin with a few of the most frequently asked interview questions on ReactJS.

  1. What is JSX?

JavaScript XML is shortened to JSX. It’s a React-specific file that blends the expressiveness of JavaScript with the template syntax of HTML. It makes the HTML file very simple to comprehend.

2. What does Virtual DOM mean? Describe how it operates.

It’s a node tree that lists elements, attributes, and content as objects with properties. React render function builds a node tree from the components. The tree is then updated in response to data model mutations caused by user or system actions.

3. Why can’t browsers read JSX?

Browsers can interpret only JavaScript objects, but JSX isn’t one of them. To allow JSX to be read by a browser, convert the JSX file into a JavaScript object with JSX transformers like Babel, and send it to the browser.

4. Explain how everything in React is a component.

Components make up the user interface of a React application. The user interface is broken down into tiny, interchangeable parts using these components. It then makes each of these components different from the rest of the UI.

5. In React, what does render() do?

A render() method is required for any React component. As a native DOM component’s representation, it returns a single React element. If there is a need to make more than one HTML element, they must group within a single enclosing tag, including form>, group>, div>, and so on. This function must remain pure, returning the same result each time it is revoked.

6.  What is Props?

In React, Props is a shorthand for Properties. They are read-only components that must be kept as pure as possible, i.e., immutable. They’re frequently passed down from the application’s parent to its child components. A prop can never be returned to the parent component by a child component. It helps in maintaining a one-way data flow and is commonly used to render dynamically generated data.

7. What is a state in React, and how is it used?

States make up React components. The data source is the state, which should be kept simple. In brief, states are the objects that decide how components are made and behave. They, unlike the props, are mutable and generate dynamic and interactive elements.

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8. What are synthetic events in React?

Synthetic events are objects that wrap the browser’s native event in a cross-browser wrapper. They create a single API that combines the functionality of various browsers. It is done to ensure that the properties of the events remain consistent across browsers.

9. What do you understand by refs in React?

References in React are shorthand as Refs. It’s an attribute that allows you to save a reference to a specific React element or object, which will be returned by the render configuration feature of the Component. It’s used to get regards to a particular component or Component that render() returned. They’re helpful when we need to make DOM measurements or add methods to elements.

10. How are forms created in React?

Forms created with React are like those made with HTML. In React, however, the state is stored in the Component’s state property and can only be modified using the setState(). As a result, the elements can’t directly change their state, so a JavaScript function handles their submission. This role has complete control over the information that a user enters a form.

11. How does React work? How Virtual-DOM works in React?

React generates a virtual DOM. When a component’s state changes, it employs a diffing algorithm to determine the difference in the virtual DOM. The second stage is reconciliation, in which the DOM is updated with the outcome of the diff.

The HTML DOM is often tree-structured, as allowed by the HTML document’s structure. Because of massive apps, DOM trees have grown enormously in size. Since we are being moved more and more into interactive web apps, we must constantly and significantly change the DOM tree. 

The Virtual DOM abstracts the HTML DOM. It is light and detached from browser-specific implementation information. React did not create it, but it makes use of it and makes it available for free. The virtual DOM is where ReactElements lives. They build the virtual nodes here. 

When a ReactComponent changes its state, React’s diff algorithm runs and determines what has changed. After that, the DOM is updated with the diff results. The point is that it takes less time to finish than it does in a regular DOM.

12. What is JSX?

JSX is a JavaScript syntax extension that includes all of JavaScript’s control and generates the elements in React. JSX can contain any JavaScript expression just by enclosing it in curly braces. JSX expressions are converted to standard JavaScript objects after compilation. It means you can use JSX within statements and loops, add it to variables, accept it as arguments, and get it back from functions. While React does not require JSX, it is the preferred method of representing the user interface in a React app.

13. What is React.createClass?

We can generate component “classes” with React.createClass. We can, however, build component classes that use ES6 JavaScript classes with ES6. The result is the same: a component class. The style, however, is unique. One uses a custom JavaScript class system, while the other uses a native JavaScript class system.

The createClass() method in React passes an object as an argument. It’s a little different than writing the same part in an ES6 class. We extend Component, an ES6 class defined by the library, instead of using a react library form.

14. How does ReactDOM differentiate from React exactly?

All ReactDOM functions were included in React before version 0.14. React, and ReactDOM was later separated into two libraries.

ReactDOM is the glue that connects React to the DOM, as its name suggests. It is frequently used for only one purpose: mounting with ReactDOM. ReactDOM.findDOMNode() is another useful function that allows us to get immediate access to DOM elements.

We can use React to describe and construct our elements, lifecycle hooks, and other React app components.

15. How does the functional Component vary from the class component?

Features like local state and lifecycle hooks may be used for class components to allow our Component immediate access to the store and maintain state.

A ‘stateless component’ receives props and takes them to the page, and for which a pure function can be used. These are also known as presentational elements or dumb components.

16. Explain how state and props differ?

The state of the Component is a data structure that begins with default when mounted. It can change over time, often because of user actions.

Props define the configuration of a Component. Props are how components interact with each other. They are obtained from the Component and are timeless in the Component’s eyes that receive them. A Component can’t adjust its props, but it is in charge of putting its child Components together. Props aren’t limited to Data; callback functions may also be used as props.

We may also use default props to ensure they are set even if a parent component does not pass them down.

Props and States both do the same thing but in different ways. Most of our components would most likely be stateless. Props are used to transfer data from a parent to a child component or by the Component itself. They are unchangeable. The term “state” is used to describe mutable data or data that will change over time. It is especially beneficial for user input.

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17. What are controlled components?

Form elements in HTML such as input, text field opt to keep and modify its state using user input. The values from the elements are sent along with the form when a user submits it. It’s different with React. When the callback function, such as onChange, is called to update the state, the Component that contains the form will keep track of the input value in its state and re-render the Component. A driven component is a form element that has its value controlled by React.

Every state mutation associated with a regulated component will have a handler function, making it simple to change or verify user input.

18. What is a higher-order component?

React’s higher-order component (HOC) technique allows you to reuse component logic. The React API doesn’t have HOCs. They’re a pattern that React’s compositional nature produces. HOCs allows for the reuse of code, logic, and bootstrap abstraction. Third-party React libraries commonly use HOCs. Redux’s connect feature is likely the most common. HOCs are perfect for sharing behaviors between react components and sharing utility libraries or elemental composition. You might be able to refactor code into a reusable HOC if you find yourself writing a lot of it in various places which does the same thing.

19. What is create-react-app?

Create-react-app is the official React CLI for creating React apps with no build setup. Webpack and Babel aren’t needed to be installed or configured. They’re all set up in advance and are hidden so we can concentrate on the code. Installing it is as easy as installing every other node module. You must run one command to get the React project started.

It has everything we’ll need to create a React app, including:

  • Language features not found in ES6, such as the object spread operator.
  • No need for -WebKit- or other prefixes for autoprefixer CSS.
  • A quick interactive unit test runner with coverage reporting built-in.
  • A real-time development server that alerts you to popular mistakes.

20. What is Redux?

Redux’s core concept is that the application’s entire state is stored in a single store. At the shop, a javascript object is used. You can update the status only by firing your app actions and then writing reducers for these measures that change the state. There can be no side effects, and a complete change of the state is used in reducers.

Redux is based on the premise that be it a UI state including the active tab or a data state such as user profile, there is only one source of truth.

Redux saves all this information in a closure called a store. It also contains a formula for building the said shop, namely create store (x).

As an argument, the create store function takes another function, x. The passed-in role is in charge of returning the application’s current state, saved in the store. The reducer is the term for the function that is passed in.

21. What is Redux Thunk used for?

Redux thunk is a middleware piece that allows one to write action creators that return a feature rather than an action. If a criterion is met, one may use the thunk to delay the dispatch of an operation. It enables us to perform asynchronous action dispatching. The methods of transmission and getState store are passed to the inner function as parameters.

22. What is PureComponent? When to use PureComponent over Component?

PureComponent is identical to Component, with the exception that it takes care of the shouldComponentUpdate process. PureComponent compares props and state as they change. On the other hand, the Component will not, out of the box, compare current props and state to the next. As a result, if shouldComponentUpdate is called, the Component will re-render by default.

A shallow comparison tests that primitives have the same value, and as compared to previous props and states, comparisons to more complex matters such as objects and arrays are the same.

23. How is Virtual-DOM better than Dirty checking in terms of efficiency?

Every Component in React has its state. It is a measurable condition. React can detect when this data changes and therefore knows when to re-render the scene. Dirty checking takes longer than observables since we have to poll the Data daily and recursively verify all values in the data structure. Setting a value on the state, on the other hand, would notify a listener that a state has changed, enabling React to listen for state change events and re-rendering to happen.

For effective DOM rendering, the Virtual DOM is used. It’s unrelated to uncleaned Data. We can re-render a virtual DOM with or without dirty checking where the diff algorithm is a dirty Cheeker.

Only when the state changes can we re-render the virtual tree. Using an observable, we can verify whether the state has changed in a good way to avoid needless re-renders and tree diffs. We should take no action if nothing has changed.

24. Is setState() is async? 

SetState() activities are asynchronous and batch-processed to improve efficiency. It is clarified in the following documentation.

The state is not automatically mutated when setState() is called; instead, a pending state change is created. After reaching this process, accessing the state can return the current value. Calls to setState cannot be assured synchronously for setState to batch for performance improvements.

It is because setState modifies the state and induces re-rendering. It is a costly process, and making it synchronous can cause the browser to become unresponsive. As a result, the setState calls are both asynchronous and batch, resulting in a better UI experience and performance.

25. In React, what is render()? Explain its purpose?

A render() method is required for each React component. It returns a single React element containing the native DOM component representation. If your work has more than one HTML element to make, a single tag must include all of them, including form>, group>, div>, etc. This function must be held pure in the context that it must consistently return the same result.

26. Explain the React controlled and uncontrolled components 

It is about stateful DOM components and the following difference:

  • A Controlled Component receives its current values via props and notifies calls such as onChange for changes. The parent component “controls” it through the callback process, manages its state, and passes new values as props into the controlled Component. It is frequently referred to as a dumb component.
  • An Uncontrolled Component keeps track of its state internally and uses a ref to find the DOM for its current value. It is more like traditional HTML.

Controlled components can be used in most cases.

27. Explain the components of Redux.

  •     Actions:Actions are data payloads that transmit information from our app to our store. They’re the store’s sole source of information. We are using the store. dispatch, we can transfer them to the store ().
  •     Reducer:Reducers define how the application’s status changes to the storage behavior. It’s important to remember that acts describe what happened, not how the application’s state changed. As a result, the place decides how the state transforms into action.
  •     Store:A Store is an object that connects the Action and the Reducer. The following are the store’s tasks: GetState() gives access to the application’s state. Allows for the updating of state via dispatch(action); Subscriber(listener) registers listeners. Subscribers are unregistered using the feature returned by subscribing (listener).

28. What is React.cloneElement? And the difference with this.props.children?

React.cloneElement creates a new React element from the passed element. The new props will be merged in shallowly with the original element’s props in the resulting element. New children will replace current children. The initial element’s key and the ref will hold.

Only if our child is a single React element will React.cloneElement work. These.props.children are the better option in almost any case. Cloning is helpful in some more advanced topics, such as when a parent component falls into an element. The child component must change some aspects or add things such as a ref to the DOM element.

29. What is the second argument, which can be forwarded to setState optionally, and how does it intend to work?

When setState is completed, and the part is re-rendered, this callback function will be invoked.

Since setState is asynchronous, a second callback function is required. After the state has been updated, we can use this feature to do whatever we want.

30. Explain the difference between React and React Native?

React is a type of JavaScript library used for creating user interfaces and web applications that work on both the front end and the server.

React Native is a mobile framework that builds native app components that lets us create mobile, native JavaScript apps that allow us to use ReactJS for element development and ReactJS implementation.

It is possible to replicate the behavior of a native app in JavaScript using React Native, and the result would be platform-specific code. If we want to optimize our application even further, we can mix native code and JavaScript.

31. What’s the React Router? 

React router is a versatile React routing library that can bring new screens and flows to the application. This keeps the URL synchronized with details on the web page. It maintains a structured form that conducts and is used on single pages for creating web applications. The React Router has a basic API.

32. Why is the router required in React?

A router is used to define different routes, and if a user types a specific URL, the user is forwarded to a particular course when the URL matches the path of some routing specified within the router. We then have to add a Router Library to our application, enabling us to create many routes with an unparalleled view.