React is a JavaScript library for building user interfaces. React is used to build single-page applications. React allows us to create reusable UI components.
ReactJS is a declarative, efficient, and flexible JavaScript library for building reusable UI components. It is an open-source and component-based front-end library that is responsible only for the view layer of the application. It was initially developed and maintained by Facebook and later used in its products like WhatsApp & Instagram.
Why do we use ReactJS?
The main objective of ReactJS is to develop User Interfaces (UI) that improve the speed of the apps. It uses virtual DOM (JavaScript object), which improves the performance of the app. The JavaScript virtual DOM is faster than the regular DOM. We can use ReactJS on the client and server sides as well as with other frameworks. It uses component and data patterns that improve readability and helps to maintain larger apps.
First of all, React is not a JavaScript framework, it’s simply a javascript library developed by Jordan Walke
that helps you to create User Interfaces(UI). React was first used in Facebook’s newsfeed in 2011 and then later in Instagram, and WhatsApp by 2012, and released to the public in 2013.
Today, most of the applications are built using Model View Controller(MVC) architecture and in this MVC architecture React is the ‘V’ that stands for view.
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In ReactJS, everything is a component and each component is responsible for outputting a small, reusable piece of HTML code. It is mostly used to build reusable components and it reduces the re-rendering of the DOM with the help of Virtual DOM.
Why do we need ReactJS?
As there are many JavaScript frameworks available in the market(like angular, node, jQuery) but what ReactJS has done to the front-end world that made it so popular and one of the most sought-out options in the world of UI development. Let’s have a brief look into some ReactJS features:-
JSX – JSX stands for JavaScript XML. It’s an XML/HTML-like syntax used by ReactJS. It extends the ECMAScript so that HTML-like code can co-exist with JavaScript react code. This format is rendered to the normal browser Javascript by the pre-processors such as Babel. It is much faster than normal JS as it performs optimizations while translating to regular JS. Download the Eduriefy app now to gain more insights into the subject.
Virtual DOM – Do you remember how Facebook’s UI looked a few years back? You had to reload the entire page for new updates repeatedly. But now it’s no longer required and this is the magic of ReactJS.
Re-render everything on every update? It sounds expensive but it’s not. React will make the browser render only if there are any differences and if there are no differences, React will make the browser render nothing. This makes the rendering super fast.
Fundamentals of React
React has exploded in popularity — and for good reason! Let’s study the fundamental building blocks of React and understand things clearly.
- Component – As I already discussed that ReactJS is all about components. Components make the task of building UIs much easier. React allows you to break your page into independent building blocks that can be created, maintained, manipulated, reused independently, and then merged to construct the entire page.
- Props – When building a react application, the UI is divided into many smaller components. Some of these components are re-used in various parts of the application and to make these components effective props are used. They attribute like entities in components, which can be passed to the children’s components. Props are immutable so we cannot modify them inside the child component.
- State – A state is an updatable object that contains data and also keeps track of changes in the component. States are mutable, which means we can modify the data with the help of the setState() method. It is responsible for making components dynamic. Whenever the state changes the setState() schedules an update to a component associated with that particular state object. Example App.js.
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Top 25 Interview Questions and Answers
Question 1: What is React?
React is a front-end JavaScript library developed by Facebook in 2011.
It follows the component-based approach which helps in building reusable UI components.
Q2: What is the use of React JS?
And:- It is used for developing complex and interactive web and mobile UI.
Even though it was open-sourced only in 2015, it has one of the largest communities supporting it.
Q3: What are the features of React?
Major features of React are listed below:
And:-It uses the virtual DOM instead of the real DOM.
It uses server-side rendering.
It follows unidirectional data flow or data binding.
Q4: List some of the major advantages of React.
Ans:-Some of the major advantages of React is:
It increases the application’s performance
It can be conveniently used on the client as well as server-side
Because of JSX, code’s readability increases
React is easy to integrate with other frameworks like Meteor, Angular, etc
Using React, writing UI test cases becomes extremely easy
Q5: What are the limitations of React?
Limitations of React are listed below:
Ans: React is just a library, not a full-blown framework
Its library is very large and takes time to understand
It can be a little difficult for the novice programmers to understand
Coding gets complex as it uses inline templating and JSX.
Q6: What is JSX?
Ans:- JSX is a shorthand for JavaScript XML. This is a type of file used by React which utilizes the expressiveness of JavaScript along with HTML-like template syntax. This makes the HTML file easy to understand.
Q7: What do you understand by Virtual DOM? Explain its works.
Ans:-A virtual DOM is a lightweight JavaScript object which originally is just a copy of the real DOM. It is a node tree that lists the elements, their attributes, and content as Objects and their properties.
Q8: Why can’t browsers read JSX?
Ans:-Browsers can only read JavaScript objects but JSX is not a regular JavaScript object. Thus to enable a browser to read JSX, first, we need to transform the JSX file into a JavaScript object using JSX transformers like Babel and then pass it to the browser.
Q9: How different is React’s ES6 syntax when compared to ES5?
Ans:-Syntax has changed from ES5 to ES6 in the import vs export scenario.
Q10: What is the purpose of using the super constructor with props argument in React?
Answer:-A child class constructor cannot make use of this reference until the super() method has been called. The same applies to ES6 sub-classes as well. The main reason for passing the props parameter to the super() call is to access this. props in your child constructors.
Q11: What’s the difference between a Controlled component and an Uncontrolled one in React?
Answer:-This relates to stateful DOM components (form elements) and the React docs explain the difference:
A Controlled Component takes its current value through props and notifies changes through callbacks like onChange. A parent component “controls” it by handling the callback and managing its state and passing the new values as props to the controlled component. You could also call this a “dumb component”.
Q12: Can you initialize the state from a function? Provide an example
Answer:- Yes, you need to have the react JS knowledge to initialize the state from a function.
Q13: Does React state Hook update immediately?
Ans:-React state and state don’t make changes directly to the state object; they create queues to optimize performance, which is why the changes don’t update immediately. The process to update React state is asynchronous for performance reasons.
Q14: Given the React code defined above, can you identify two problems?
Answer:- The constructor does not pass its props to the superclass.
Q15: How can I make use of Error Boundaries in functional React components?
Answer
As of v16.2.0, there’s no way to turn a functional component into an error boundary. The componentDidCatch() method works like a JavaScript catch {} block, but for components. Only class components can be error boundaries. In practice, most of the time you’ll want to declare an error boundary component once and use it throughout your application.
Q16: What are the lifecycle methods of ReactJS class components?
Answer
- componentWillMount: Executed before rendering and is used for App-level configuration in your root component.
- componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
Q17: What are the different phases of the ReactJS component lifecycle?
Answer:- There are two different phases of React component’s lifecycle:
- Initialization: In this phase react component is prepared to set up the initial state and default props.
- Mounting: The react component is ready to mount in the browser DOM. This phase covers componentWillMount and componentDidMount lifecycle methods
Q18: What do these three dots (…) in React do?
Ans:- That’s property spread notation. It was added in ES2018 (the spread for arrays/iterables was earlier, ES2015).
Q19: What is equivalent to the following using React.create Element?
Answer
const element = (
<h1 className=”greeting”>
Hello, world!
</h1>
Q20: What is prop drilling and how can you avoid it?
Answer
When building a React application, there is often the need for a deeply nested component to use data provided by another component that is much higher in the hierarchy. The simplest approach is to simply pass a prop from each component to the next in the hierarchy from the source component to the deeply nested component. This is called prop drilling.
Q21: What is StrictMode in React?
Answer
React’s StrictMode is sort of a helper component that will help you write better react components, you can wrap a set of components with <StrictMode /> and it’ll basically:
- Verify that the components inside are following some of the recommended practices and warn you if not in the console.
- Verify the deprecated methods are not being used, and if they’re used strict mode will warn you in the console.
- Help you prevent some side effects by identifying potential risks.
Q22: What is the difference between ShadowDOM and VirtualDOM?
Answer
Virtual DOM
Virtual DOM is about avoiding unnecessary changes to the DOM, which are expensive performance-wise because changes to the DOM usually cause re-rendering of the page. Virtual DOM also allows the collecting of several changes to be applied at once.
Q23: What’s wrong with the state code?
Ans:-It is so because of this. props and this. the state may be updated asynchronously, you should not rely on their values for calculating the next state. To fix it, use the second form of setState() that accepts a function rather than an object.
Q24: Why do class methods need to be bound to a class instance?
Answer
In JavaScript, the value of this changes depending on the current context. Within React class component methods, developers normally expect this to refer to the current instance of a component, so it is necessary to bind these methods to the instance.
Q25: Why should we not update the state directly?
Answer
If you try to update the state directly then it won’t re-render the component.
Frequently Asked Questions (FAQs)
Question:- What is ReactJs used for?
Answer:-React JS is a JavaScript library used in web development to build interactive elements on websites.
Question:- Is React hard to learn?
Answer:- Both HTML and CSS are integral to any web development project. If you have these skills already, then learning React should be a relatively straightforward process. It has its own unique set of challenges, but it is an excellent tool to have to start or further your career as a web developer.
Question:- Are ReactJs and JavaScript the same?
Answer:-React is just JavaScript, there is a very small API to learn, just a few functions and how to use them. After that, your JavaScript skills are what make you a better React developer. There are no barriers to entry. A JavaScript developer can become a productive React developer in a few hours.
Question:-Which language is used in React?
Answer:-JavaScript
Question:- Does React replace HTML?
Answer:- What makes React such a desirable library to learn is that it doesn’t replace HTML. It takes advantage of HTML’s popularity and strength as the most popular programming language, by letting you use a very similar syntax to HTML.
