React.js interview questions and answers for 2022

This is a warm-up React.js interview question that builds into the other tougher questions. Answering this React.js interview question with some practical example that illustrates its application can earn you some brownie points. React.js is an open-source JavaScript library that Facebook developed in 2011. It is used to build user interfaces. Since it uses components to build interfaces, these components can be replicated for different UIs. It is helpful when interactive and complex UIs are built for websites or mobile apps. React.js was released for use in 2015 and since then it has become one of the most trusted and used technologies of recent time. It has built one of the largest developer communities around itself.

Important features of React.js include:

• Instead of a real DOM, there is Virtual DOM
• It completely renders the HTML files on the server into client HTML pages, in other words, server-side rendering
• In React.js, the data can be passed to the other parts of an application only in one direction. In other words, there is a unidirectional flow of data.

The main advantages of React.js are:

• It enhances the performance of the application
• It can be used from the client-side as well as the server-side
• The readability of code is higher in React.js because of JSX
• It offers easy integration with frameworks such as Angular, Meteor, etc.
• It is easy to write UI test cases with React.js

If you can include some practical experience demonstrating the advantages of React.js in this React.js interview question, you are likely to impress the recruiter.

JSX is the syntax extension for Javascript in React.js. With the help of JSX, it is easy to define how the user interface should look. Additionally, with JSX, the file containing the Javascript code can be used to write the HTML structures as well.

Web browsers are built to understand and read original JavaScript or JS objects. Since JSX is not a regular JS object, web browsers are unable to read it. For web browsers to read JSX, it must be made compatible by first converting it into regular JS objects. This is achieved by using a transcompiler, such as Babel. While this may seem like a simple React.js interview question, knowing answers to such questions can be critical.

DOM is the abbreviated version of the Document Object Model. The DOM shows the logical tree structure for an HTML document. There is a node at the end of each tree branch, with every tree node containing one or more objects.

In React.js, a lighter version of real DOM is stored in the memory. It is called the Virtual DOM. When there are any changes in specific objects, instead of updating all objects, only those specific changes are updated in the real DOM. This increases the efficiency and speed of the program. For a React.js interview question like this, if you have the opportunity to illustrate through a diagram, you must, as it shows the depth of your understanding.

We can use the following steps:

• Install the React.js app on the machine as the React.js library installation requires npm. This is because npm or node package manager has several JavaScript libraries including React.js
• Install Sublime text or any text editor similar to it.
• Use the command prompt or terminal to install the create-React.js-app package
• Start the React.js app by going to the newly generated directory (my app) and running npm start
• Set up the component structure and define what components - functional or classes - you will use for your application
• Set up the back-end structure with Ruby-on-Rails API
• Define the App component that will contain two child components - MainContainer and Nav
• Set up the Nav component
• Set up the CreateCard component in the controlled form to then set up the MainContainer component as a class component
• Create a fetch request to the back-end using React.js’s component lifecycle methods
• Set up the ToDoCardContainer component to render all the ToDoCard components
• Render the cards with the ToDoCard component
• Create lists using the addList callback function
• Render lists for appropriate cards by rendering the ToDoList component
• Click on the list item to check it off once it is completed

Be sure to mention all points in detail for such a React.js interview question.

When a user presses a key, clicks the mouse, or performs any action on the machine or when the machine itself triggers an action, these actions are registered as events in React.js.

• In React.js, we use camelCase to name events, instead of the lowercase in HTML
• In React.js, because of JSX, a function is passed as an event handler, instead of the string in HTML

Lists in React.js are created similar to how they are created in regular Javascript. With lists, data can be displayed in an orderly manner and is useful in displaying menus on websites. For traversing lists, the map() function is used. For example,

• An array of numbers is taken by the map() function and their value is multiplied by 5
• var numbers = [2,4,6,8,10]
• const multiplyNums = numbers.map((number => { return (number*5); });
• console.log (multiplyNums);

Output: The output in Javascript will be logged in the console. The output in the above case is [10, 20, 30, 40, 50]

Keys are used in React.js Lists because:

• Key helps in identifying what objects have been altered, moved, or deleted from lists
• With the help of keys, it is easier to determine which objects must be re-rendered instead of all components being re-rendered all the time.

React.js interview questions that ask about the use of a certain function or component in React.js programming are common.