10 Most Important React.js Interview Questions [2022 Updated]
Today, this library has become much popular and also widely used due to various advantages provided by react.js.
In this blog, I will be covering the top 10 most important React.Js interview questions.
What is React.js?
What Are Some Features of React.js?
- Components: It is the building block of React elements. using components lets you split the UI into independent, reusable pieces. components are like JS functions.
- Virtual DOM: is a “virtual” representation of UI that is kept in memory and synchronized with the “real DOM” by a library using ReactDOM.
- One-way data-binding: React uses one-way data binding. In one-way data-binding one of the following conditions can be followed:
- Component to View: any change in component data would get reflected in the view.
- View to Component: any change in view would get reflected in the component’s data.
- High performance: React updates only those components that have changed rather than updating the whole component. This leads to react.js working faster. Also, react uses virtual DOM which makes the app runs faster.
What Is the Difference Between Real DOM and React’s Virtual DOM?
- DOM stands for “Document Object Model” and is also an application programming interface for manipulating HTML documents.
- It represents an HTML document as a tree of nodes. DOM also provides you features to add, remove, update and delete the parts of the document.
- In the DOM, all the HTML documents are Objects and the programming interface is the properties and methods of each object. (properties – is the value that changes the content of HTML elements, method – is the action that we performed like add or delete)
React Virtual DOM
- React use the virtual DOM and it is a lightweight copy of the actual DOM.(i.e it is a virtual representation of the DOM). So, every object that is present in the original DOM, is also present in React Virtual DOM.
- The Virtual DOM is an abstraction of the HTML DOM. It is lightweight and detached from the browser-specific implementation details. Since the DOM itself was already an abstraction, the virtual DOM is, in fact, an abstraction of an abstraction.
What Is the Difference Between React Component State and Props?
|In-state, we cannot pass data from one component to another component.||In props, data can be passed from one component to another component.|
|In-state, we can modify the data.||Props cannot be modified.|
|In-state, both read and write operations can be done.||In props, are read-only.|
|States are mutable.||Props are immutable.|
Explain the Reconciliation Process in React
Reconciliation is the process through which react updates the Real DOM(browser DOM). It uses the diffing algorithm to update the react component faster.
These two important concepts are behind the working of the reconciliation process: –
- Virtual DOM: React renders JSX components to the Browser DOM and keeps the copy of the real DOM. This copy is the virtual DOM.
- Diffing Algorithm: React uses the diffing algorithm to update the react nodes.
What Are Hooks in React?
While using hooks you need to follow a few rules:
- Don’t call Hooks inside loops, conditions or nested functions.
- Hooks can be used only in functional components.
What is the Life Cycle of a React.js Class Component?
Each component in react goes through the life cycle of React which keeps monitoring and updating the component. The lifecycle of the component is divided into four phases.
- Mounting: – Mounting is the first phase in the life cycle. Mounting means putting the elements into the DOM. It has four built-in methods that are used during mounting.
- The constructor for react component is used before it is mounted.
- In class component, while implementing the constructor for a React. component subclass, always you should use
super(props)before any other statement.
this.propswill be undefined in the constructor.
getDerivedStateFromProps() the method is called right before rendering the element(s) in the DOM.
render() a method is the only required method in a class component. render() should always be pure, which means it does not modify the component state, it returns the same result every time it gets called.
componentDidMount() gets invoked immediately after the component gets inserted (mounted).
2. Updating:- In this phase when the component gets rendered whenever there is a change in the component’s state or properties.
React has five built-in methods that get called, in this order, when a component is updated:
getDerivedStateFromProps() are invoked right before invoking the render method, both on the initial mount and on subsequent updates.
shouldComponentUpdate() let’s react to know if a component’s output is not affected by the current change in state or props.
render() the method is of course called when a component gets updated, it has to re-render the HTML to the DOM, with the new changes.
getSnapshotBeforeUpdate() is invoked right before the recently rendered output is done.
componentDidUpdate() is invoked after the updating occurs in the DOM.
3. Updating: -in this phase, when a component is removed from the DOM or unmounted then React calls it.
componetWillUnmount() is called immediately before the component is unmounted or removed.
What Is PureComponent in React?
Based on the concept of purity in function programming paradigms, a function is said to be pure if it meets the following two conditions:
- Its return values are only determined by its input values.
- Its return values are always the same for the same input values.
React Component is considered pure, it renders the same output for the same state and props.
What Are a Few Differences Between a Class and Functional Component?
|Functional Components||Class Components|
|Earlier referred to as stateless components as they simply accept data and display them in some form; that is they are mainly responsible for rendering UI. Now they can also be stateful thanks to hooks.||Sometimes called stateful components as they tend to implement logic and state.|
|React lifecycle methods (for example, componentDidMount()) cannot be used in functional components. It hooks like ||React lifecycle methods can be used inside class components (for example, componentDidMount()).|
|There is no render method used in functional components. Whatever is returned from the function is the component code.||We pass props down to class components and access them with |
|Functional components should be favoured moving forward.|
What is Higher-order-Component (HoC) in React?
- It is a function that takes a component and returns a new component.
- It is an advanced technique in React for reusing the component logic.
- They are the pattern that emerges from React’s compositional nature. The component transforms props into UI, and a higher-order component converts a component into another component.
Learn basics about React UI library and understand how to work with React for small to mid-sized projects
Unlimited access to all platform courses
100+ practice projects included
ChatGPT Based Instant AI Help
Structured React.js/Next.js Full-Stack Roadmap To Get A Job
Exclusive community for events, workshops
Sharing is caring
Did you like what Prateek Singh wrote? Thank them for their work by sharing it on social media.
No comments so far
- What are React JS Class Components?
- What is React.js?
- What Are Some Features of React.js?
- What Is the Difference Between Real DOM and React’s Virtual DOM?
- What Is the Difference Between React Component State and Props?
- Explain the Reconciliation Process in React
- What Are Hooks in React?
- What is the Life Cycle of a React.js Class Component?
- What Is PureComponent in React?
- What Are a Few Differences Between a Class and Functional Component?
- What is Higher-order-Component (HoC) in React?