Frequently asked React JS Interview Questions and Answers
What is React?
React. js is an open-source JavaScript library that is used for building user interfaces specifically for single-page applications. … React allows developers to create large web applications that can change data, without reloading the page. The main purpose of React is to be fast, scalable, and simple.
Why is React so fast to load the data?
React is a JavaScript library, thus it technically isn’t “faster” at the act of DOM manipulation than JavaScript itself. React isn’t “faster than JavaScript”. React is approximately as fast as JavaScript.
React Virtual DOM is not faster than real DOM. The real DOM itself is fast, it can search, remove and modify elements from the DOM tree quickly. The real benefit from Virtual DOM is it allows calculation the different between each changes and make make minimal changes into the HTML document.
What is state?
React components has a built-in state object. The state object is where you store property values that belongs to the component. When the state object changes, the component re-renders.
Tell me about the react hook?
React Hooks are functions that let us hook into the React state and lifecycle features from function components. Hooks allow us to easily manipulate the state of our functional component without needing to convert them into class components.
What are the features of React?
· JSX
· Components
· One-way Data Binding
· Virtual DOM
· Simplicity
· Performance
Tell me about the reduce
You have an array of items and you want to compute some new value by iterating over each item. The result can be anything, another array, a new object, a boolean value etc.
What is JSX?How does JSX work?
JSX is the extension of javascript syntax. Its syntax is similar to HTML. ReactJS components are written in JSX. JSX can be seen as a combination of javascript and XML. Its syntax is very simple that makes writing components very easy.
JSX you can write HTML code in JavaScript then Babel transform these expressions into actual JavaScript code.It allows us to put HTML into JavaScript.
Difference between JSX and JS?
JS is standard javascript, JSX is an HTML-like syntax that you can use with React to (theoretically) make it easier and more intuitive to create React components. … Without JSX, creating large, nested HTML documents using JS syntax would be a large pain in the rear; JSX simply makes that process easier.
Why can’t browsers read JSX?
Browsers cannot read JSX because there is no inherent implementation for the browser engines to read and understand it. You can use babel to transform your jsx into native javascript and HTML which browser can understand.
JSX is not valid JavaScript, web browsers cant read it directly. So, if JavaScript files contains JSX, that that file will have to be changed. That means that before the file gets to the web browser, a JSX compiler will translate any JSX into regular JavaScript. JSX produces React “elements”.
Tell me about the virtual dom?
DOM stands for Document Object Model. It is the hierarchical representation of your web page(UI). virtual DOM is just a copy of the original DOM kept in the memory and synced with the real DOM by libraries such as ReactDOM. This process is called Reconciliation.
Virtual DOM has the same properties that of the Real DOM, but it lacks the power to directly change the content of the screen.
Think of Virtual DOM as the blueprint of a machine, changes made to the blueprint doesn’t reflects on the machine itself.
So when there is a update in the virtual DOM, react compares the virtual DOM with a snapshot of the virtual DOM taken right before the update of the virtual DOM.
With the help of this comparison React figures out which components in the UI needs to be updated. This process is called diffing. The algorithm that is used for the diffing process is called as the diffing algorithm.
Once React knows which component has been updated, then it replaces the original DOM nodes with the updated DOM node.
What are the differences between VDOM and DOM?
A virtual DOM object has the same properties as a real DOM object, but it lacks the real thing’s power to directly change what’s on the screen. Manipulating the DOM is slow. Manipulating the virtual DOM is much faster
What is Props?
Props are arguments passed into React components. Props are passed to components via HTML attributes.
What is default Props?
defaultProps is a property in React component used to set default values for the props argument. It will be changed if the prop property is passed.
defaultProps can be defined as a property on the component class itself, to set the default props for the class.
What is an event in React?
An event is an action that could be triggered as a result of the user action or system generated event. For example, a mouse click, loading of a web page, pressing a key, window resizes, and other interactions are called events.
- function ActionLink() {
- function handleClick(e) {
- e. preventDefault();
- console. …
- }
- return (
- <a href=”#” onClick={handleClick}>
- Click_Me.
What is the significance of keys in React?
Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity.
Can a child component modify its own props?
. A component cannot update its own props unless they are arrays or objects (having a component update its own props even if possible is an anti-pattern), but can update its state and the props of its children.
What are the different phases of React component’s lifecycle?
· Initial Phase
· Mounting Phase
· Updating Phase
· Unmounting Phase
What are Higher Order Components(HOC)?
A higher-order component (HOC) is an advanced technique in React for reusing component logic. … They are a pattern that emerges from React’s compositional nature. Concretely, a higher-order component is a function that takes a component and returns a new component.
What are Pure Components?
Pure Components do not depend or modify the state of variables outside their scopeThe output is solely dependent on Input parameters and no other external variable. A React component is considered pure if it renders the same output for the same state and props.
What are hooks in React?
React Hooks are functions that let us hook into the React state and lifecycle features from function components. Hooks allow us to easily manipulate the state of our functional component without needing to convert them into class components.
Will React hooks work inside class components?
Hooks don’t work inside classes — they let you use React without classes. In fact, they can only be used in functional components.
How to test components that use Hooks?
If you need to test a custom Hook, you can do so by creating a component in your test and using your Hook from it. Then you can test the component you wrote.
How do I implement getDerivedStateFromProps?
It is used when the state of the component depends on changes in props over time.
Why do we need a Router in React?
React router routes the web pages in react. Without exact routing, it is not possible to visit the expected page.