React Js Interview Questions And Answers

41. What is Props?
Answer: Props are Properties of React. It is immutable Properties of React, In another word you can’t change the value of props. They are always passed down from the parent to the child components throughout the application. A child component can never send a prop back to the parent component. we can also say, Props are communication channel of two different component. It is using to render the dynamically generated data and helping to maintain the unidirectional data flow.

42. What is Higher-Order Components in React?
Answer: A higher-order component is an advanced way to re-using component logic and it is a function that takes a component and returns a new component that is known as Higher-Order Components.

43. Why virtual dom is faster?
Answer:

Virtual dom is faster because it uses:

• Efficient diff algorithm
• Batched update operations
• Efficient update of subtree only
• Uses observable instead of dirty checking to detect a change

Note: ReactJS maintains two virtual DOM, one of the updated state Virtual DOM and another with the prior state Virtual DOM.

44. Why should we use ReactJS?
Answer: As you can probably tell, this is one of the more opinion-based ReactJS interview questions. You would do well, however, if you simply state some of the best features that are accredited to React.

These include, but are not limited to:

Virtual DOM instead of a real DOM;

Fast and scalable;

JSX provides a code that’s easy to both read and write;

React is easily integrated with other JavaScript frameworks;
etc.

45. What Are the differences in between Real DOM and Virtual DOM?
Answer:

Real DOM:

• It updates slow as compare to Virtual DOM
• We can directly update the HTML on DOM at the run time.
• Will creates a new DOM if element updates
• DOM manipulation is a bit expensive
• There is too much memory wastage

Virtual DOM:

• It updates faster as compare to Real DOM
• We can’t directly update the HTML on DOM at the run time.
• Will need to updates the JSX if element updates
• DOM manipulation is easy as compare to the real DOM.
• There is no memory wastage

46. What Are the lifecycle methods of Reactjs components?
Answer:

The life cycle methods of the Reactjs are:

Component Will Mount: This method executed just before rendering takes place both on the client and server.

Component Did Mount: This method executed on the client-side only after the first render.

Component Will Receive Props: This method invoked as soon as the props are received from the parent class and before another render is called.

Should Component Update: This method returns true or false value based on certain conditions.

Component Will Update: This method called just before rendering takes place in the DOM.

Component Did Update: This method called immediately after rendering takes place.

Component Will Unmount: This method called after the component is unmounted from the DOM and used to clear the memory spaces.

47. Mention the key benefits of Flux?
Answer: Applications that are built on Flux have components which can simply be tested. By simply updating the store, developers are able to manage and test any react component. It cut down the overall risk of data affection. All the applications are highly scalable and suffer no compatibility issues.

48. What are the differences between a class component and functional component?
Answer:

1. Class components allow us to use additional features such as local state and lifecycle hooks. Also, to enable our component to have direct access to our store and thus holds state.
2. When our component just receives props and renders them to the page, this is a ‘stateless component’, for which a pure function can be used. These are also called dumb components or presentational components.
3. From the previous question, we can say that our Booklist component is functional components and are stateless.

49. What do you understand by Virtual DOM? Explain its working?
Answer: A virtual DOM is a lightweight JavaScript object which originally is just the copy of the real DOM. It is a node tree that lists the elements, their attributes, and content as Objects and their properties. React’s render function creates a node tree out of the React components. It then updates this tree in response to the mutations in the data model which is caused by various actions done by the user or by the system.

50. Can you use ref inside a functional component?
Answer: No, you cannot use ref inside a functional component since functional components are stateless and they produce the same output. However, ref attribute is used to get the reference to a DOM node or an instance of a component in React and then manipulate its instance.

51. How are forms created in React?
Answer: React forms are similar to HTML forms. But in React, the state is contained in the state property of the component and is only updated via setState(). Thus the elements can’t directly update their state and their submission is handled by a JavaScript function. This function has full access to the data that is entered by the user into a form.

52. Explain the role of Reducer?
Answer: Reducers are pure functions which specify how the application’s state changes in response to an ACTION. Reducers work by taking in the previous state and action, and then it returns a new state. It determines what sort of update needs to be done based on the type of the action, and then returns new values. It returns the previous state as it is if no work needs to be done.

53. What is render() to React? And explain its purpose?
Answer: Each React component must have a render() mandatorily. It returns a single React element which is the representation of the native DOM component. If more than one HTML element needs to be rendered, then they must be grouped together inside one enclosing tag such as This function must be kept pure i.e., it must return the same result each time it is invoked.