React JS Interview Questions [Part – 1]

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1) What is React.js and how does it differ from other JavaScript libraries?

  • React.js is a JavaScript library for building user interfaces.
  • It allows developers to create reusable UI components and manage the state and props of those components.
  • It differs from other JavaScript libraries in that it focuses specifically on the view layer of an application, making it a great choice for building complex, large-scale user interfaces.

2) What are the advantages of using React.js?

  • Reusable components.
  • Virtual DOM for efficient updates and rendering.
  • Good performance.
  • Strong developer community and support.
  • Easy integration with other libraries and frameworks.
  • Can be used on the client and server sides.

3) How does the virtual DOM in React.js work?

  • React uses a virtual DOM (Document Object Model) to optimize updates and rendering.
  • The virtual DOM is a lightweight in-memory representation of the actual DOM.
  • When the state of a component changes, React compares the virtual DOM with the actual DOM and only makes changes to the actual DOM where necessary, which is much more efficient than rerendering the entire page.

4) How does React.js handle updates and rendering?

  • When a component’s state changes, React will re-render that component and its child components to reflect the new state.
  • React uses a virtual DOM to optimize updates by only re-rendering the specific parts of the actual DOM that has changed.
  • This helps to improve the performance of the application.

5) What are the components in React.js and how are they used?

  • Components in React.js are the building blocks of a React application.
  • They are used to create reusable UI elements.
  • Components can be either functional or class-based and can be nested to create more complex UI elements.
  • Components accept inputs called props and manage their own state.

6) How does React.js handle state and props?

  • State in React.js refers to the data or variables that determine a component’s behavior and render its content.
  • The state can be changed within a component, which will trigger a re-render.
  • Props (short for properties) are inputs passed to a component from its parent.
  • They are read-only and cannot be changed within the component.

7) What is JSX and how is it used in React.js?

  • JSX is a syntax extension for JavaScript that allows developers to write HTML-like elements in their JavaScript code.
  • It is used in React to describe the structure and content of a component.
  • JSX is transpiled to plain JavaScript before being executed, so it is compatible with all web browsers.

8) What is the component lifecycle in React.js?

  • The component lifecycle in React.js refers to the different stages a component goes through, from its creation to its destruction.
  • The main lifecycle methods include
    1. componentDidMount: executed after the first render.
    2. componentDidUpdate: executed after each update.
    3. componentWillUnmount: executed before the component is removed from the DOM.

9) How do you use event handling in React.js?

  • Event handling in React.js is done using the onEventName syntax, where EventName is the name of the event you want to handle, such as onClick or onSubmit.
  • Event handlers are passed as props to the component and are typically defined as arrow functions or bound methods.

10) What is the significance of props in React.js?

  • Props are used to pass data from a parent component to a child component.
  • Props provide a way to make components reusable and configurable.
  • Props components are read-only components.

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