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Interview Questions and Answers

Context API provides a way to pass data through the component tree without having to pass props down manually at every level. Its useful for sharing data that is considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language.

Performance optimization techniques include: using `React.memo` (or `PureComponent` for class components) to prevent unnecessary re-renders, memoizing expensive calculations with `useMemo` and `useCallback`, code splitting, lazy loading, and optimizing images and other assets.

Styled-components is a library that allows you to write CSS-in-JS, meaning you can define CSS styles directly within your React components using template literals. It offers features like automatic vendor prefixing, theming, and component-level styling.

Popular testing libraries include Jest (a testing framework), React Testing Library (for testing components from a users perspective), and Enzyme (for more detailed component testing, though React Testing Library is generally preferred). Cypress is often used for end-to-end testing.

React Router is a standard library for routing in React applications. It allows you to navigate between different views or components based on the URL.

Forms in React are typically handled using controlled components, where the components state is the single source of truth for the form input values. You use event handlers (like `onChange`) to update the state when the user interacts with the form, and the state is used to render the form elements.

Redux is a predictable state container for JavaScript apps. Its used to manage application state in a centralized store, making it easier to reason about state changes and debug issues, especially in large and complex applications.

Common hooks include: `useState` (for managing state), `useEffect` (for side effects), `useContext` (for accessing context), `useReducer` (for managing complex state), `useRef` (for persisting values across renders), and `useMemo`/`useCallback` (for performance optimization).

`useEffect` is a hook that allows you to perform side effects in functional components. Side effects include data fetching, subscriptions, manual DOM manipulations, and setting up event listeners. It runs after every render (or after specific dependencies change).

Props (short for properties) are read-only arguments passed to React components from their parent components. They are used to pass data and configuration options down the component tree.

State is a plain JavaScript object used by React components to manage and store data that may change over time. When state changes, the component re-renders to reflect the updated data. Its private to the component.

You update state using the `setState()` method (for class components) or the `useState` hook (for functional components). `setState()` merges the new state with the existing state and triggers a re-render. For functional components, the setter function returned by `useState` replaces the state value.

Lifecycle methods are special methods that are called at different stages of a components lifecycle, such as when it is mounted (added to the DOM), updated, or unmounted (removed from the DOM). Examples include `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount` (in class components).

Hooks are functions that let you “hook into” React state and lifecycle features from function components. They allow you to use state and other React features without writing a class.

React components are independent, reusable bits of code that render HTML elements. They can be functional or class-based. They accept arbitrary inputs called "props" and return React elements describing what should appear on the screen.

Functional components are simple JavaScript functions that accept props as an argument and return a React element. Class components are ES6 classes that extend `React.Component` and must define a `render()` method that returns a React element. Class components can also manage state and have lifecycle methods.

JSX is a syntax extension to JavaScript that allows you to write HTML-like code within your JavaScript files. It makes your React components easier to read and write. Its not HTML, but it gets transformed into JavaScript calls by a compiler (like Babel).

The Virtual DOM is a lightweight in-memory representation of the actual DOM. React uses it to efficiently update the UI by calculating the differences between the virtual DOM and the real DOM, then applying only the necessary changes to the real DOM.

React.js is a JavaScript library for building user interfaces, particularly for single-page applications where the UI updates frequently. It allows developers to create reusable UI components.

Key features include: Component-based architecture, Virtual DOM, JSX (JavaScript XML), unidirectional data flow, and declarative programming style.