React.js is a JavaScript library for building user interfaces, particularly single-page applications where you can update data without refreshing the entire page. It allows developers to create reusable UI components.

Components are the building blocks of a React application. They allow you to split the UI into independent, reusable pieces that can be managed separately.

JSX stands for JavaScript XML. It is a syntax extension that allows you to write HTML elements in JavaScript and place them in the DOM without using createElement() or appendChild() methods.

The virtual DOM is a lightweight representation of the real DOM. React uses the virtual DOM to improve performance by updating only the parts of the DOM that have changed, rather than re-rendering the entire UI.

Class components are ES6 classes that extend from React.Component, while functional components are simple functions that return JSX. Functional components are simpler and are often used with React Hooks to manage state.

React Hooks are functions that let you use state and other React features in functional components. Examples include useState, useEffect, and useContext.

The useState Hook allows you to add a state to a functional component. It returns a state variable and a function to update that state.

The useEffect Hook lets you perform side effects in functional components, such as fetching data, directly manipulating the DOM, or setting up subscriptions. It runs after every render by default but can be controlled using dependency arrays.

Props (short for "properties") are read-only inputs that are passed from a parent component to a child component. They allow components to communicate with each other.

State is a built-in object that stores data about the component that can change over time. Unlike props, state is managed within the component and can change based on user interaction.

setState is a method used to update the state of a component. When the state changes, React re-renders the component to reflect the updated state.

The render method is required in class components and is used to describe what the UI should look like. It returns a React element that represents the UI.

.container {

  display: grid;

  grid-template-columns: 1fr 2fr;

}

React Router is a standard library for routing in React. It allows you to manage navigation between different components and pages in a React application.

Events in React are handled similarly to DOM events but with some differences. Event handlers are passed as props to components and are written in camelCase. For example:

<button onClick={handleClick}>Click me</button>

The key prop is used to uniquely identify elements in an array of components. It helps React optimize rendering by determining which items have changed, been added, or removed.

A higher-order component is a function that takes a component and returns a new component with enhanced functionality. HOCs are used for reusing component logic.

Controlled components are form elements that are controlled by the React state. The form data is handled by the React component, and changes are made using event handlers

Uncontrolled components are form elements that maintain their state and are not controlled by React. They can be accessed using refs to retrieve their values.

Prop drilling occurs when you pass data through multiple levels of components to reach a deeply nested component. It can be avoided using context, Redux, or other state management solutions.

The context API is a way to share data between components without passing props manually through every level of the component tree. It is useful for managing the global state.

A context can be created using the React.createContext() method. Example:

const MyContext = React.createContext(defaultValue);

React.Fragment is a wrapper that allows you to group multiple elements without adding an extra node to the DOM. It is useful when you need to return multiple elements from a component.

@keyframes example {

  from {background-color: red;}

  to {background-color: yellow;}

}

div {

  animation: example 5s infinite;

}

useMemo is used to memoize the result of a function, preventing expensive calculations on every render.

useCallback is used to memoize a function itself, preventing it from being recreated on every render.

Data can be passed from a child to a parent component by passing a function as a prop to the child, which the child can then call with the data as an argument.

React.lazy and Suspense are used for code splitting and lazy loading in React.

React.lazy allows you to dynamically import components, and Suspense is used to wrap lazy-loaded components and show a fallback while they are loading.

Performance in a React application can be optimized by using techniques such as memoization (React.memo), code splitting, lazy loading, using the shouldComponentUpdate lifecycle method, and avoiding unnecessary re-renders.

Redux is a state management library that is often used with React to manage global state in complex applications. It centralizes the state in a store and allows components to access and update the state in a predictable way.

The connect function is used to connect a React component to the Redux store. It allows you to map state and dispatch actions to the component's props.

React Portals allow you to render a component outside of its parent component's DOM hierarchy. This is useful for rendering modals, tooltips, and other UI elements that need to appear above other elements.

The useRef Hook is used to persist a value across renders without causing a re-render when the value changes. It is often used to access DOM elements directly or to store mutable values.

Forms in React can be handled using controlled or uncontrolled components. Controlled components use state to manage form data, while uncontrolled components use refs.

The shouldComponentUpdate method is a lifecycle method in class components that allows you to control whether a component should re-render when its state or props change. It is used to optimize performance.

defaultProps is a property used to define default values for props in a component. It ensures that the component has default values for props if they are not provided.

PropTypes is a type-checking feature in React that allows you to specify the expected types of props passed to a component. It helps catch bugs by ensuring that components receive the correct type of data.

The forwardRef API allows you to forward a ref from a parent component to a child component, enabling the parent to access the child's DOM node or component instance.

A custom Hook is a reusable function that encapsulates stateful logic and can be shared between components. It starts with the word "use" and can be created by extracting logic from existing components.

Reconciliation is the process by which React updates the DOM to match the virtual DOM. React compares the virtual DOM with the previous version and only applies changes to the actual DOM where necessary.

React.StrictMode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants, helping developers identify unsafe lifecycle methods, deprecated APIs, and other issues.

Inline styles in React are written as objects with camelCase properties. For example:

<div style={{ backgroundColor: 'blue', color: 'white' }}>Hello</div>

React.createElement is a method used to create React elements manually, while JSX is a syntactic sugar that simplifies the creation of elements by allowing you to write HTML-like code directly in JavaScript.