React is widely used by many popular websites and applications, including Facebook, Instagram, and Netflix. Other companies that use React include Airbnb, Dropbox, and Salesforce.

No, it is not recommended to dispatch actions in a reducer function. This violates the principle of unidirectional data flow in Redux, in which actions flow in a single direction from the view to the reducer. Dispatching an action in a reducer can lead to unexpected behavior and make the application more difficult to reason about.

In React, Imperative and Declarative are two different approaches to building user interfaces. Imperative programming involves giving explicit instructions on how to accomplish a task, while declarative programming involves describing what the outcome should be without specifying how to achieve it. React uses a declarative approach, which means that developers describe what the user interface should look like, and React takes care of the details of how to render it. This makes it easier to reason about and maintain complex UIs.

To use Polymer in React, you can use the @polymer/reactive-elements library. This library provides React components that wrap Polymer elements, allowing you to use them in your React applications. The @polymer/reactive-elements library also provides a way to create new React components that extend existing Polymer elements.

The suspense component is a component for delaying rendering in React. The suspense component allows components to wait for asynchronous data to load before rendering, improving the user experience and reducing loading times. The suspense component is typically used with code splitting and lazy loading to enable on-demand loading of code.

ReactDOM is separated from React in order to improve performance and reduce the bundle size of React applications. Separating the rendering logic from the component logic allows for more efficient updates and reduces the amount of JavaScript that needs to be downloaded by the client. This separation also allows for easier integration with other rendering targets, such as native mobile apps or desktop applications.

Strict mode in React is a mode that highlights potential problems in the code, such as deprecated lifecycle methods or unsafe practices. It can help identify issues early on and improve the overall quality of the code. Strict mode can be enabled globally or for individual components.

The main goal of React Fiber is to improve client-side rendering performance. It does this by introducing a new algorithm for rendering updates that is more efficient and flexible than the previous algorithm. With React Fiber, React can break up large updates into smaller chunks, prioritize updates, and pause and resume updates as needed.

JSX is a JavaScript syntax extension that allows developers to write HTML-like code in JavaScript. JSX is not required to use React, but it is commonly used because it makes writing and managing UI components easier.

Higher-Order Components (HOCs) are a powerful pattern in React that allow developers to reuse code between components. HOCs can also be implemented using render props, which involves returning a new component from the higher-order component that renders the children using a function prop. This pattern is known as "render prop HOCs" and is a flexible and powerful way to share code between components.