Keyed Fragments are a feature in React that allows elements to be grouped together without adding extra nodes to the DOM. This can improve performance and reduce the complexity of the component hierarchy. Keyed Fragments are created by using the element and adding a unique key prop to each child element.

Relay is a library for managing network requests in React applications. It is often used in conjunction with GraphQL APIs and provides a number of features for managing data fetching and caching, such as automatic batching, optimistic updates, and pagination. Relay also provides a way to declare data dependencies for your components, which can help to reduce unnecessary re-renders.

In React, components can be prevented from rendering unnecessarily by using the shouldComponentUpdate() lifecycle method. By implementing this method, components can determine whether an update is necessary before rendering. This can help improve performance by minimizing the number of unnecessary re-renders.

Redux selectors are functions that transform the Redux state into a more useful format for the application. They allow you to abstract the details of the state shape and provide a simpler interface for accessing and manipulating the state. This helps to keep the code more maintainable and reduces the risk of bugs when the state shape changes.

Higher-Order components (HOCs) are components that enhance the behavior of other components by adding additional functionality or props. HOCs take a component as input and return a new component that includes the additional behavior. This allows developers to reuse code and separate concerns in their applications.

redux-saga is a middleware library for Redux that allows you to handle side effects, such as asynchronous data fetching or complex state updates, in a declarative and testable way. It uses ES6 Generators to provide a more readable and maintainable way to handle complex logic in your Redux application.

Forward refs are a way to pass a ref from a parent component to a child component. This allows the parent component to access and manipulate the child component's DOM node. Forward refs are created using the React.forwardRef() function.

It is not mandatory to define a constructor for a React component. If the component does not need to set its initial state or bind methods to the component, the constructor can be omitted. Additionally, if the component does not have any props, a constructor is not necessary.

The proper way to access the Redux store is to use the connect() function provided by React Redux. This allows you to connect a component to the store and access its state and dispatch functions.

The mental model of redux-saga is that of a finite state machine. It allows you to define a sequence of steps for handling a specific side effect and provides a way to manage the state of that sequence. This makes it easier to reason about and test complex logic in your Redux application.