Synthetic events in React are events that are cross-browser compatible and behave consistently across different platforms. They are also optimized for performance by pooling event objects, which allows them to be reused and prevents excessive memory allocation. Synthetic events have the same interface as native events, but they are implemented differently behind the scenes.

The componentWillUnmount() method is called immediately before a component is unmounted and destroyed. It is not safe to call setState() within this method, as the component's state is no longer available after unmounting. Attempting to update the state after unmounting can cause errors or unexpected behavior in the application.

The displayName class property in a React component is used to set the name of the component for debugging purposes. This name is used in error messages and in the React Developer Tools to help identify the component in the component hierarchy. If the displayName property is not set explicitly, React will attempt to infer the name of the component from the name of the component class or function.

In React, state refers to a component's internal data that can change over time. State is managed using the setState method, which allows developers to update the state of a component and trigger a re-render of the UI. State is used to keep track of data that changes in response to user input or other events.

It is not recommended to use arrow functions in render methods because it can cause unnecessary re-renders. Arrow functions are created each time the component renders, which can cause a new reference to be passed down to child components, triggering unnecessary re-renders.

Render props in React are a technique for sharing code between components. A render prop is a function that a component uses to render its children, allowing the children to access the component's state or other data. The render prop can be named anything, and the name of the prop does not affect its functionality. The convention of naming the prop "render" is simply a convention and is not required by React.

Reselect is a library for optimizing Redux selectors. It provides a way to create memoized selectors that only recompute when their input selectors have changed. This can help to improve performance and reduce unnecessary re-renders in React applications. Reselect works by caching the results of previous computations and only re-executing the computation if the input selectors have changed.

The callback function as an argument of the setState method in React is used to update the state asynchronously. When the setState method is called, the state update may not happen immediately, which can cause issues when trying to access the updated state. The callback function is called after the state has been updated, ensuring that the updated state is available for use.

Using props in initial state will cause the initial state to ignore the props. This is because the initial state is only set once, when the component is first created. If you want to use props to initialize state, you should set the state in the constructor instead.

String refs, which allow developers to set refs using a string identifier, are considered legacy because they can cause naming conflicts and bugs. They have been replaced with callback refs and the React.createRef() API.