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.

Shadow DOM and Virtual DOM are two different concepts. Shadow DOM is a browser feature that allows developers to encapsulate styles and markup within components. Virtual DOM, on the other hand, is a React feature that allows for performance optimization by minimizing changes to the actual DOM.

In React, state should not be updated directly because it can cause the component to re-render improperly. Instead, the setState method should be used to update state, which triggers a re-render of the component and ensures that the updated state is properly reflected in the UI.

Fragments are a way to group multiple elements without adding an extra DOM node. Fragments are useful when you want to return multiple elements from a component, but don't want to add an unnecessary container element to the DOM.

Events in React are different from native events in several ways. React events are synthetic events that are cross-browser compatible and have the same API across all browsers. React events are also camelCase instead of lowercase (e.g. onClick instead of onclick). Finally, React events have a different API than native events, including the ability to call preventDefault and stopPropagation on the event object.

Memoization is a technique used to optimize functions by caching the results of expensive function calls and returning the cached result when the same inputs occur again. By using memoization, a function can be prevented from being called multiple times with the same inputs.