JavaScript URLs are not allowed in React16.9 or any modern web development framework due to security concerns. JavaScript URLs are URLs that begin with "javascript:", and they allow for the execution of arbitrary JavaScript code when clicked. This can be used for malicious purposes, such as stealing user data or injecting malware. Instead of using JavaScript URLs, developers should use event handlers and other safe mechanisms to handle user interactions.

The "constructor" method and "getInitialState" method are both used to initialize the state of a component in React, but they are used in different class styles. The "constructor" method is used in ES6 classes, while "getInitialState" is used in ES5 classes. In general, it is recommended to use the "constructor" method in modern React code.

Pure components in React are components that implement the shouldComponentUpdate lifecycle method to improve performance. This method compares the current props and state to the next props and state, and determines whether the component needs to be re-rendered. If the props and state have not changed, the component can skip the rendering process.

When a component's state changes in React, it updates the Virtual DOM and calculates the most efficient way to update the actual DOM based on the differences between the two versions. This approach minimizes the number of changes that need to be made to the actual DOM and improves performance.

Stateful components, also known as class components, are components that maintain state. Stateful components are useful for creating more complex UI components that need to maintain state, but they are more difficult to test than stateless components.

Whether or not to use Redux to manage state in a React application depends on the complexity and size of the application. For small and simple applications, it may be sufficient to use React's internal state management. However, for larger and more complex applications, Redux can be a helpful tool for managing application state.

The ref argument is only available for class components in React. Function components do not have an instance, so refs cannot be attached to them. Refs can be attached to DOM elements, class components, and functional components that are created using the forwardRef() method.

In React, the "setState" method is used to update component state, and it merges the new state with the old state. The "replaceState" method is similar, but it overwrites the old state completely with the new state. However, "replaceState" is deprecated in React and should not be used. Instead, you should use the "setState" method to update state in a React component.

In Create React App, you can use HTTPS instead of HTTP by setting the "HTTPS=true" environment variable. This will start the development server with HTTPS enabled, and will allow you to test your application with SSL/TLS encryption.

In React Router v4, you can programmatically navigate by using the "history" object provided by the router. To navigate to a new route, you can use the "this.props.history.push" method with a route path as a string. This method adds a new entry to the history stack and navigates to the specified route. For example: this.props.history.push('/new-route');.