Route-based code splitting is a technique for splitting code based on the location of the component in the application. Route-based code splitting allows components to be loaded on-demand based on the user's navigation, reducing the initial load time of the application. Route-based code splitting is typically used with libraries like React Router to enable on-demand loading of code.

In React, you can use the dangerouslySetInnerHTML prop to set the inner HTML of a component. The dangerouslySetInnerHTML prop is used to bypass React's built-in sanitization and allow arbitrary HTML to be injected into a component. However, this should be used with caution, as it can pose a security risk.

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.

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');.

In React, an element is a plain JavaScript object that represents a single HTML element. A component, on the other hand, is a function or class that returns an element (or multiple elements). Components are used to create reusable UI elements, while elements are used to represent the actual DOM elements that make up the UI.

Portals in React provide a way to render a component's children in a different part of the DOM. This can be useful for creating modal dialogs, tooltips, and other UI components that need to be positioned outside the normal flow of the page.