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.

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.

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.

Error boundaries were not supported in React v15. Error handling in React v15 was less robust and could lead to the entire application crashing if an error occurred during rendering.

CRA stands for Create React App, which is a boilerplate for creating React applications. It provides a pre-configured setup for building, testing, and deploying React applications, allowing developers to focus on writing code rather than setting up the build toolchain. Some benefits of using CRA include easy setup, automatic configuration, and a built-in development server.

In React Router v4, you can pass params to the history.push method by using the "state" property of the location object. This property can contain any data that you want to pass along with the route, such as query parameters, form data, or session information. For example: this.props.history.push({ pathname: '/new-route', state: { foo: 'bar' } });.

In React, a component's props will default to true if the prop value is undefined. This can happen if the prop is not passed explicitly in the component declaration or if it is explicitly set to undefined in the parent component. To avoid this behavior, default values can be set for props using the defaultProps property in the component class.

In React, you can focus an input element on page load by using the "focus()" method in the "componentDidMount()" lifecycle method. This will set the focus to the input element after the component has been mounted in the DOM. For example: componentDidMount() { this.myInput.focus(); }.

React Hooks were introduced based on community proposals and discussions, as well as official React documentation and blog posts. They were also presented and discussed at React conferences and meetups.

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.

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.

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');.