Constants in Redux are used to define action types, which are used to identify the type of action being dispatched to the Redux store. By using constants, you can prevent unintended changes to the action types and make it easier to understand the purpose of each action.

The ReactTestUtils package provides a set of utilities for testing React components. It allows developers to simulate events, perform component rendering, and find components in the rendered output. It is commonly used in combination with Jest or another testing framework.

React supports a subset of HTML attributes, as not all attributes are applicable or relevant to React components. Some HTML attributes, such as "class" and "for", are reserved words in JavaScript and cannot be used directly in JSX. Instead, React uses the "className" and "htmlFor" attributes, respectively. Additionally, some HTML attributes may have different names or syntax in React, such as "tabindex" being spelled "tabIndex" in React.

In React and ES6, you can import and export components using the "import" and "export" keywords. To import a component, you can use the "import" keyword followed by the component name and file path. To export a component, you can use the "export" keyword before the component declaration. For example: import React from 'react'; export class MyComponent extends React.Component { ... }.

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' } });.

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.

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.

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.