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.

The "key" prop in React is a special prop that provides a unique identifier for each element in an array of elements. It is used to optimize the performance of rendering by allowing React to identify which elements have changed, and to update only those elements instead of re-rendering the entire list. The "key" prop should be a unique and stable identifier for each element, such as an ID or index.

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.

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.