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(); }.

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' } });.

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');.

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.

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.