Alexander

Верстка:

• https://styled-components.com
  

Тестирование:

• https://www.cypress.io/
  

Нет.

Иди учить.

https://reactjs.org/docs/react-component.html#cons...

The constructor for a React component is called before it is mounted. When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement. Otherwise, this.props will be undefined in the constructor, which can lead to bugs.

На счет PureComponent, встречал такой термин когда использовал Redux почитать можно тут:
https://medium.com/@dan_abramov/smart-and-dumb-com...

import {BrowserRouter, Route} from 'react-router-dom';
<BrowserRouter>
    <Route path="/" exact component={Home}/>
</BrowserRouter>

В экшенах. redux.js.org/docs/basics/Actions.html