For your first question I need to say: No, you don't need react-router in Nextjs it will use something called file-system based router which you can read more about it here

So after you set up your routes if you want to Navigate to them you have two options:

first using the Link Component from next/link: more about it here

second using the router from next/router which you can Navigate around like useHistory from react-router: more about it here

example from the doc:

import { useRouter } from 'next/router'



function ActiveLink({ children, href }) {

  const router = useRouter()

  const style = {

    marginRight: 10,

    color: router.asPath === href ? 'red' : 'black',

  }



  const handleClick = (e) => {

    e.preventDefault()

    router.push(href)

  }



  return (

    <a href={href} onClick={handleClick} style={style}>

      {children}

    </a>

  )

}



export default ActiveLink

So in your case, using this is how you can redirect:

import { withRouter } from 'next/router'



Class Login extends Component {



  state = {

    user: {},

  }



  loginUser = (e) => {

      loginUser(this.state.user)

        .then(response => {

          console.log(response);

          if (response['username']) {

            console.log('yippee!');

            //here is what you need:

            this.props.router.push('/your-route');

          }

      });

  }



}



export default withRouter(Login)