Next.js Redirect from / to another page

Update: Next.js >= 12
Now you can do redirects using middleware, create a _middleware.js file inside the pages folder (or any sub folder inside pages)

import { NextResponse, NextRequest } from 'next/server'
export async function middleware(req, ev) {
    const { pathname } = req.nextUrl
    if (pathname == "https://stackoverflow.com/") {
        return NextResponse.redirect('/hello-nextjs')
    }
    return NextResponse.next()
}

Update: Next.js >= 10

From Next.js 10 you can do server side redirects (see below for client side redirects) with a redirect key inside getServerSideProps or getStaticProps :

export async function getServerSideProps(context) {
  const res = await fetch(`https://.../data`)
  const data = await res.json()
  // or use context.resolvedUrl for conditional redirect
  // if(context.resolvedUrl == "/")
  if (!data) {
    return {
      redirect: {
        destination: '/hello-nextjs',
        permanent: false,
      },
    }
  }

  return {
    props: {}, // will be passed to the page component as props
  }
}

Note : Using getServerSideProps will force the app to SSR,also redirecting at build-time is not supported , If the redirects are known at build-time you can add those inside next.config.js

In next.js you can redirect after the page is loaded using Router ex :

import Router from 'next/router'

componentDidMount(){
    const {pathname} = Router
    if(pathname == "https://stackoverflow.com/" ){
       Router.push('/hello-nextjs')
    }
}

Or with Hooks :

import React, { useEffect } from "react";
import Router from 'next/router'

...
useEffect(() => {
   const {pathname} = Router
   if(pathname == "https://stackoverflow.com/" ){
       Router.push('/hello-nextjs')
   }
 });

If you want to prevent the flashing before the redirect you can use a simple trick :

import React, { useEffect,useState } from "react";
import Router from 'next/router'
const myPage = ()=>{
    const [loaded,setLoaded] = useState(false)
    useEffect(() => {
        const {pathname} = Router
        // conditional redirect
        if(pathname == "https://stackoverflow.com/" ){
            // with router.push the page may be added to history
            // the browser on history back will  go back to this page and then forward again to the redirected page
            // you can prevent this behaviour using location.replace
            Router.push('/hello-nextjs')
           //location.replace("/hello-nextjs")
        }else{
            setLoaded(true)
        }
      },[]);

    if(!loaded){
        return <div></div> //show nothing or a loader
    }
    return ( 
        <p>
            You will see this page only if pathname !== "/" , <br/>
        </p> 
    )
}
export default myPage

I would say that in general is not a good/elegant approach to do client redirects when you can use next.config.js redirects or even better use conditional render of components.

I have create a simple repo with all the examples above here.