next.js/packages/next/src/client/route-announcer.tsx

import React from 'react'
import { useRouter } from './router'

const nextjsRouteAnnouncerStyles: React.CSSProperties = {
  border: 0,
  clip: 'rect(0 0 0 0)',
  height: '1px',
  margin: '-1px',
  overflow: 'hidden',
  padding: 0,
  position: 'absolute',
  top: 0,
  width: '1px',

  // https://medium.com/@jessebeach/beware-smushed-off-screen-accessible-text-5952a4c2cbfe
  whiteSpace: 'nowrap',
  wordWrap: 'normal',
}

export const RouteAnnouncer = () => {
  const { asPath } = useRouter()
  const [routeAnnouncement, setRouteAnnouncement] = React.useState('')

  // Only announce the path change, but not for the first load because screen
  // reader will do that automatically.
  const previouslyLoadedPath = React.useRef(asPath)

  // Every time the path changes, announce the new page’s title following this
  // priority: first the document title (from head), otherwise the first h1, or
  // if none of these exist, then the pathname from the URL. This methodology is
  // inspired by Marcy Sutton’s accessible client routing user testing. More
  // information can be found here:
  // https://www.gatsbyjs.com/blog/2019-07-11-user-testing-accessible-client-routing/
  React.useEffect(
    () => {
      // If the path hasn't change, we do nothing.
      if (previouslyLoadedPath.current === asPath) return
      previouslyLoadedPath.current = asPath

      if (document.title) {
        setRouteAnnouncement(document.title)
      } else {
        const pageHeader = document.querySelector('h1')
        const content = pageHeader?.innerText ?? pageHeader?.textContent

        setRouteAnnouncement(content || asPath)
      }
    },
    // TODO: switch to pathname + query object of dynamic route requirements
    [asPath]
  )

  return (
    <p
      aria-live="assertive" // Make the announcement immediately.
      id="__next-route-announcer__"
      role="alert"
      style={nextjsRouteAnnouncerStyles}
    >
      {routeAnnouncement}
    </p>
  )
}

export default RouteAnnouncer