Next.js version of `activeClassName` support.

ActiveLink.js

import { withRouter } from 'next/router';
import Link from 'next/link';
import React, { Children } from 'react';

const ActiveLink = ({ router, children, ...props }) => {
  const child = Children.only(children);

  let className = child.props.className || '';
  if (router.pathname === props.href && props.activeClassName) {
    className = `${className} ${props.activeClassName}`.trim();
  }

  delete props.activeClassName;

  return <Link {...props}>{React.cloneElement(child, { className })}</Link>;
};

export default withRouter(ActiveLink);

example.md

import Link from './Link'; // our version of link

export default () => (
  <header className="Header">
    <nav>
      <Link activeClassName="active" href="/">
        <a className="some-other-class">Home</a>
      </Link>
      <Link activeClassName="active" href="/about">
        <a>About</a>
      </Link>
      <Link activeClassName="active" href="/contact">
        <a>Contact</a>
      </Link>
    </nav>
  </header>
);

I rather have my own function that creates a pure next/link component:

import Link from "next/link";
import {useRouter} from "next/router";
import styles from './Toolbar.module.css'

const Toolbar = () => {
    const router = useRouter();
    const activeLink = (path, content, activeClass = styles.active, normalClass = '') => {
        let className = router.pathname === path ? activeClass : normalClass;
        return <Link href={path}><a className={className}>{content}</a></Link>
    }

    return <div className={styles.toolbar}>
        <nav>
            {activeLink('/', 'Home')}
            {activeLink('/about', 'About')}
        </nav>
    </div>
}

export default Toolbar;

If you need a more reusable function, you can pass the router:

import Link from "next/link"

const createActiveLink = (router, path, content, activeClass = 'active', normalClass = '') => {
      let className = router.pathname === path ? activeClass : normalClass;
      return <Link href={path}><a className={className}>{content}</a></Link>
  }

export default createActiveLink

typed. 21/10/22
Thank you very much remy san XD

import { withRouter, NextRouter  } from 'next/router';
import React, { ReactElement } from 'react';
import Link from 'next/link';

type Props = {
  router: NextRouter;
  children: ReactElement;
  href: string;
  activeClassName: string;
}

const ActiveLink = ({ router, children, ...props }: Props) => {
  const child = children;
  
  let className: string = child.props.className;
  if (router.pathname == props.href) {
    className = `${className} ${props.activeClassName}`;
  }

  return (
  <Link {...props}>{React.cloneElement(child, { className })}</Link>
  );
}

export default withRouter(ActiveLink);

Here's my version of ActiveLink for Next.js + TypeScript:

import Link, { LinkProps } from "next/link";
import { NextRouter, useRouter } from "next/router";
import { FC, PropsWithChildren } from "react";

type ActiveLinkProps = LinkProps & {
  activeClassName: string;
  className: string;
};

export const ActiveLink: FC<ActiveLinkProps> = ({ children, ...props }: PropsWithChildren<ActiveLinkProps>) => {
  const router: NextRouter = useRouter();
  const className: string =
    props.className + ((router.pathname === props.href && props.activeClassName) ? " " + props.activeClassName : "");
  return (
    <Link {...props}>
      <a className={className}>{children}</a>
    </Link>
  );
};

Example of usage:

          <ActiveLink href="/" className="menulink" activeClassName="active">
            Home
          </ActiveLink>

The difference is you no longer need <a> tag inside.

Benefits:

• TypeScript-friendly
• You no longer need to find child element and its type with Children.only
• You no longer need to do React.cloneElement
• You won't forget to put <a> inside anymore.