Language switching UI in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Next.js language switcher component render?

Consider this React functional component in Next.js that switches languages using the useRouter hook.

import { useRouter } from 'next/router';

export default function LanguageSwitcher() {
  const router = useRouter();
  const { locale, locales, pathname } = router;

  return (
    <div>
      <p>Current locale: {locale}</p>
      <ul>
        {locales.map((loc) => (
          <li key={loc}>
            <a href={pathname} onClick={(e) => {
              e.preventDefault();
              router.push(pathname, pathname, { locale: loc });
            }}>
              Switch to {loc}
            </a>
          </li>
        ))}
      </ul>
    </div>
  );
}

Assuming the current locale is 'en' and available locales are ['en', 'fr'], what will the component display?

NextJS

import { useRouter } from 'next/router';

export default function LanguageSwitcher() {
  const router = useRouter();
  const { locale, locales, pathname } = router;

  return (
    <div>
      <p>Current locale: {locale}</p>
      <ul>
        {locales.map((loc) => (
          <li key={loc}>
            <a href={pathname} onClick={(e) => {
              e.preventDefault();
              router.push(pathname, pathname, { locale: loc });
            }}>
              Switch to {loc}
            </a>
          </li>
        ))}
      </ul>
    </div>
  );
}

A A paragraph showing 'Current locale: en' and a list with two links: 'Switch to en' and 'Switch to fr'.

B A paragraph showing 'Current locale: en' and no links because locales is empty.

C A paragraph showing 'Current locale: en' and a list with one link: 'Switch to fr' only.

D A paragraph showing 'Current locale: fr' and a list with two links: 'Switch to en' and 'Switch to fr'.

💡 Hint

Check the locale and locales values from the router object.

The component reads the current locale as 'en' and the available locales as ['en', 'fr']. It renders the current locale and a list of links for each locale. So it shows 'Current locale: en' and two links: 'Switch to en' and 'Switch to fr'.

📝 Syntax

intermediate

Which option correctly uses Next.js router to change locale on click?

In Next.js, you want to change the language without reloading the page by clicking a link. Which code snippet correctly uses router.push to change locale?

A onClick={e => { e.preventDefault(); router.push(router.pathname, { locale: 'fr' }); }}

B onClick={e => { e.preventDefault(); router.push('/fr'); }}

C onClick={e => { e.preventDefault(); router.push(router.pathname, router.pathname, { locale: 'fr' }); }}

D onClick={e => { router.push(router.pathname, router.pathname, { locale: 'fr' }); }}

💡 Hint

Remember to prevent default link behavior and pass the correct arguments to router.push.

Option C prevents the default link behavior and calls router.push with the current pathname for both URL and asPath, and passes the locale option correctly. Other options either miss preventing default, use wrong arguments, or incorrect method signature.

🔧 Debug

advanced

Why does this Next.js language switcher cause a full page reload?

This Next.js language switcher uses anchor tags with href set to the current path and an onClick handler that calls router.push with locale. However, clicking the links causes a full page reload. Why?

const LanguageSwitcher = () => {
  const router = useRouter();
  return (
    <ul>
      {router.locales.map(loc => (
        <li key={loc}>
          <a href={router.pathname} onClick={() => router.push(router.pathname, router.pathname, { locale: loc })}>
            {loc}
          </a>
        </li>
      ))}
    </ul>
  );
};

NextJS

const LanguageSwitcher = () => {
  const router = useRouter();
  return (
    <ul>
      {router.locales.map(loc => (
        <li key={loc}>
          <a href={router.pathname} onClick={() => router.push(router.pathname, router.pathname, { locale: loc })}>
            {loc}
          </a>
        </li>
      ))}
    </ul>
  );
};

A Because the href attribute is missing, so the browser reloads the page.

B Because router.push is called with wrong arguments causing a reload fallback.

C Because Next.js does not support locale changes via router.push.

D Because the onClick handler does not call e.preventDefault(), so the browser follows the href and reloads the page.

💡 Hint

Think about default link behavior in browsers.

The anchor tag's default behavior is to navigate to the href URL, causing a full reload. Without calling e.preventDefault() in the onClick handler, the browser performs the default navigation after the router.push call.

❓ state_output

advanced

What is the value of the locale state after clicking 'Switch to fr' link?

Given this Next.js language switcher component, what will be the value of locale state after clicking the 'Switch to fr' link?

import { useRouter } from 'next/router';
import { useState } from 'react';

export default function LanguageSwitcher() {
  const router = useRouter();
  const [currentLocale, setCurrentLocale] = useState(router.locale);

  const changeLocale = (loc) => {
    router.push(router.pathname, router.pathname, { locale: loc });
    setCurrentLocale(loc);
  };

  return (
    <div>
      <p>Current locale: {currentLocale}</p>
      <button onClick={() => changeLocale('fr')}>Switch to fr</button>
    </div>
  );
}

NextJS

import { useRouter } from 'next/router';
import { useState } from 'react';

export default function LanguageSwitcher() {
  const router = useRouter();
  const [currentLocale, setCurrentLocale] = useState(router.locale);

  const changeLocale = (loc) => {
    router.push(router.pathname, router.pathname, { locale: loc });
    setCurrentLocale(loc);
  };

  return (
    <div>
      <p>Current locale: {currentLocale}</p>
      <button onClick={() => changeLocale('fr')}>Switch to fr</button>
    </div>
  );
}

A The original locale (e.g., 'en'), because router.push triggers a page reload resetting state.

B 'fr', because setCurrentLocale is called immediately after router.push.

C Undefined, because currentLocale is not updated properly.

D An error occurs because setCurrentLocale is called inside changeLocale.

💡 Hint

Consider what happens to React state when Next.js changes locale via router.push.

Option B is correct. The changeLocale function calls setCurrentLocale(loc) synchronously right after router.push. Since the state setter is synchronous and router.push starts an asynchronous navigation, the currentLocale updates to 'fr' immediately after clicking.

🧠 Conceptual

expert

Which Next.js feature enables automatic locale detection and routing?

Next.js supports internationalized routing with locale detection. Which configuration enables automatic locale detection and locale-aware routing?

A Setting <code>i18n.locales</code> and <code>i18n.defaultLocale</code> in <code>next.config.js</code> with <code>localeDetection: true</code>.

B Using <code>getStaticProps</code> to fetch locale data manually for each page.

C Adding a custom server to parse Accept-Language headers and redirect accordingly.

D Using <code>next-i18next</code> package without any Next.js config changes.

💡 Hint

Check Next.js official docs on internationalized routing configuration.

Next.js supports automatic locale detection and routing by configuring the i18n object in next.config.js with locales, defaultLocale, and localeDetection: true. This enables Next.js to detect user locale and serve the correct localized page automatically.