I would suggest using https://github.com/iamvishnusankar/next-sitemap instead of using this library going forward, as it is not being maintained anymore.

Higher order Next.js config to generate sitemap.xml and robots.txt. Read more at Medium. Heavily inspired from IlusionDev/nextjs-sitemap-generator.

yarn add next-with-sitemap

Update or create next.config.js with

const withSitemap = require("next-with-sitemap");

module.exports = withSitemap({
  sitemap: {
    baseUrl: "https://www.example.com",
  },
  // .
  // ..
  // ... other Next.js config
});

Add sitemap.xml and robots.txt to your .gitignore

robots.txt
sitemap.xml

There are options you can use to customize the behavior of this plugin by adding sitemap object in the Next.js config in next.config.js. Alongside those given sitemap options, this library would also rely on your Next.js config values exportPathMap, exportTrailingSlash, and pageExtensions to come up with the correct sitemap.xml content.

const withSitemap = require("next-with-sitemap");

module.exports = withSitemap({
  sitemap: {
    baseUrl: "https://www.example.com",
    alternateBaseUrls: [
      {
        lang: "jp",
        url: "https://example.jp",
      },
      {
        lang: "es",
        url: "https://example.com/es",
      },
    ],
    dest: "public",
    excludedPaths: ["/login", "/signup"],
    extraPaths: ["/extra/path"],
    pages: "pages",
    pageTags: [
      {
        path: "/",
        priority: 1.0,
      },
      {
        path: "/extra/path",
        changefreq: "monthly",
      },
      {
        path: "/about",
        changefreq: "weekly",
        priority: 0.5,
      },
    ],
    robots: true,
    sitemap: true,
  },
});

• baseUrl: string (required) - the baseUrl name including protocol and subdomain (e.g. https://www.example.com).
• alternateBaseUrls: array - an array of alternative base urls for given languages.
  • defaults to [].
  • for each object in array:
    • set lang (required) to the language identifier,
    • set url (required) to the base url that should be used for that lang.
• dest: string - the destination folder to put generated files, relative to the project root.
  • defaults to public.
• excludedPaths: array - an array of paths that should be excluded from the sitemap.
  • defaults to [].
• extraPaths: array - an array of extra paths that cannot be found at pages folder and that should be included to the sitemap.
  • defaults to [].
• pages: string - the folder that contains page files, relative to the project root.
  • defaults to pages.
• pageTags: array - an array of additional tags for any given path.
  • defaults to [].
  • for each object in array:
    • set path (required) to the relative path of the page,
    • set changefreq to one of always, hourly, daily, weekly, monthly, yearly, never values,
    • set priority to a number value between 0 and 1 (inclusive).
• robots: boolean - whether to enable robots.txt generation.
  • defaults to true,
  • set robots: false, so it won't generate a robots.txt,
  • set robots: true to generate a robots.txt.
• sitemap: boolean - whether to enable sitemap.xml generation.
  • defaults to true,
  • set sitemap: false, so it won't generate a sitemap.xml,
  • set sitemap: true to generate a sitemap.xml.