@carrotsearch/gatsby-transformer-html/src/rewrite-internal-links.js

A Gatsby transformer plugin for authoring content in HTML. Supports ToC generation, responsive images, section anchors and HighlightJS code highlighting.

const rewriteInternalUrl = url => {
  return "/" + url.replace(/(.*)\.html(#.*)?$/i, "$1/$2");
};

const isRelativeUrl = url => {
  return url && !/^(https?:\/\/)|^\/|^#/i.test(url);
};

exports.rewriteInternalUrl = rewriteInternalUrl;

exports.rewriteInternalLinks = $ => {
  // The $ wrapper is a mutable DOM/jQuery-like representation, so
  // we only need to select and modify links, returning the original $ reference.
  $("a:not([data-external])")
    .filter((i, link) => {
      const href = link.attribs.href;

      // Must end with .html
      return isRelativeUrl(href) && /\.html(#.*)?$/i.test(href);
    })
    .attr("href", (i, href) => {
      return rewriteInternalUrl(href);
    });

  return $;
};