How to resolve relative links to sibling documents with myst-parser?

[fhg-isi]

I have a set of online documents that I would like to render using myst-parser.

The documents might include links to each other:

foo.md
[[baa]]

baa.md
[foo](./foo.md)

Here is a discussion on how to use myst in the browser:

executablebooks/mystmd#824

    const pipe = unified()
      .use(mystParser)
      .use(transform, new State())
      .use(mystToHast)
      .use(formatHtml)
      .use(rehypeStringify);

=> Is there an extra step for the pipe to resolve links?
=> How can I specify a base url or prefix for relative links?

Edit

Just saw that MyST does not support double bracket notation for sibling documents like[[baa]].
And I could implement text replacements to replace [foo](./foo.md) and [[foo]] with [foo](http://my-base-url.com/current_folder/foo.md).
If there is an already existing solution, please let me know.

[fhg-isi]

Here is some code that worked for me, using vanilla TypeScript and regular expressions. The functions _baseUrlForRelativeLinks and _targetUrl work for my context of a SharePoint WebPart.

function _replaceRelativeLinks(url: string, markdownContent: string): string {
  const baseUrl = _baseUrlForRelativeLinks(url)

  const targetUrlDelegate = _targetUrl;

  // Replace [[foo]] with [foo]({https://baseUrl.com/}foo.md)
  let updatedContent = markdownContent.replace(
    /\[\[([^\]]+)\]\]/g, 
    '[$1](' + targetUrlDelegate(baseUrl + '/$1.md') +')'
  );

  // Replace [foo](./foo.md) with [foo]({https://baseUrl.com/}foo.md)
  updatedContent = updatedContent.replace(
    /\[([^\]]+)\]\((\.\/)?([^)]+)\)/g, 
    function(match, p1, p2, p3) {
      if (p2 === './') {
          //replace realtive urls
          return '[' + p1 + '](' + targetUrlDelegate(baseUrl + '/' + p3) + ')';
      } else {
          //keep absolute urls
          return match;
      }
  });

  return updatedContent
}

function _baseUrlForRelativeLinks(url: string): string {
  //const currentUrl = window.location.href;
  const lastSlashIndex = url.lastIndexOf('/');
  const urlWithoutFileName = url.substring(0, lastSlashIndex);
  return urlWithoutFileName;
}

function _targetUrl(url: string): string{
  // An example target/edit url for a document url 
  // https://fraunhofer.sharepoint.com/sites/eis/Shared%20Documents/foo.md
  // is
  // https://fraunhofer.sharepoint.com/sites/eis/Shared%20Documents/Forms/AllItems.aspx?
  // id=/sites/eis/Shared%20Documents/foo.md&parent=/sites/eis/Shared%20Documents

  const lastSlashIndex = url.lastIndexOf('/');
  const urlWithoutFileName = url.substring(0, lastSlashIndex);
  const fileName = url.substring(lastSlashIndex+1);

  const siteIndex = url.indexOf('/sites');
  const tenantUrl = url.substring(0, siteIndex);  // https://fraunhofer.sharepoint.com
  const idUrl = url.replace(tenantUrl, ''); // /sites/eis/Shared%20Documents/foo.md
  const folderUrl = idUrl.replace(fileName,''); // /sites/eis/Shared%20Documents/

  const editPrefix = '/Forms/AllItems.aspx?id='
  const parentPrefix = '&parent='

  const targetUrl = urlWithoutFileName + editPrefix + idUrl + parentPrefix + folderUrl;
  return targetUrl;

}