Add a string to the start of all internal links on a page.

add_string_to_href.js

//get base url
 const getUrl = window.location;
 const baseUrl = getUrl .protocol + "//" + getUrl.host + "/";
 // the string you want to append
 const stringToAdd = "test/"
 // get all a tags
 let elements = document.getElementsByTagName('a');
  for (let node of elements) {
  // is this internal to the site? If so update the link
  if(node.href.startsWith(baseUrl)){
    // update the link with the stringToAdd
    node.href = baseUrl + stringToAdd + node.href.replace(baseUrl,"")
  }
 }