copyNodes.js

const allowedNodeTypes = [
  'p',
  'span',
  'div',
  'a',
  'img',
  'h1',
  'h2',
  'h3',
  'h4',
  'h5',
  'strong',
  'b',
  'em',
  'pre',
  'blockquote',
  'table',
  'tr',
  'td',
  'tbody',
  'u',
  'br',
  'i',
  'ul',
  'ol',
  'li',
  'figure',
  'code',
  'hr',
  'italic',
  'font',
  'sup',
  'sub'
]

const allowedAttributeTypes = [
  'style',
  'href',
  'src',
  'alt',
  'title'
]

const urlTransformAttributeTypes = [
  'src', 
  'href'
]
  
function recursivelyCopyContent(elem1, elem2, keep_tags) {
  while (elem2.firstChild){
    elem2.removeChild(elem2.firstChild);
  }

  if (keep_tags === undefined) {
    keep_tags = true;
  }

  elem1.childNodes.forEach(child=>{
    //console.log(child);
    if (child.nodeType === Node.TEXT_NODE){
      elem2.appendChild(
        document.createTextNode(child.data)
      );
    }
    else if (child.nodeType === Node.ELEMENT_NODE)  {
      let tag = child.tagName.toLowerCase();
      if (allowedNodeTypes.indexOf(tag)>=0){
        let element = document.createElement(keep_tags?tag:'span');

        if (keep_tags){
          for (var name of child.attributes){
            if (urlTransformAttributeTypes.indexOf(name.nodeName)>=0){
              element.setAttribute(name.nodeName, new URL(name.value, 'https://thedailywtf.com/articles/'));
            }
            else if (allowedAttributeTypes.indexOf(name.nodeName)>=0){
              element.setAttribute(name.nodeName, name.value);
            } else {
              console.log("not alled attribute", name.nodeName);
            }
          }
        }

        recursivelyCopyContent(child, element);
        elem2.appendChild(element);
      } else {
        console.log("Not allowed", tag);
      }
    }
  });
}