serialize.js

function serialize(node) {
  if (node.nodeType === 3) {
    return node.data;
  }
  else if (node.nodeType === 8) {
    return '<!--' + node.data + '-->';
  }
  else if (node.nodeType === 1 || node.nodeType === 9) {
    let str = '<' + node.nodeName;
    const attrs = [];
    for (let i=0; i<node.attributes.length; i++) {
      attrs.push(node.attributes[i].name);
    }
    attrs.sort();
    for (let i=0; i<attrs.length; i++) {
      const name = attrs[i];
      const value = node.getAttribute(name);
      str += ' ' + name;
      if (value !== '') {
        str += '="' + value + '"';
      }
    }
    str += '>';
    for (let i=0; i<node.childNodes.length; i++) {
      str += serialize(node.childNodes[i]);
    }
    str += '</' + node.nodeName + '>';
    return str;
  }
}