Javascript заменяет дочерние узлы, заставляя их изменяться на [object HTMLTableSectionElement]

function prepHTMLforExport(element) {
  //recursively goes into an HTML element, preparing it for export by:
  //removing thead and tbody statements but leaving their contents in the parent element
  //removing floattext tags
  //replacing hyperlink a tags with ...

  //go through child elements and prep each for export
  var newchildren = []
  var children = element.childNodes
  for (let i = 0; i < children.length; i++) {
    var newkid = prepHTMLforExport(children[i])

    //if the new kid is actually a list, concat the lists
    if (newkid instanceof Array) {
      newchildren = newchildren.concat(newkid);
    } else {
      newchildren.push(newkid)
    }
  }
  //if to be removed, return a list of children
  if (element.tagName in ["floattext", "table-body", "table-head"]) {
    return newchildren
  } else {
    //else add children to self and return self
    if (newchildren.length > 0) {
      element.replaceChildren(...newchildren);
    }
    return element
  }
}

prepHTMLforExport(document.getElementById("tbl"));

<table id='tbl'>
  <th>sample text</th>
  <th>some more sample text</th>
</table>

function fancyTableOnClick( event ) {
      prepHTMLforExport( FancyTableID )
    }


    function prepHTMLforExport(element){
    //recursively goes into an HTML element, preparing it for export by:
    //removing thead and tbody statements but leaving their contents in the parent element
    //removing floattext tags
    //replacing hyperlink a tags with ...

    //go through child elements and prep each for export
    var newchildren = []
    // var children = element.childNodes
    var children = element.children  // Just elements, no text nodes
    for (var i=0;i<children.length;i++){
        var newkid = prepHTMLforExport(children[i])

        //if the new kid is actually a list, concat the lists
        if (newkid instanceof Array){
            newchildren = newchildren.concat(newkid)
        } else {
            newchildren.push(newkid)
        }
    }
    //if to be removed, return a list of children
    // if (element.tagName in ["floattext", "table-body", "table-head"]){
    if (["FLOATTEXT", "TBODY", "THEAD"].includes(element.tagName)){  // [].includes() not "in []"
        return newchildren
    } else {
        //else add children to self and return self
        if (newchildren.length > 0) {
            // element.replaceChildren(newchildren)
            element.replaceChildren(...newchildren)  // Takes individual elements, not an array
        }
        return element
    }
}

table {
  border: 2px solid red;
}
th {
  border: 1px solid black;
  padding: 0px 4px;
}
tbody {
  background-color: green;
}
thead {
  background-color: cyan;
}

<button type = "button" onclick = "fancyTableOnClick(event)">Fix Table</button>

      <table id = "FancyTableID">
        <thead>
          <th>sample text</th>
          <th>Something</th>
        </thead>
        <tbody>
          <h>Table body</th>
          <th>some more sample text</th>
        </tbody>
      </table>