How to create a .docx file and not a .doc word using JavaScript

I am using:

var header = "<html xmlns:o='urn:schemas-microsoft-com:office:office' "+
    "xmlns:w='urn:schemas-microsoft-com:office:word' "+
    "xmlns='http://www.w3.org/TR/REC-html40'>"+
    "<head><meta charset='utf-8'><title>Export HTML to Word Document with JavaScript</title></head><body>";
    var footer = "</body></html>";
    //@ts-ignore
    if(ev.target.id === "screenword"){
      this.setState({
        typeOfPrint: "screenword",
      }, () => {
        var sourceHTML1 = header+document.getElementById("spage").innerHTML+footer;
        var source1 = 'data:application/vnd.ms-word;charset=utf-8,' + encodeURIComponent(sourceHTML1);
        var fileDownload1 = document.createElement("a");
        document.body.appendChild(fileDownload1);
        fileDownload1.href = source1;
        fileDownload1.download = this.state.PPName+'.doc';
        fileDownload1.click();
        document.body.removeChild(fileDownload1);

To create a doc file but want to create a docx. I've searched and cannot find any guidance on how to update this code. I'm trying to avoid external libraries as much as possible.