gistfile1.txt

/* http://webapps.stackexchange.com/questions/23861/header-numbering-in-google-docs */

var pars = DocumentApp.getActiveDocument().getBody().getParagraphs();
  for(var i=0; i<pars.length; i++) {
    var par = pars[i];
    var hdg = par.getHeading();
    if (hdg == DocumentApp.ParagraphHeading.HEADING1) {
      counterh1 = counterh1+1; counterh2 = 0; counterh3 = 0; counterh4 = 0; counterh5 = 0; counterh6 = 0;
      var content = par.getText();
      var chunks = content.split('\t')
      var result = 'ok'
      if(chunks.length > 1) { par.setText(counterh1+'.\t'+chunks[1]); }
      else { par.setText(counterh1+'.\t'+chunks[0]); }
    }
... idem for HEADING2,3,4,5,6
}