Things will go wrong...the Javascript Console is your friend

• United States Senate Table - Plain
• United States Senate Table - Striped with JS
• United States Senate Table - Striped with JS and Console Messages

function stripeTable(tableid) {
  var mytable = document.getElementById(tableid);
  console.log("mytable is:");
  console.log(mytable);
  // get tbody (i.e. we dont' want to stripe the thead)
  var mytbody = mytable.getElementsByTagName('tbody');
  console.log("mytbody is:");
  console.log(mytbody);
  /*   getElementsByTagName gives us a Node List, so we need
     to access it as a list (Array), arrayname[i].
     Find all the tr within the tbody
  */                                          
  var myrows =  mytbody[0].getElementsByTagName('tr');
  console.log("myrows is:");
  console.log(myrows);
  console.log("myrows length is: " + myrows.length);
  
  /*  Iterate over the node list of "tr" returned, setting
      the class appropriately
    */  
  for(var i=0; i<myrows.length; i++) {
      var senatorname = myrows[i].getElementsByTagName('td')[0].firstChild.nodeValue;
      console.log(senatorname);
    if ((i + 1) % 2 == 0) {
      // even rows
      console.log("row with index of " + i + " is evenRow");
      myrows[i].setAttribute('class','evenRow');
    } else {
      // odd rows
      console.log("row with index of " + i + " is oddRow");
      myrows[i].setAttribute('class','oddRow');
    }
  }
}

console.log() allows you output messages to a console.