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) {
         let mytable = document.getElementById(tableid);
         console.log("mytable is:");
         console.log(mytable);
         // get tbody (i.e. we dont' want to stripe the thead)
         let 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
         */
         let 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(let i=0; i<myrows.length; i++) {
             let 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.