Me quedé sin definir en la primera iteración. Cualquier comentario me es útil. Gracias de antemano.

 for (i = 0; i <= nummonth; i++) { var res = compound * (Math.pow(interest, i)); res = res.toFixed(0); var tableHTML; var interestgained = (+res * interest)-res; interestgained = interestgained.toFixed(0); var powint = compound * (Math.pow(interest, i)); powint = powint.toFixed(0); var tr = powint - compound; var ointerest = (interest - 1)*100; ointerest = ointerest.toFixed(0); tableHTML = tableHTML + "<tr><td>Compound: " + i + "</td><td> " + "$"+res + "</td><td> " + ointerest + "%"+"</td><td> " + "$"+interestgained + "</td><td>"+ "$"+powint +"</td></tr>"; }