Form element values are returned as text:
Code:
var principle = parseFloat(document.forms["Myform"]["principle"].value);
var period = parseInt(document.forms["Myform"]["period"].value);
var rate = parseFloat(document.forms["Myform"]["rate"].value);
Code:
onclick="createRow ()" /> <!-- delete the space before () -->
I assume you've omitted 'function createRow() {' from your JavaScript when copying and pasting?