Assuming that the value we are testing is a number, this is a simple if (...) condition testing whether the value is less than 0.
<script type="text/javascript"> <!-- function isPositiveNumber(field) { var n = field.value; if (n < 0) { alert("This field is required to be a positive number."); // if you want to change the number without // user interaction, put your code here field.focus(); field.select(); return false; } return true; } // --> </script>