Form clears after alert

I have three buttons included in my form. I have a javascript onclick event for one of the submit buttons. Here is the javascript associated with the onclick event:

function chkSave(){
if(document.form1.txtjobnum.value == "")
{
window.alert("please enter a job number");
document.form1.txtjobnum.focus();
return false;
}
}

The alert box works correctly but whenever I click on OK in the alert box my all the data in my form is cleared. Can anybody explain why this is happening?

Any refreshing going on that isn't being shown?

No refreshing. I thought return false was the solution but it doesn't work.

If you click on Cancel on the Alert button, does the same thing happen?

There is no choice for a cancel, there is just an OK button.