the situation is :
alert('this form is about to be submitted...');
<form id="form1" runat="server" onsubmit="DoThis();">
Through a series of hit-n-trials, I finally figured out that I was missing the "return" keyword to add in my form tag. So, I modified my form tag as :
<form id="form1" runat="server" onsubmit="return DoThis();">
Silly things eat a lot of time. Probably, in-experience.