How do I cancel form submission in submit button onclick event?

You are better off doing...

<form onsubmit="return isValidForm()" />

If isValidForm() returns false, then your form doesn't submit.

You should also probably move your event handler from inline.

document.getElementById('my-form').onsubmit = function() {
    return isValidForm();
};

Sometimes onsubmit wouldn't work with asp.net.

I solved it with very easy way.

if we have such a form

<form method="post" name="setting-form" >
   <input type="text" id="UserName" name="UserName" value="" 
      placeholder="user name" >
   <input type="password" id="Password" name="password" value="" placeholder="password" >
   <div id="remember" class="checkbox">
    <label>remember me</label>
    <asp:CheckBox ID="RememberMe" runat="server" />
   </div>
   <input type="submit" value="login" id="login-btn"/>
</form>

You can now catch get that event before the form postback and stop it from postback and do all the ajax you want using this jquery.

$(document).ready(function () {
            $("#login-btn").click(function (event) {
                event.preventDefault();
                alert("do what ever you want");
            });
 });

You need to change

onclick='btnClick();'

to

onclick='return btnClick();'

and

cancelFormSubmission();

to

return false;

That said, I'd try to avoid the intrinsic event attributes in favour of unobtrusive JS with a library (such as YUI or jQuery) that has a good event handling API and tie into the event that really matters (i.e. the form's submit event instead of the button's click event).

Change your input to this:

<input type='submit' value='submit request' onclick='return btnClick();'>

And return false in your function

function btnClick() {
    if (!validData())
        return false;
}