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();
};

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;
}

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).

you should change the type from submit to button:

<input type='button' value='submit request'>

instead of

<input type='submit' value='submit request'>

you then get the name of your button in javascript and associate whatever action you want to it

var btn = document.forms["frm_name"].elements["btn_name"];
btn.onclick = function(){...};

worked for me hope it helps.