Catch form submit event in JavaScript / jQuery

I'm trying to catch a submit request for a form

<form action='/post' name='submitform' id="submitform" method='post' class='pure-form'>
<textarea columns="40" rows="4" name='entry[body]' id="statement"></textarea>
<input type='submit' id="submitbutton" name="btnSubmit" value="save" class="pure-button pure-button-primary">
</form>

using

 document.querySelector('#submitform').addEventListener('submit', function(e) { /* code */ });

which works fine if I click a submit button

but when I call

$('#submitform').submit();

or

document.forms["submitform"].submit();

the form submits bypassing my event listener.

Why?