Submit form via JavaScript to the correct Post action URL in the background

I have the following 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>

When the user clicks on the submit button I want the form to be processed first by a script that will check the contents of #statement and then do a Post request for the form in the background. I prefer to do it in JavaScript (not jQuery).

I tried using

 document.querySelector('#submitform').addEventListener('submit', function(e) {
        e.preventDefault();
        console.log('attempt');
        // so smth to submit the form in background - what?
 });

but it doesn't seem to "catch" the form and it still submits...

What am I doing wrong?

UPDATE - had a typo in html