How can I select a button with a type of submit using Javascript?

I have two buttons like this:

<button class="medium" id="register" type="button"
        onclick="location.href='/Account/Register'">Register</button>
<button class="medium default" id="login" type="submit">Login</button>

My Javascript selects the button by id:

<script type="text/javascript">
    (function () {
        document.getElementsByClassName('form')[0]
            .addEventListener("submit", function (ev) {
                document.getElementById('login').innerHTML += '&nbsp;<i class="fa fa-spin fa-spinner" data-ng-show="fetching.length > 0"></i>';
            }, false);
    })();
</script>

How can I change this so the javascripts selects the button with a type of "submit" or a button with a class of "submit" if that's not possible?

Note that I would like to do this because the javascript is used by many forms. I need a common way to find the "submit" button.