gistfile1.phtml

<form id="my-form">
    <input type="text" name="in" value="some data" />
    <button type="submit">Go</button>
</form>


<script>
// Should only be triggered on first page load
alert('ho');

function processForm() {
    /* do what you want with the form */
    
    // Should be triggered on form submit
    alert('hi');
    // You must return false to prevent the default form behavior
    return false;
}

var form = document.getElementById('my-form');
if (form.attachEvent) {
    form.attachEvent("submit", processForm);
} else {
    form.addEventListener("submit", processForm);
}
</script>