How can I set the form action through JavaScript?

You cannot invoke JavaScript functions in standard HTML attributes other than onXXX. Just assign it during window onload.

<script type="text/javascript">
    window.onload = function() {
        document.myform.action = get_action();
    }

    function get_action() {
        return form_action;
    }
</script>

<form name="myform">
    ...
</form>

You see that I've given the form a name, so that it's easily accessible in document.

Alternatively, you can also do it during submit event:

<script type="text/javascript">
    function get_action(form) {
        form.action = form_action;
    }
</script>

<form onsubmit="get_action(this);">
    ...
</form>

Very easy solution with jQuery:

$('#myFormId').attr('action', 'myNewActionTarget.html');

Your form:

<form action=get_action() id="myFormId">
...
</form>

Plain JavaScript:

document.getElementById('form_id').action; //Will retrieve it

document.getElementById('form_id').action = "script.php"; //Will set it

Using jQuery...

$("#form_id").attr("action"); //Will retrieve it

$("#form_id").attr("action", "/script.php"); //Will set it