How to Set the Form Action Through JavaScript

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>

Set form action with JavaScript

A simpler solution might be to add names to the hidden inputs then append them to the form so they get submitted by default process.

HTML

<input type="hidden" id="firstParameter" name="firstParameter" value="parameter">
<input type="hidden" id="secondParameter" name="secondParameter" value="anotherParameter">

JS

$('form').submit(function(){
   const $form = $(this);
   $('#firstParameter, #secondParameter').each(function(){
       if(this.value){
          $form.append(this)
       }
   });
}); 

How to use JavaScript to change the form action

function chgAction( action_name )
{
    if( action_name=="aaa" ) {
        document.search-theme-form.action = "/AAA";
    }
    else if( action_name=="bbb" ) {
        document.search-theme-form.action = "/BBB";
    }
    else if( action_name=="ccc" ) {
        document.search-theme-form.action = "/CCC";
    }
}

And your form needs to have name in this case:

<form action="/"  accept-charset="UTF-8" method="post" name="search-theme-form" id="search-theme-form">

form action with javascript

A form action set to a JavaScript function is not widely supported, I'm surprised it works in FireFox.

The best is to just set form action to your PHP script; if you need to do anything before submission you can just add to onsubmit

Edit turned out you didn't need any extra function, just a small change here:

function validateFormOnSubmit(theForm) {
    var reason = "";
    reason += validateName(theForm.name);
    reason += validatePhone(theForm.phone);
    reason += validateEmail(theForm.emaile);

    if (reason != "") {
        alert("Some fields need correction:\n" + reason);
    } else {
        simpleCart.checkout();
    }
    return false;
}

Then in your form:

<form action="#" onsubmit="return validateFormOnSubmit(this);">

Form action using javascript

Going out on a limb here. Going to assume you're really just asking why the form isn't submitting, in which case it's because you're missing the form-submit:

var form = document.getElementById("form4");
form.action = "http://localhost/profile_book/login_key.php";
form.submit()

change forms action with javascript

Firstly, the value in your JavaScript should be quoted, as it is a string:

function submitForm(){
    document.getElementById('form1').action = '<?echo base_url().'index.php/folder/controller/controller_function/';?>';
    document.getElementById('form1').submit();
}

Secondly, you're triggering the function on the change event of a button, which probably won't work because buttons do not change. Try binding to the submit event instead, on the form itself:

<form id="form1" name="form1" action="" method="post" onsubmit='submitForm();'>

But I'd do it in the JavaScript, using an event listener:

document.getElementById('form1').addEventListener('submit', function(e) {
    e.preventDefault();
    this.action = '<?echo base_url().'index.php/folder/controller/controller_function/';?>';
    this.submit();
}, false);

How do I conditionally set a form action via Javascript?

Your code should work

In any case here is simpler version

document.getElementById("confirmForm").addEventListener("submit",function() {
  this.method="post"
  this.action = document.getElementById('confirmOptionCompare').checked ? "comparison.php": "bookConfirm.php";
})

<form id="confirmForm">
  <p>Click confirm to go to the next step:</p>
  <button type="submit" name="confirm">Confirm</button>
</form>

<input type="checkbox" name="confirmOptionCompare" id="confirmOptionCompare"/>