Not sure if that's what you need, but if I understood correctly, it'd be something like this:

let form = // get the form somehow (for example, with querySelector)
form.addEventListener('submit', function(e) {
  e.preventDefault(); // don't submit the form
  fetch('backend.url')
   .then(function(response) {
      // do whatever you need with the response
   });
});

For the backend URL, you can also use form.getAttribute("action"), if you want to use the same URL that is in the action attribute.

After the last edit it's still unclear what you mean by "apply backend behavior".

Do you have a form action="URL A" and want to send all its data to URL B using JavaScript?

hkotsubo‭

I don't think that's what I want to do.

I want to send all form data to an email address according to the directives in a backend file ("behavior.php"), with:

<?php
	$topic = $_POST['topic']; # Required;
	$name = $_POST['name'];   # Required;
	$email = $_POST['email']; # Required;
	$url = $_POST['url']; 
	# Name attribute is "url" but input type is "text" because "url" obligates http:// or https:// which users normally don't input;
	$phone = $_POST['phone'];
	$notes = $_POST['notes']; # Normally turned off to prevent spam;

	$to = 'example@example.com';
	$subject = 'הודעה חדשה מאת: ' . $name;
	include 'message.php';
	$headers[] = 'MIME-Version: 1.0';
	$headers[] = 'Content-type: text/html; charset=utf-8';
	mail($to, $subject, $message, implode("\r\n", $headers));
?>

deleted user Then I suggest you to edit the question and add this information there.

But anyway, if you want to execute behavior.php, you'll have to send data to it (which is basically what I described above)

hkotsubo‭ I don't know what you meant by "you'll have to send data to it".

By the way, given the heavy down voting and the edits, I invite you to backup your code as I might just delete this thread.

deleted user "send data" means making a http request, using fetch with POST method (there's an example in the docs). And you can use FormData to send the form fields (if I understood correctly what you need)

"send data" means making a http request

I knew that.

using fetch with POST method (there's an example in the docs) And you can use FormData to send the form fields (if I understood correctly what you need)

But I want to keep the email address in a backend file (behavior.php) so it won't be detectable by the browser (as far as I know any JavaScript file affecting a webpage is available via the developer tools).

In the PHP file you're getting $_POST['email'], which means that it's getting the email sent by the client (if the backend will get another email that the client side can't know, then where/how it will be done?). Well, it seems that there is a lot of missing information, please edit the question and add all this context to it

hkotsubo‭ I swear that I have no idea what is the "a lot" of missing information.

No sarcasm (and not anything like it), I just really don't know how to further improve this question...

The answer below is basically doing what I suggested (using fetch and FormData to send the data via POST).

As you didn't say if it worked, I assumed it didn't work and therefore there might be some missing details (because based on what I could understand, that code should work)