concatenate fields in another fields with auto refresh - ruby-on-rails

Question

I m new in rails and html....
I want to concatenate first name and last name in a field fullname in a view. I want that when you modify last name or first name, full name is automatically refreshed with 'first name' +" " + 'lastname'
I tried this but Ive an error. because, seems that full name field stays empty and I tried to make an alert in function to see if concatenation is working. But seems not to do it....
<%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
<%= devise_error_messages! %>
<script type="text/javascript">
function test ()
{
$('fullname').value <= $('firstname').value + " " + $('lastname').value;
}
</script>
<div class="form-group">
<%= f.label :prenom %><br />
<%= f.text_field :firstname, autofocus: true, placeholder: "Prenom", class: "form-control",:onchange => "test()" %>
</div>
<div class="form-group">
<%= f.label :lastname %><br />
<%= f.text_field :nom, autofocus: true, placeholder: "Nom", class: "form-control" %>
</div>
<div class="form-group">
<%= f.label :fullname %><br />
<%= f.text_field :fullname, autofocus: true, placeholder: "Full Name", class: "form-control", :readonly => true %> <!-- ajout de readonly -->
</div>
Thanks for helping

Answer 1

You should check (or post to your question) your resulting html so I'll may help you more.
This is incorrect:
$('fullname').value <= $('firstname').value + " " + $('lastname').value;
Here the working example.
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form>
<script type="text/javascript">
$( document ).ready(function() {
$('input.firstname, input.lastname').on('input', function(e) {
var changedFullName = $('input.firstname').val() + " " + $('input.lastname').val()
$('input.fullname').val(changedFullName);
});
});
</script>
<label for="firstname">FirstName</label><br>
<input type="text" name="firstname" class="firstname" /><br>
<label for="lastname">LastName</label><br>
<input type="text" name="lastname" class="lastname" /><br>
<hr>
<span>FullName</span><br>
<input type="text" name="fullname" class="fullname" readonly />
</form>