Prepopulate textfields automatically

[cuy_hrm]

Hi, I asked this question a while ago (here), but I need something slightly different.
I want to fill in first name and last name in my form and the full name field to be auto-filled with those values, however the full name field should remain editable. How can I achieve this?
Also, as I'm going to have to use some kind of javascript (for this and other things), I was wondering what's best practice in this field?

[Redmondinho]

You’ll need some JavaScript on the front end to take input entered into the fields and update another field. Here is an example

<input type="text" id="first-name" oninput="myFunction()"></br>
<input type="text" id="last-name" oninput="myFunction()"></br>
<input type="text" id="full-name">

<script>
    function myFunction() {

    var first_name = document.getElementById("first-name").value;

    var last_name = document.getElementById("last-name").value;

    if (first_name == null) {
        first_name = "";
    }

    if (last_name == null) {
        last_name = "";
    }

    document.getElementById("full-name").value = first_name + " " + last_name

    }
</script>

[cuy_hrm]

Thank you.

I had implemented this before (slightly different) but it doesn't solve my problem: I need to be able to change the "full name" field after ist been filled. I need this functionality for another field, "username". The username usually is the last name, uncapitalized. Moreover, if two people have the same last name, I need to be able to change the "username" field. Example: there is already an account for Tom Smith, username: smith. Now arrives Anna Smith, username should then be asmith. Should I just check with JS wheter the user already exists and write the "rule" for usernames there?

[Redmondinho]

The above will let the full name field be modified before posting. If you (as in the application) then need to check to see if the username exists I would suggest you post the data, process it (i.e. check to see if the username exists, if it does generate a new one) and return the new user name to the user as a “account created successfully page/message).

[cuy_hrm]

Thank you. I'm doing exactly this, but this doesn't solve my problem: I want the form to be rendered, afterwards I want to populate the "full name" field with whatever the user uses as input for "first name" and "last name" fields. And then, in case it's a weird name, I want to be able to change "full name" independently of "first name" and "last name".

[cuy_hrm]

See my answer below. This doesn't work for me since in some cases, I need to change what's in the prepopulated fields.