$('form').submit(function(){
    const $favDevices = $("#favDevices")
    const unvalidated = $favDevices.val()
    const validated = unvalidated
                        .split("n")
                        .map(line => `${line}-VALIDATE`)
                        .join("")
    $favDevices.val(validated)
})

The value from the textarea is a string. You need to split the string into lines, add -VALIDATED to the end of each line, then recombine it into a single string.

I usually use a regular expression like this to split into lines:

let lines = $("#favDevices").val().split(/(?:rn|r|n)/);

Then map is useful for processing each line:

lines = lines.map(line => line + "-VALIDATED");

Then join recombines into a single string; choose your delimiter, I usually use a single n.

const str = lines.join("n");

Live Example:

const str =
  $("#favDevices").val().split(/(?:rn|r|n)/)
  .map(line => line + "-VALIDATED")
  .join("n");
console.log(JSON.stringify(str));

<textarea id="favDevices" class="t">Laptop
Computer
PlayStation 4</textarea>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

In a comment you’ve said:

But I want to replace those content inside of textarea to device-VALIDATED so the new value of device is device-VALIDATED

You’d add a fourth step: Writing back to the textarea:

$("#favDevices").val(str);

Live Example:

const favDevices = $("#favDevices");
const str =
  favDevices.val().split(/(?:rn|r|n)/)
  .map(line => line + "-VALIDATED")
  .join("n");
favDevices.val(str);

<textarea id="favDevices" class="t">Laptop
Computer
PlayStation 4</textarea>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

How can I replace the one content inside of textarea. I have a textarea for the user’s favorite devices.

<textarea id="favDevices" class="t"></textarea>

Let’s say users give this three devices:

Laptop
Computer
PlayStation 4

After submitting it (using the jquery ajax) I want to replace one of the content to device-VALIDATED. So the results would be

Laptop-VALIDATED
Computer-VALIDATED
PlayStation 4-VALIDATED

I try figure out how can I do it using this code:

$('form').submit(function(){
    f = $("#favDevices").val().split("n")
    for(i in f){
        device = f[i];
        // function here to send data to back end
        f.replace(device, device + "-VALIDATED")
    }
})