$ .post函数不会将更改附加到textarea

So I'm new to Javascript and after searching a long time I did not find the answer I was looking for (maybe because I don't have the right terms)

So here's my script:

function changeStyle() {
    var type = $('select[name=style_website]').val();
    var numwebsite = $('select[name=style_website]').attr('data-numwebsite');

    $.post("getstyle",{'type': type,'numwebsite': numwebsite},
        function(retour){
            $('.websitestyle_js').val(retour);
        });
}

And here's my html:

<strong><?php echo "Modify your style";?></strong>
<select onchange="changeStyle()" style="float: right;" name="style_website" data-numwebsite="<?php echo $numwebsite;?>">
    <option value="style" selected>Your style</option>
    <option value="mini">Mini</option>
    <option value="bmw">Bmw</option>
</select>
<form method="post" action="<?php echo base_url();?>websites/modificationStyle?numwebsite=<?php echo $numwebsite;?>">
    <div class="well">
        <p><strong><?php echo "Code Editor";?></strong></p>
        <textarea name="websitestyle" class="websitestyle_js" id="code1"><?php echo $websitedata['websitestyle'];?></textarea>
    </div>
    <button type="submit" class="btn btn-success form-control m-b"><?php echo "Modify Style";?></button>
</form>

What I wanted to do is change the content of my textarea depending on the select, every time the user click on a different option it is suppose to change the textarea. The problem is that it doesn't change anything, so I was wondering if my syntax was wrong or if I was missing something.

I printed "retour" with alert() and it contains the data that I want to append to the textarea, so it doesn't come from that. And I also try to add this line:

$('.websitestyle_js').val('test');

Jut alone at the end of my html to see if it was a syntax error, but it changes the textarea.

Thank you for your answers.

EDIT1 (result of console.log as asked):

console.log(retour)

EDIT2 (what it looks like at the end for those of you who are confused):

code_editor

Try workaround via jquery on change. Here is a demo that changes textarea when you change select

<strong><?php echo "Modify your style";?></strong>
<select style="float: right;" name="style_website" data-numwebsite="numwebsite">
<option value="style" selected>Your style</option>
<option value="mini">Mini</option>
<option value="bmw">Bmw</option>
</select>
<form method="post">
<div class="well">
    <p><strong><?php echo "Code Editor";?></strong></p>
    <textarea name="websitestyle" class="websitestyle_js" id="code1"></textarea>
</div>
<button type="submit" class="btn btn-success form-control m-b">Modify Style</button>
</form>

JS

$('select[name=style_website]').on('change', function (e) {
var optionSelected = $("option:selected", this);
var valueSelected = this.value;
$('.websitestyle_js').val(valueSelected);
});

Maybe this is not really that you need, but it generally answers your question "What I wanted to do is change the content of my textarea depending on the select, every time the user click on a different option it is suppose to change the textarea".

Live Demo

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script>   
 function changeStyle() {
        var type = $('select[name=style_website]').val();
        var numwebsite = $('select[name=style_website]').attr('data-numwebsite');

        $.post("getstyle.php",{'type': type,'numwebsite': numwebsite},
            function(retour){
                $('.websitestyle_js').val(retour);
            });
    }
</script>  

<strong><?php echo "Modify your style";?></strong>
<select onchange="changeStyle()" style="float: right;" name="style_website" data-numwebsite="<?php echo $numwebsite;?>">
    <option value="style" selected>Your style</option>
    <option value="mini">Mini</option>
    <option value="bmw">Bmw</option>
</select>
<form method="post" action="websites/modificationStyle?numwebsite=<?php echo $numwebsite;?>">
    <div class="well">
        <p><strong><?php echo "Code Editor";?></strong></p>
        <textarea name="websitestyle" class="websitestyle_js" id="code1"><?php echo $websitedata['websitestyle'];?></textarea>
    </div>
    <button type="submit" class="btn btn-success form-control m-b"><?php echo "Modify Style";?></button>
</form> 

Try this...It is working..

Try console logging retour, maybe you should use responseText, another option is to try html(retour) instead of val

It was CodeMirror which is what I use to have the Code style that was creating a conflict. I fixed my problem, thank you all for you anwers