txt.replace </blockquote> in textarea js – Jquery

The first problem in your code was how you were adding the event listener to the ready event. Being it something invented by jQuery, and not a native event, the correct way to do it should be as of now (v.3.3.1 the version I used in this demo) $(document).ready(()=>{/*code here*/}).

As a further reference:
https://api.jquery.com/ready/

There is also $(document).on( "ready", handler ), deprecated as of
jQuery 1.8 and removed in jQuery 3.0. Note that if the DOM becomes
ready before this event is attached, the handler will not be executed.

But… it’s not perfectly clear how did you wish to transform your text before setting the value of the textarea. So I just better factored your logic so that you have some clear steps:

• grabbing the blockquote element text content and trimming it (being the origin)

• applying the transform newline to whitespace (with the regex that I left untouched)

• build the final string as a template literal that will include the quote content, the meta tags wrapping it, AND anything else you wish to add like for example a new line (n) that in this example is exacerbated by a text following it.

There’s a hint in your words that put me in the position to say something superflous but still deserving an attempt: the value of a inner text is just plain text and doesn’t render html content. So the <br> itself would remain as you read it and wouldn’t have any rendering effect on the textarea content. That’s why I focused my demonstration on putting a newline with the escaping sequence. It works both on double quoted strings and template literals: "n" `n`

Further notes

It seems the original approach of processing the blockquote html was preferred. It’s worth saying that it was appearently a terrible strategy for several reasons:

• It grabs the blockquote content as html despite that’s not how it’s
  rendered on the page.
• It takes the effort to consider the whole outerHTML removing the
  wrapping blockquote tags instead of fetching directly the innerHTML.
• It adds the newline as newline instead of embedding it as <br> so
  at this point I ask myself if the content in the textarea was
  supposed to be encoded html or not.. and the added br would then
  belong to something meta?
• It’s harder to deal with in case you want to further customize the
  string processing

But… maybe there’s something I didn’t get and I’m doing weak assumptions.

//since you are using the ready event with jquery, that's the correct syntax
$(document).ready(function() {
  $('.quoteMsg').click(function() {

    //grabs the text content of the blockquote element (trimming it)
    var quoteTextContent = $(this).closest('.replyBox').find('.replyMsg').text().trim();
    
    //performs the transform already in place in your code.. replacing newlines with white spaces
    quoteTextContent = quoteTextContent.replace(/(?:rn|r|n)/g, ' '); //working great    
    
    //builds the string to set the textarea value with, using a template literal
    //here you can add anything you want.. like a new line but that's just an example
    const encoded = `[quote]${quoteTextContent}[/quote]nand something following to show the new line happening`;

    console.log(encoded);
    $("textarea[name='quote']").val( encoded );
  });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<textarea id="comment" name="quote" placeholder="quote" style="width:80%;height:200px;"></textarea>
<section class="replyBox" style="width: 100%;"><br/>
  <a href="#napisz" class="quoteMsg"> [ click for quote ] </a>
  <div class="replyMsg">
    <blockquote>this is a quote for comment&#128526;
    <br>
    Having new lines also ... since you perform a regex transform newline=>whitespace
    </blockquote><br />
  </div>
</section>