JSFiddle - React, Tailwind, and code Playground

HTML

<form method="POST" name="comment" class="setCommentBox">
    <div class="form-group">
        <div class="bs-example">
            <textarea class="form-control" rows="2" name="txtcomment" maxlength="140" style="resize: none;" placeholder="Compose comment"></textarea>
            <input name="txtHiddenMusicPostID" value="37" type="hidden" />
        </div>
    </div>
    <input type="submit" class="btn btn-success pull-right" name="post-Comment" value="post-Comment" />
</form>
<form method="POST" name="comment" class="setCommentBox">
    <div class="form-group">
        <div class="bs-example">
            <textarea class="form-control" rows="2" name="txtcomment" maxlength="140" style="resize: none;" placeholder="Compose comment"></textarea>
            <input name="txtHiddenMusicPostID" value="37" type="hidden" />
        </div>
    </div>
    <input type="submit" class="btn btn-success pull-right" name="post-Comment" value="post-Comment" />
</form>
<form method="POST" name="comment" class="setCommentBox">
    <div class="form-group">
        <div class="bs-example">
            <textarea class="form-control" rows="2" name="txtcomment" maxlength="140" style="resize: none;" placeholder="Compose comment"></textarea>
            <input name="txtHiddenMusicPostID" value="37" type="hidden" />
        </div>
    </div>
    <input type="submit" class="btn btn-success pull-right" name="post-Comment" value="post-Comment" />
</form>

CSS

.btn{width:120px;height:30px;}

JavaScript

$('.btn').prop('disabled',true);

$("textarea[name='txtcomment']").keyup(function () {
        if ($(this).val() != '') {
            $(this).closest('form').find('.btn').prop('disabled', false);
        }else{
            $(this).closest('form').find('.btn').prop('disabled', true);
        }
});