您需要设置文本区域的价值。

You need to set the text areas value.

$("#textareaID").val("value of text area");

我把这个成头部引用查询一个空白的HTML文档，它能正常工作。

I put this into a blank HTML doc referenced query in the head and it worked fine.

<form>
  <textarea id="test"></textarea>
</form>
<script>
    $('#test').val('testing');
</script>