How to display an error if the content of a <textarea></textarea> is empty using javascript?

Question

如果用户在填写表单时将必填的输入字段留空，我知道如何显示错误消息，但是我无法对textarea进行同样的操作。

Answer 1

i think on change fires for text areas as well so assuming it does

theTextArea.onChange = function(){
  if(this.value == ''){
     alert('You must supply a value for this field');
  }
};

or with jquery

$(document).ready(
  function(){
    $('#mytextarea').change(function(){
      var $this = $(this);
      if($this.val() == ''){
         alert('You must supply a value for this '+$("label[for='mytextarea']").text());
      }
    });
});