Checking for attachments in JavaScript

Prior to submitting a form, I would like to check if a file has been attached and pop up a warning message saying that a file needs to 开发者_如何学Gobe attached if it hasn't been. I was wondering how to accomplish this using JavaScript or Prototype or JQuery etc?

Assuming you are using an <input type="file"> field, you can simply check if the element's value is a non-empty string:

<form method="POST">
   <input type="file" id="attachment" />
   <input type="button" onClick="checkAttachment();" value="Send" />
</form>

<script type="text/javascript"> 
   function checkAttachment() {
      if (document.getElementById('attachment').value !== '') {
         alert('File Attached');
      }   
      else {
         alert('No File Attached');
      }
   }
</script>