I am using the below javascript to set the source image file. But I could not set ALT tag with this script. Can anyone help?

  function labnolIframe(div) {
    var iframe = document.createElement('iframe');
    iframe.setAttribute(
      'src',
      '/image/directory/img.jpg'
    );

I tried using below to set Alt but not working. iframe.setAttribute('alt', 'test alt');