// fire event when window is resized
$( window ).resize( function() {
  $body = $( 'body' );
  $documentWidth = $( '#document-width' );
  if ($documentWidth.length===0){
    $documentWidth = $( '<p id="document-width" />' );
    $body.prepend( $documentWidth );
  }
  $documentWidth.text( $body.width() );

} );