jQuery clarification

[mnaseersj]

Hi - I'm working my way through jQuery book, Novice to Ninja (p94), on an exercise which is illustrating how to pop up a simple lightbox for an image when it's clicked.

I would be grateful if someone could explain to me in the code at the bottom, how is it that, in the line :

var top = ($(window).height() - $('#lightbox').height()) / 2;

there is a value for the 'height' for the #lightbox at this stage

since the img element is if I'm not mistaken is in limbo land at this stage and is only appended to the id 'lightbox' later down the code.

$(document).ready(function(){
  $('a.lightbox').click(function(e) {
    $('body').css('overflow-y', 'hidden'); // hide scrollbars!
 
    $('<div id="overlay"></div>')
      .css('top', $(document).scrollTop())
      .css('opacity', '0')
      .animate({'opacity': '0.5'}, 'slow')
      .appendTo('body');
 
    $('<div id="lightbox"></div>')
      .hide()
      .appendTo('body');
 
    $('<img />')
      .attr('src', $(this).attr('href'))
      .load(function() {
        positionLightboxImage();      //   at this stage how is the 'height' for the
      })                             //   #lightbox populated with any value
      .click(function() {
        removeLightbox();
      })
      .appendTo('#lightbox');
 
    return false;;
  });
});
 
function positionLightboxImage() {
  var top = ($(window).height() - $('#lightbox').height()) / 2;
  var left = ($(window).width() - $('#lightbox').width()) / 2;
  $('#lightbox')
    .css({
      'top': top + $(document).scrollTop(),
      'left': left
    })
    .fadeIn();
}
 
function removeLightbox() {
  $('#overlay, #lightbox')
    .fadeOut('slow', function() {
      $(this).remove();
      $('body').css('overflow-y', 'auto'); // show scrollbars!
    });
}

thanking in advance.

[paul_wilkins]

since the img element is if I'm not mistaken is in limbo land at this stage and is only appended to the id 'lightbox' later down the code.

The image does not have to be attached to the page for information about its height or width to be known. It only has to be loaded for that info to be available.

[Raffles]

To add to what Paul said, this is happening all over the place. For example, the "overlay" DIV is having CSS and even event handlers applied to it before it's actually appended to the body.

[mnaseersj]

Hi Paul , Raffles - thank you for your replies.

The image does not have to be attached to the page for information about its height or width to be known. It only has to be loaded for that info to be available.

To add to what Paul said, this is happening all over the place. For example, the "overlay" DIV is having CSS and even event handlers applied to it before it's actually appended to the body.

$('<img />')
      .attr('src', $(this).attr('href'))
      .load(function() {
        positionLightboxImage();      //   at this stage how is the 'height' for the
      })                              //   #lightbox populated with any value
      .click(function() {
        removeLightbox();
      })
      .appendTo('#lightbox');
 
    return false;;
  });

If it's not attached to the page then I assume you are saying that the height is retrieved through the this keyword for the image click event, but that is not the case here from what i can make out of the code....


Unfortunately, I am still unclear though - because the way in which the height is being retrieved is through the #lightbox node i.e: $('#lightbox').height() , yet the node gets assigned to the DOM after the call to positionLightboxImage() ? ....Is it the case that the .load event only fires after the image get's fully loaded and by that time the DOM is complete and includes the #lightbox node as well.

Or am I not catching something very basic that I am over looking or perhaps reading into this more than I should....

.

[paul_wilkins]

Or am I not catching something very basic that I am over looking or perhaps reading into this more than I should....

Here's a simplification:

$('<img />')
      .load(function() {
          ...
      })
      .appendTo('#lightbox');

The image is added to the lightbox.
The events assigned when the image was in limbo-land remain with the image after it's attached to the document.

Once the image has finished been added to the lightbox, the image starts to load.
When the image has finished loading, the load event of that image is triggered.
That is when the function then runs.

[mnaseersj]

thanks, that makes it crystal clear