What purpose does generating a closure inside of the $(document).ready() function accomplish?

Question

Debugging one of my projects I noticed another developer had changed the $(document).ready() function to generate a closure inside of itself. E.G. $(document).ready(function($) { }); I am curious as to the point of doing this, as well as it's usage.

Note: By removing the $ from the function my code works again. $(document).ready(function() { })

Original/Fixed Code

$(document).ready(function() {
    var id = //pull session variable from asp session (yuck)
    var img = $('.photoLink');

    $('.photoLink').click(function() {
        $(this).photoDialog({
            id: id,
            onClose: function() {
                img.attr('src', img.attr('src') + '&rand=' + (new Date()).getTime()); //prevent caching of image
            }
        });
    });
});

Modified/Broken Code

$(document).ready(function($) {
    var id = //pull session variable from asp session (yuck)
    var img = $('.photoLink');

    $('.photoLink').click(function() {
        $(this).photoDialog({
            id: id,
            onClose: function() {
                img.attr('src', img.attr('src') + '&rand=' + (new Date()).getTime()); //prevent caching of image
            }
        });
    });
});

The modified code would produce errors in FireBug stating that the custom plugin function that I was calling did not exist. I am assuming this is because the $ argument is overriding or conflicting with any of the jQuery functions trying to use it.

I'm really confused as to why someone would have changed this, in the current context it makes no sense as that plugin call is the only javascript on the page.

Can someone explain to me why you would use this and possibly an example of it's usage?

Edit

Below is the code for my custom plugin, I also modified the examples above to display how I am calling it:

(function($) {
    var link = $('<link>');
    link.attr({
        type: 'text/css',
        rel: 'stylesheet',
        href: 'http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.7/themes/black-tie/jquery-ui.css'
    }).appendTo('head');

    var script = $('<script>');
    script.attr({
        type: 'text/javascript',
        src: 'https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.7/jquery-ui.min.js'
    }).appendTo('head');

    $.fn.photoDialog = function(options) {

        var defaults = {
            autoOpen: false,
            title: 'Photo Tool',
            minHeight: 560,
            minWidth: 540,
            url: '/photo_form.aspx',
            onClose: function(){}
        };
        var opts = $.extend(defaults, options);

        return this.each(function() {
            $this = $(this);
            that =$(this);
            var $dialog = $('<div>')
                .html('<iframe src="' + opts.url + '?sn=' + opts.id + '" width="' + (opts.minWidth - 20) + '" height="' + (opts.minHeight - 20) + '" style="border: none;" scrolling="no"></iframe>')
                .dialog({
                    autoOpen: opts.autoOpen,
                    title: opts.title,
                    minHeight: opts.minHeight,
                    minWidth: opts.minWidth,
                    modal: true,
                    close: function() {
                        opts.onClose.call(that);
                    }
                });

            $this.click(function() {
                $dialog.dialog('open');
                return false;
            });
        });
    };
})(jQuery);

Answer 1

When you write a jQuery plugin, to support the noConflict feature, you usually do:

(function($) {
    // Plugin code...
})(jQuery);

That allows you to use $ as an alias for jQuery within the plugin code, regardless of the noConflict settings.

Maybe the other developer added the $ argument by reflex.

On second thought, strike that. The other developer was probably trying to improve the situation.

Your code should still work, even with a $ argument passed to the ready handler. You say the custom plugin function that you were calling does not exist anymore. Can you tell us more about that custom plugin?

I suspect $ is changing between the call to document.ready() and the actual execution of the handler, and you were taking advantage of that before, but you can't anymore since the original $ is now passed to the handler.