Filtering script for products by category
I don't see how this is scalable if you have to hard code product id's into your jQuery code and duplicate handlers for every category you may want to add.

I also see no need to have separate logic for "first time" execution vs. every other time.

I would consider putting a data- attribute on each checkbox that can indicate the target category the checkbox filter is to work against. And then put a data- attribute on each product to indicate what category it belongs to.

So let's assume your products are like this:

&lt;div class="product" data-category="1"&gt;...&lt;/div&gt;
&lt;div class="product" data-category="2"&gt;...&lt;/div&gt;


And your category checkboxes are like:

&lt;input class="category_filter" data-category-target="1" type="checkbox" value="first-checkbox" /&gt;
&lt;input class="category_filter" data-category-target="2" type="checkbox" value="second-checkbox" /&gt;


Then your jQuery might be simplified down to something like:

// cache references to DOM elements to eliminate need to requery
var $categoryFilters = $('.category_filter');
var $allProducts = $('.product');
// build out selectors for each category
var productsByCategory = {};
$categoryFilters.each(function()) {
     var category = $(this).data('category-target');
     productsByCategory[category] = $allProducts.filter('[data-category=' + category + ']');
});

// now attach change handler to filters
$categoryFilters.on('change', function() {
    $allProducts.hide();
    $categoryFilters.each(function() {
        var target = $(this).data('category-target');      
        if($(this).is(':checked')) {
            productsByCategory[target].show();
        }
    }
});




Why are you loading two versions of jQuery?  If this is absolutely unavoidable,  you should be aliasing these versions appropriately (i.e. to something other than $) so that you can write code against whichever version is pertinent to that section of code.



Anytime you find yourself duplicating code for cases like you have with #example-category-*, this should be an immediate red flag to you that have established a coding antipattern.  For this specific example, what you really have is class level behavior (all of the category checkboxes exhibit similar behavior in this case).  That typically mean you should be writing code against the class of elements, not against individual unique id's.