get_query_var没有返回值（wordpress）

So im using isotope to filter categories on my blog page, when you click on a blog I still have my menu of categories at the top of the single blog post so people can click on a category and it will take them back to the blog page but on the category selected. So on the single blog page ive added an add_query_arg to the link to return a custom url.

<a href="<?php echo add_query_arg( 'cat', $category->name, '/blog' );?>">

this gives me the url on my blog page as mysite.com/blog/?cat=information

then on my blog page i call the query like this

<?php $cat = get_query_var('cat');?>

Then in my javascript for filtering the categories with isotope i have

filter: "<?php echo $cat;?>"

but this is not returning anything in my javascript when i view source is just

filter: ""

i have also added this to my functions.php

 function add_query_vars_filter( $vars ){
   $vars[] = "cat";
   return $vars;
 }

//Add custom query vars
add_filter( 'query_vars', 'add_query_vars_filter' );

anyone got an idea where im going wrong?