Function : js_push_if_not_exists()

Description : enqueue JavaScript file to the global JavaScript queue if this file hasn't yet been included. It a clever way to add js script once. This function check if a file path has already been recorded, if so, the file is not included. Files will been loaded in order of they are called. This function should also been used only on theme files.

Example :

    js_push_if_not_exists( theme_assets_url( '/js/jquery.js' ) ); /* First to load */   
    js_push_if_not_exists( theme_assets_url( '/js/custom.js' ) ); /* second and so on */