How to dynamically get eBay ItemID in JavaScript

header.php

<script type="text/javascript">
function wpl_process_ebay_item_id_jsonp(ebayID) {
    // console.log('jsonp: ', ebayID);
    window.ebayItemID = ebayID;
    document.getElementsByClassName('wpl_ebay_item_id_wrapper')[0].innerHTML = ebayID;
}

if ( typeof window.ebayItemID == 'undefined' ) {
    var script = document.createElement('script');
    script.src = '[[admin_ajax_url]]' + '?action=wpl_ebay_item_query&callback=wpl_process_ebay_item_id_jsonp&id=' + '[[wpl_listing_id]]';

    document.getElementsByTagName('head')[0].appendChild(script);
    // or document.head.appendChild(script) in modern browsers
} else {
    wpl_process_ebay_item_id_jsonp( window.ebayItemID );
}
</script>