Get related item id from a workflow task list

​​​​<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js" type="text/javascript"></script>
<script type="text/javascript">
ExecuteOrDelayUntilScriptLoaded(retrieveListItems, "sp.js");
function retrieveListItems() {
    var clientContext = new SP.ClientContext.get_current();
    var oList = clientContext.get_web().get_lists().getByTitle('Workflow Tasks');
    var camlQuery = new SP.CamlQuery();
    camlQuery.set_viewXml('<View><Query><Where></Where></Query><RowLimit>10</RowLimit></View>');
    this.collListItem = oList.getItems(camlQuery);
    clientContext.load(collListItem);
    clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));            
}

function onQuerySucceeded(sender, args) {
    var listItemEnumerator = collListItem.getEnumerator();
    while (listItemEnumerator.moveNext()) {
        oListItem = listItemEnumerator.get_current();
        obj=JSON.parse(oListItem.get_item('RelatedItems'));
        alert(obj[0].ItemId);
    }
}

function onQueryFailed(sender, args) {
    alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}
</script>​​​​​​​​​​​​​​​​​​​​​​​​​​