Ecma to GetListItem and fill in dropdown

<script type=”text/ecmascript” language=”ecmascript”>
   
    ExecuteOrDelayUntilScriptLoaded(retrieveListItems, "sp.js");
    var ItemContainer = { ItemList: [] };

   function retrieveListItems() {

        var clientContext = SP.ClientContext.get_current();
        // Get the SharePoint list by giving the display name of the list.
        var oList = clientContext.get_web().get_lists().getByTitle(‘Categories’);
        // To retreive all items use the predefined createAllItemsQuery operation.
        var camlQuery = SP.CamlQuery.createAllItemsQuery();
        this.collListItem = oList.getItems(camlQuery);
        clientContext.load(collListItem);
        clientContext.executeQueryAsync(
            Function.createDelegate(this, this.onListDataLoadQuerySucceeded),
            Function.createDelegate(this, this.onListDataLoadQueryFailed));
    }



 // Callback function if the item retrieval Async call get successful.
    function onListDataLoadQuerySucceeded(sender, args) {
        var listItemInfo = ”;
        var listItemEnumerator = collListItem.getEnumerator();
        while (listItemEnumerator.moveNext()) {
            var oListItem = listItemEnumerator.get_current();
            // Fill a json object with Id and Value properties.
            var tempItem = { Id: oListItem.get_id(), Value: oListItem.get_item(‘Title’) };
            ItemContainer.ItemList.push(tempItem);
        }
        // Fill the drop down with retrieved data.
        fillDropDown();
    }
    // Callback function if item retrieval failes.
    function onListDataLoadQueryFailed(sender, args) {
        alert(‘Request failed. ‘ + args.get_message() + ‘\n’ + args.get_stackTrace());
    }


 // Fill  the drop down with the retrieved list item data.
    function fillDropDown() {
        var ddlCategory = document.getElementById(‘ddlCategory’);
        if (ddlCategory != null) {
            for (var i = 0; i < ItemContainer.ItemList.length; i++) {
                var theOption = new Option;
                theOption.value = ItemContainer.ItemList[i].Id;
                theOption.text = ItemContainer.ItemList[i].Value;
                ddlCategory.options[i] = theOption;
            }
        }
    }


</script>