Get "CreatedBy" and "ModifiedBy" from list using client coding

<script language="ecmascript" type="text/ecmascript">

        var listItem;

        var list;

        var clientContext;

        function getFieldUserValue() {

            this.clientContext = SP.ClientContext.get_current();

            if (this.clientContext != undefined && clientContext != null) {

                var webSite = clientContext.get_web();

                this.list = webSite.get_lists().getByTitle("CustomList");

                this.listItem = list.getItemById(1);

                clientContext.load(this.listItem);

                this.clientContext.executeQueryAsync(Function.createDelegate(this, this.OnLoadSuccess), Function.createDelegate(this, this.OnLoadFailed));

            }

        }

        function OnLoadSuccess(sender, args) {

            var fieldUserValueCreatedBy = this.listItem.get_item("Author");

            var fieldUserValueModifiedBy = this.listItem.get_item("Editor");

            alert("Created By: " + fieldUserValueModifiedBy.get_lookupValue() + "\n Modified By: " + fieldUserValueModifiedBy.get_lookupValue() + "\n");

        }

        function OnLoadFailed(sender, args) {

            alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());

        }</script>

    <input id="btnGetFieldUserValue" onclick="getFieldUserValue()" type="button" value="Get Created by and Modified by" />