Get user properties using javascript

<script type="text/javascript">
    ExecuteOrDelayUntilScriptLoaded(getProfile, "sp.js");
    var context = null;
    var web = null;      
    function getProfile() {
        context = SP.ClientContext.get_current();
        web = context.get_web();
        userInfoList = web.get_siteUserInfoList();
        camlQuery = new SP.CamlQuery();
        camlQuery.set_viewXml('<View><Query><Where><Eq><FieldRef Name=\'ID\'/><Value Type=\'Number\'>' + _spPageContextInfo.userId +
'</Value></Eq></Where></Query><RowLimit>1</RowLimit></View>');
        this.listItems = userInfoList.getItems(camlQuery);
        context.load(listItems);
        context.executeQueryAsync(Function.createDelegate(this, this.onProfileSuccessMethod), Function.createDelegate(this,

this.onFailureMethod));
    }

    function onProfileSuccessMethod(sender, args) {
        var item = listItems.itemAt(0);
        var obj = item.get_item('FirstName');
        alert(obj);
        }
 
    function onFailureMethod(sender, args) {
        alert('Error: ' + args.get_message() + '\n' + args.get_stackTrace());
    }
</script>