How to get Host web Url from App web in sharepointhosted app

Hi all,

          I am doing a sharepoint hosted app with client webpart using rest API.I can't get app web and host web url using decodeURIComponent(getQueryStringParameter("SPHostUrl")); since the webpart is loading in iframe then how to get App web URL and Host web URL in client webpart page of sharepoint hosted app.Thanks in advance.