Host app using rest service Sharepoint 2013 , Sharepoint 2016 , Office 365

'use strict';

var context;
var user;
var hostweburl;
var appweburl;
var ListName;
// This code runs when the DOM is ready and creates a context object which is needed to use the SharePoint object model
$(document).ready(function () {
    getListItem();
});

// This function prepares, loads, and then executes a SharePoint query to get the current users information
function getListItem() {
    hostweburl = decodeURIComponent(getQueryStringParameter('SPHostUrl'));
    appweburl = decodeURIComponent(getQueryStringParameter('SPAppWebUrl'));
    ListName = decodeURIComponent(getQueryStringParameter('ListName'));
    var scriptbase = hostweburl + '/_layouts/15/';
    //Call method after loading required JS files from Host Web
    $.getScript(scriptbase + 'SP.Runtime.js',
        function () {
            $.getScript(scriptbase + 'SP.js',
                function () { $.getScript(scriptbase + 'SP.RequestExecutor.js', getItem); }
            );
        }
    );
}

//Fetching List Item
function getItem() {
   /* var context = SP.ClientContext.get_current();
    var user = context.get_web().get_currentUser();*/
    debugger;
    $.ajax({
        url: appweburl + "/_api/SP.AppContextSite(@target)/web/lists/getbyTitle('"+ ListName + "')/Items?@target='" + hostweburl + "'",
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: function (data) {
            var result = data.d.results;
            var item="";
            for (var i = 0; i < result.length; i++) {
                item += "<span>" + result[i].dept + "</span>";
                item += "<span>" + result[i].TRS + "</span>";
                item += "<span>" + result[i].PTD + "</span>";
            }
            $(".DetailsItem").html(item)
        },
        error: function (data) {
            failure(data);
        }
    });
}
//Get query string parameters
function getQueryStringParameter(paramToRetrieve) {
    var params = document.URL.split("?")[1].split("&");
    var strParams = "";
    for (var i = 0; i < params.length; i = i + 1) {
        var singleParam = params[i].split("=");
        if (singleParam[0] == paramToRetrieve) return singleParam[1];
    }
}