__REQUESTDIGEST is undefined in sharepoint hosted app ( Client WebPart ) but its working in full page app

Below is the way to use. Note its working in both Single Page App and Client App Part as well

var formDigest; //Declare the variable

//Document Ready
$(document).ready(function () {
    hostweburl = decodeURIComponent(getQueryStringParameter('SPHostUrl'));
    appweburl = decodeURIComponent(getQueryStringParameter('SPAppWebUrl'));
    scriptbase = hostweburl + '/_layouts/15/';
    CharacterAnimation();
    $.getScript(scriptbase + 'SP.Runtime.js', function ()
    {
        $.getScript(scriptbase + 'SP.js', function ()
        {
            $.getScript(scriptbase + 'SP.RequestExecutor.js', getFormDigest);
        });
    });
});

//Get  Form Digest Value
function getFormDigest() {
    var appweburl = decodeURIComponent(getQueryStringParameter('SPAppWebUrl'));
    $.ajax({
        url: appweburl + "/_api/contextinfo",
        type: "POST",
        headers: {
            "accept": "application/json;odata=verbose",
            "contentType": "text/xml"
        },
        success: function (data) {
            requestdigest = data;
            var formDigest = data.d.GetContextWebInformation.FormDigestValue;
            DoSomething(formDigest);
        },
        error: function (err) {
            alert(JSON.stringify(err));
        }
    });
}

//Do Something Method
function  DoSomething(formDigest) {
    var urltest = appweburl + "/_api/SP.AppContextSite(@target)/web/lists/getByTitle('List1')/getitems(<Query><Where><BeginsWith><FieldRef">query=@v1)?@v1={\"ViewXml\":\"<View><Query><Where><BeginsWith><FieldRef Name='Title'/><Value Type='Text'>A</Value></BeginsWith></Where></Query><RowLimit>1</RowLimit></View>\"}&@target='" + hostweburl + "'";
    $.ajax({
        url: urltest,
        type: "POST",
        headers: {
            "Accept": "application/json; odata=verbose",
            "Content-Type": "application/json; odata=verbose",
            "X-RequestDigest": formDigest
        },
        contentType: 'application/json',
        success: function (data) {
            alert(data); // Finally found [Object][Object] :)
        },
        error: function (data) {
            alert(data.responseText);
        }
    });
}

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