Alert elements form returned JSON

I am using the following to set the json returned to a global variable

 $('#dtInvoiceDetails')
    .on('xhr.dt', function ( e, settings, json, xhr ) {
           selectedPrintData = JSON.stringify(json);

    } );

the console shows

selectedPrintData = {"data":[{"DT_RowId":"row_10108","inv_list":{"lineID":"10108","InvID":"3113","quantity":"23","barCode":"5555","stockCode":"8686856","size":"77","colour":"","description":"more jjjjjjj","costPrice":"0.00","sellPrice":"5.00","lineTotalNett":"115.00","lineTotal":"0.00","vatRate":"0.00"}}],"options":[],"files":[]}

I have tried

console.log(selectedPrintData[0]['data']['inv_list']['quantity']);
console.log(selectedPrintData[0][data][inv_list][quantity]);

What is the correct way to get thhe returned data into an array then be able to alert the 'quantity' value.

Cheers

Steve Warby