var json = localStorage.getItem('theData');
var blob = new Blob([json], {type: "application/json"});
var url = URL.createObjectURL(blob);

var a = document.createElement('a');
a.download = "backup.json";
a.href = url;
a.textContent = "Download backup.json";

($ == undefined ) ? document.body.appendChild(a) : $("body").append(a);