You can use a Blob
and the HTML5 a[download]
feature to provide a JSON backup download:
var data = {a:1, b:2, c:3};
var json = JSON.stringify(data);
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";
Here is a jsfiddle example: http://jsfiddle.net/potatosalad/yuM2N/