webkitStorageInfo.queryUsageAndQuota() is used to find out the usage stats of the files that have been stored in the file system using the HTML5 file system API I suppose. C
// Request storage usage and capacity left
window.webkitStorageInfo.queryUsageAndQuota(webkitStorageInfo.TEMPORARY, //the type can be either TEMPORARY or PERSISTENT
function(used, remaining) {
console.log("Used quota: " + used + ", remaining quota: " + remaining);
}, function(e) {
console.log('Error', e);
} );
Where used and remaining are in bytes
Taken from google devlopers