Calculate size of used LocalStorage space

gistfile1.js

/**
 * Returns the total amount of disk space used (in MB) by localStorage for the current domain.
 */
var getLocalStorageSize = function() {
    var total = 0;
    for (var x in localStorage) {
        // Value is multiplied by 2 due to data being stored in `utf-16` format, which requires twice the space.
        var amount = (localStorage[x].length * 2) / 1024 / 1024;
        total += amount;
    }
    return total.toFixed(2);
};

See also navigator.storage.estimate() https://developer.mozilla.org/en-US/docs/Web/API/StorageManager/estimate