Storage Typescript adapter layer. Use localStorage when useable, otherwise cookies

Storage.ts

namespace Util {

    // Contract for storage
    export interface Storage {
        getItem(key: string): any;
        removeItem(key: string): void;
        setItem(key: string, data: string): void;
    }

    // Borrowed from https://gist.github.com/wittnl/2890870
    // and a little of https://gist.github.com/remy/350433
    class CookieStorage implements Storage {

        createCookie(name, value, days) {
            if (days) {
                var date = new Date();
                date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
                var expires = "; expires=" + date.toUTCString();
            }
            else var expires = "";
            document.cookie = name + "=" + value + expires + "; path=/";
        }

        readCookie(name) {
            var result = null;
            var nameEQ = name + "=";
            var ca = document.cookie.split(';');
            for (var i = 0; i < ca.length; i++) {
                var c = ca[i];
                while (c.charAt(0) == ' ') {
                    c = c.substring(1, c.length);
                }
                if (c.indexOf(nameEQ) == 0) {
                    result = c.substring(nameEQ.length, c.length);
                    return result;
                }
            }
            return result;
        }


        getItem(key: string): any {
            return this.readCookie(key);
        }
        removeItem(key: string): void {
            this.setItem(key, '');
        }
        setItem(key: string, data: string): void {
            if (data != null) {
                this.createCookie(key, data, 3000);
            }
        }
    }

    // Checks if localStorage is usable
    // Borrowed from https://github.com/marcuswestin/store.js/blob/master/src/store.js
    // This detects Safari iOS private mode
    // see http://apple.stackexchange.com/a/131589
    function isLocalStorageUsable() {
        try {
            var testKey = '__localstoragetest__'
            localStorage.setItem(testKey, testKey)
            if (localStorage.getItem(testKey) != testKey) {
                return false;
            }
            localStorage.removeItem(testKey)
        } catch (e) {
            return false;
        }
        return true;
    }

    // Loads a concrete Storage 
    // When localStorage isn't useable, use cookies
    export function provideStorageLayer(): Storage {

        // Check availability
        if (!localStorage) {
            return new CookieStorage();
        }
        // Check safari private mode
        if (!isLocalStorageUsable()) {
            return new CookieStorage();
        }

        // Default to localStorage
        return localStorage as Storage;
    }
}