This is the best localStorage polyfill in the world

best-localStorage-polyfill-evar.js

// I mean, seriously, localStorage is supported even by your mum. How about instead of
// casing the feature out, you give users in-memory (stale) storage instead?
// If they close your application, they deserve to lose data anyway.

// if (!('localStorage' in window)) {
if (!Modernizr.localstorage) {
  window.localStorage = {
    _data       : {},
    setItem     : function(id, val) { return this._data[id] = String(val); },
    getItem     : function(id) { return this._data.hasOwnProperty(id) ? this._data[id] : undefined; },
    removeItem  : function(id) { return delete this._data[id]; },
    clear       : function() { return this._data = {}; }
  };
}

For those that want to "polyfill" the localStorage object due to it being blocked by an error like this: DOMException: Failed to read the 'localStorage' property from 'Window': Access is denied for this document. (usually due to an embedded frame when your site is used in incognito), you'll need to use defineProperty. For example, here's a super minimal polyfill that doesn't allow usage like localStorage.setItem("key", value), but does allow localStorage.key=value:

Object.defineProperty(window, 'localStorage', {value: new Proxy({}, {set:(obj,k,v)=>obj[k]=String(v)}) });