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 = {}; }
  };
}

Not sure if this addresses the fact that private Safari threw an error on setItem in some versions... which is stupid, but still relevant. I'm just going to conveniently ignore it at this point though, and mute all my sentry/rollbar issues :-)

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)}) });