Can I be notified of cookie changes in client side JavaScript?

Question

Can I somehow follow changes to cookies (for my domain) in my client side JavaScript? For example, is there a function that gets called if a cookie gets changed, deleted or added?

In order of preference:

• standard cross browser
• cross browser
• browser specific
• extension / plugin

Why? Because cookies I depend on in window / tab #1 can get changed in window / tab #2.

I found out that Chrome allows extensions to be notified of cookie changes. But that's my least favorite option.

Answer 1

Method 1: Periodic Polling

Poll document.cookie

function listenCookieChange(callback, interval = 1000) {
  let lastCookie = document.cookie;
  setInterval(()=> {
    let cookie = document.cookie;
    if (cookie !== lastCookie) {
      try {
        callback({oldValue: lastCookie, newValue: cookie});
      } finally {
        lastCookie = cookie;
      }
    }
  }, interval);
}

Usage

listenCookieChange(({oldValue, newValue})=> {
  console.log(`Cookie changed from "${oldValue}" to "${newValue}"`);
}, 1000);

document.cookie = 'a=1; Path=/';

Method 2: API Interception

Intercept document.cookie

(()=> {
  let lastCookie = document.cookie;
  // rename document.cookie to document._cookie, and redefine document.cookie
  const expando = '_cookie';
  let nativeCookieDesc = Object.getOwnPropertyDescriptor(Document.prototype, 'cookie');
  Object.defineProperty(Document.prototype, expando, nativeCookieDesc);
  Object.defineProperty(Document.prototype, 'cookie', {
    enumerable: true,
    configurable: true,
    get() {
      return this[expando];
    },
    set(value) {
      this[expando] = value;
      // check cookie change
      let cookie = this[expando];
      if (cookie !== lastCookie) {
        try {
          // dispatch cookie-change messages to other same-origin tabs/frames
          let detail = {oldValue: lastCookie, newValue: cookie};
          this.dispatchEvent(new CustomEvent('cookiechange', {
            detail: detail
          }));
          channel.postMessage(detail);
        } finally {
          lastCookie = cookie;
        }
      }
    }
  });
  // listen cookie-change messages from other same-origin tabs/frames
  const channel = new BroadcastChannel('cookie-channel');
  channel.onmessage = (e)=> {
    lastCookie = e.data.newValue;
    document.dispatchEvent(new CustomEvent('cookiechange', {
      detail: e.data
    }));
  };
})();

Usage

document.addEventListener('cookiechange', ({detail: {oldValue, newValue}})=> {
  console.log(`Cookie changed from "${oldValue}" to "${newValue}"`);
});

document.cookie = 'a=1; Path=/';

Notes

1. not for IE
2. require BroadcastChannel polyfill for Safari

Conclusion

| Metric \ Method  | Periodic Polling            | API Interception |
| ---------------- | --------------------------- | ---------------- |
| delay            | depends on polling interval | instant          |
| scope            | same-domain                 | same-origin      |