can i be notified of cookie changes in client side javascript

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';

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';

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      |

One option is to write a function that periodically checks the cookie for changes:

var checkCookie = function() {

    var lastCookie = document.cookie; // 'static' memory between function calls

    return function() {

        var currentCookie = document.cookie;

        if (currentCookie != lastCookie) {

            // something useful like parse cookie, run a callback fn, etc.

            lastCookie = currentCookie; // store latest cookie

        }
    };
}();

window.setInterval(checkCookie, 100); // run every 100 ms

• This example uses a closure for persistent memory. The outer function is executed immediately, returning the inner function, and creating a private scope.
• window.setInterval