Values of this type are strings. Possible values are:

evicted

A cookie has been automatically removed due to garbage collection.

expired

A cookie has been automatically removed due to expiry.

explicit

A cookie has been inserted or removed via an explicit call to cookies.remove().

expired_overwrite

A cookie has been overwritten by a cookie with an already-expired expiration date.

overwrite

A call to cookies.set() overwrote this cookie with a different one.

You can listen to the cookies.onChanged event to be notified when cookies change. The listener is passed a changeInfo object that contains a cause property, whose value is the OnChangeCaused string:

browser.cookies.onChanged.addListener((changeInfo) => {
  console.log(`Cookie changed: \n`
    + ` * Cookie: ${JSON.stringify(changeInfo.cookie)}\n`
    + ` * Cause: ${changeInfo.cause}\n`
    + ` * Removed: ${changeInfo.removed}`);
});