The MediaKeySession.close() method notifies that the current media session is no longer needed, and that the content decryption module should release any resources associated with this object and close it. Then, it returns a Promise.
The MediaKeySession.closed read-only property returns a Promise signaling when a MediaKeySession closes. This promise can only be fulfilled and is never rejected. Closing a session means that licenses and keys associated with it are no longer valid for decrypting media data.
The MediaKeySession.expiration read-only property returns the time after which the keys in the current session can no longer be used to decrypt media data, or NaN if no such time exists. This value is determined by the CDM and measured in milliseconds since January 1, 1970, UTC. This value may change during a session lifetime, such as when an action triggers the start of a window.