The onclose event handler of the IDBDatabase interface handles the close event, which is fired when the database is unexpectedly closed. This can happen, for example, when the application is shut down or access to the disk the database is stored on is lost while the database is open. 
 The close event is fired after all transactions have been aborted and the connection has been closed. 
Note: This feature is available in Web Workers
IDBDatabase.onclose = function(event) { ... };
A function which is called when the close event is fired.
db.onclose = function(event) { myAppShowAlert('The database "' + db.name + '" has unexpectedly closed.'); };
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onclose | 
 31 
approx 
 | 
79  | 
50  | 
No  | 
Yes  | 
10.1  | 
≤37  | 
31  | 
50  | 
Yes  | 
10.3  | 
2.0  | 
IDBDatabase
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    https://developer.mozilla.org/en-US/docs/Web/API/IDBDatabase/onclose