In what cases will a websocket close event tell you wasClean is false?

Question

In what cases will a websocket close event tell you wasClean is false? For example:

connection.onclose = function(event) {
   if(!event.wasClean) {
      // whyyyyyyy???
   }
}

Answer 1

wasClean property is set to false when the WebSocket connection did not close via the close handshake.
i.e. by not receiving a valid Close frame from the server.

And from documentation: "CloseEvent.wasClean Returns a Boolean that Indicates whether or not the connection was cleanly closed"