CloseEvent.reason

The reason read-only property of the CloseEvent interface returns the WebSocket connection close reason the server gave for closing the connection; that is, a concise human-readable prose explanation for the closure.

A string.

The following example prints the value of reason to the console.

WebSocket.onclose = function(event) {
  console.log(event.reason);
};

• RFC 6455 (the WebSocket Protocol specification)