BSON decoding from Blob

I am using Tornado to work with a client application using Javascript. BSON is used for data exchange. Since Tornado uses JSON to send data, I wrote my function to send over Websocket:

def write_bson(self, message):
    message = BSON.encode(message)
    self.ws_connection.write_message(message, binary=True)

      

        
        
        
      

    

Since "binary = True" in the browser I am getting Blob and don't understand how to do BSON, decode the received message.

I tried the following way to do the decoding, in the comments I indicated the console.log output:

    t = new WebSocket(url);
    t.onmessage = function(event) {
        console.log(event.data); // Blob { size: 390, type: "" }

        console.log(BSON.deserialize(event.data)); // Error: corrupt bson message

        var reader = new FileReader();
        reader.onload = function(e) {
             console.log(e.target.result); // ArrayBuffer { byteLength: 390 }
        };
        var data = reader.readAsArrayBuffer(event.data);

        console.log(BSON.deserialize(data)); // Error: corrupt bson message

      

        
        
        
      

    

How to decode BSON?