Using xhr-streaming, xhr-polling, or jsonp-polling, if an error is raised in the socket.onmessage event handler, then the next time a message is received SockJS will replay the message that caused the error.

For example, message A is received and the client onmessage event handler throws an uncaught error; later, message B is received, and onmessage is called again with message A.

Repro repo: https://github.com/jcheng5/sockjs-error-test

The problem appears to be here:

When this.emit('message', msg); throws an exception, this.bufferPosition doesn't get updated (third clause of the for-loop). If you update this.bufferPosition before emitting the message, the problem goes away.