If set, this will be called when a client opens a connection using the ALPN extension. One argument will be passed to the callback: an object containing `servername` and `protocols` fields, respectively containing the server name from the SNI extension (if any) and an array of ALPN protocol name strings. The callback must return either one of the strings listed in `protocols`, which will be returned to the client as the selected ALPN protocol, or `undefined`, to reject the connection with a fatal alert. If a string is returned that does not match one of the client's ALPN protocols, an error will be thrown. This option cannot be used with the `ALPNProtocols` option, and setting both options will throw an error.

Type

((arg: { servername: string; protocols: string[]; }) => string | undefined) | undefined