The read-only RTCDataChannel property negotiated indicates whether the RTCDataChannel's connection was negotiated by the Web app (true) or by the WebRTC layer (false). false

See Creating a data channel for further information about this property.

true if the RTCDataChannel's connection was negotiated by the Web app itself; false if the negotiation was handled by the WebRTC layer. The default is false.

The code snippet below checks the value of negotiated; if it's true, a function called shutdownRemoteChannel() is called with the channel's id; presumably this would be implemented to transmit a shutdown signal to the remote peer prior to terminating the connection.

if (dataChannel.negotiated) {
  shutdownRemoteChannel(dataChannel.id);
}

• WebRTC
• Using WebRTC data channels
• RTCDataChannel
• RTCPeerConnection.createDataChannel()