setLocalDescription

setLocalDescription is a method of the RTCPeerConnection interface in the WebRTC API. It sets the local session description for the connection, typically an SDP offer or answer produced by createOffer or createAnswer. The local description defines the parameters for the local endpoint and is used during signaling to negotiate media capabilities with the remote peer.

Usage generally follows the offer/answer flow. You create an offer with createOffer, then set it as the local description with setLocalDescription, and send the offer to the remote peer via your signaling channel. The remote peer applies the offer with setRemoteDescription and may reply with an answer, which you apply using setRemoteDescription. The actual transmission of the local description to the remote party is handled by your signaling layer; setLocalDescription itself does not send data over the network.

The call is asynchronous. In modern browsers it returns a Promise that resolves once the local description has been installed, and it can reject with errors such as InvalidStateError if the connection is closed or not in a suitable state. Using asynchronous syntax, for example: const offer = await pc.createOffer(); await pc.setLocalDescription(offer).

The description argument is an RTCSessionDescriptionInit (or RTCSessionDescription) object and must include a type ('offer' or 'answer') and an sdp string. Setting the local description may affect the connection’s signalingState and can trigger negotiation-needed events depending on the browser’s implementation and current state.

See also: setRemoteDescription, createOffer, createAnswer, RTCPeerConnection.