The localDescription read-only property of the RTCPeerConnection interface returns an RTCSessionDescription describing the session for the local end of the connection. If it has not yet been set, this is null.
On a more fundamental level, the returned value is the value of RTCPeerConnection.pendingLocalDescription if that property isn't null; otherwise, the value of RTCPeerConnection.currentLocalDescription is returned. See Pending and current descriptions in the WebRTC Connectivity page for details on this algorithm and why it's used.
This example looks at the localDescription and displays an alert containing the RTCSessionDescription object's type and sdp fields.
const pc = new RTCPeerConnection();
// …
const sd = pc.localDescription;
if (sd) {
alert(`Local session: type='${sd.type}'; sdp description='${sd.sdp}'`);
} else {
alert("No local session yet.");
}