The read-only signalingState property on the RTCPeerConnection interface returns a string value describing the state of the signaling process on the local end of the connection while connecting or reconnecting to another peer. See Signaling in our WebRTC session lifetime page.
Because the signaling process is a state machine, being able to verify that your code is in the expected state when messages arrive can help avoid unexpected and avoidable failures. For example, if you receive an answer while the signalingState isn't "have-local-offer", you know that something is wrong, since you should only receive answers after creating an offer but before an answer has been received and passed into RTCPeerConnection.setLocalDescription(). Your code will be more reliable if you watch for mismatched states like this and handle them gracefully.
This value may also be useful during debugging, for example.
In addition, when the value of this property changes, a signalingstatechange event is sent to the RTCPeerConnection instance.
The allowed string values are:
stable-
There is no ongoing exchange of offer and answer underway. This may mean that the RTCPeerConnection object is new, in which case both the localDescription and remoteDescription are null; it may also mean that negotiation is complete and a connection has been established.
have-local-offer-
The local peer has called RTCPeerConnection.setLocalDescription(), passing in SDP representing an offer (usually created by calling RTCPeerConnection.createOffer()), and the offer has been applied successfully.
have-remote-offer-
The remote peer has created an offer and used the signaling server to deliver it to the local peer, which has set the offer as the remote description by calling RTCPeerConnection.setRemoteDescription().
have-local-pranswer-
The offer sent by the remote peer has been applied and an answer has been created (usually by calling RTCPeerConnection.createAnswer()) and applied by calling RTCPeerConnection.setLocalDescription(). This provisional answer describes the supported media formats and so forth, but may not have a complete set of ICE candidates included. Further candidates will be delivered separately later.
have-remote-pranswer-
A provisional answer has been received and successfully applied in response to an offer previously sent and established by calling setLocalDescription().
closed-
The RTCPeerConnection has been closed.
const pc = new RTCPeerConnection(configuration);
const state = pc.signalingState;