WebRTC: force peers to use TURN server

I have no idea if or when the browsers will support this but have a look at the "ICE candidate policy" in section 4.1.1 of draft-ietf-rtcweb-jsep-08, you can see how setting the policy to "relay" will do what you want. In the current W3C API draft this is set using an RTCIceTransportPolicy value of "relay" for the iceTranportPolicy field in the configuration. Search for RTCIceTransportPolicy in https://w3c.github.io/webrtc-pc/

relay: The ICE Agent uses only media relay candidates such as candidates passing through a TURN server.

You can now force turn by using the iceTransportPolicy attribute

let pc = new RTCPeerConnection({ iceTransportPolicy : "relay" })

https://developer.mozilla.org/en-US/docs/Web/API/RTCConfiguration/iceTransportPolicy

just adding this to close the question,

function onIceCandidate(event, targetSessionID, targetUserName) {
    if (event.candidate) {
    var candidate = event.candidate.candidate;
    if(candidate.indexOf("relay")<0){ // if no relay address is found, assuming it means no TURN server
        return;
    }...

The above code works, checked to wireshark,

after adding the if(candidate.indexOf("relay")<0) condition, communication takes place only through TURN server, if server is not present/ incorrect details, connection state get's struck at new

Edit: like cullen has said in his answer, according to w3 webrtc, passing relay as iceTransportPolicy should work, but I haven't checked if it is implemented in Firefox and Chrome yet...