.js

(async () => {
  // Offer (actually, from SFU server)
  const offerSdp =
        "v=0\r\n" +
        "o=- 3163441866893267132 2 IN IP4 127.0.0.1\r\n" +
        "s=-\r\n" +
        "t=0 0\r\n" +
        "a=group:BUNDLE 0\r\n" +
        "a=msid-semantic: WMS\r\n" +
        "m=video 9 UDP/TLS/RTP/SAVPF 96 97\r\n" +
        "c=IN IP4 0.0.0.0\r\n" +
        "a=rtcp:9 IN IP4 0.0.0.0\r\n" +
        "a=ice-ufrag:og7u\r\n" +
        "a=ice-pwd:F1+FjtmwP0s+GWjvwxYBfpwq\r\n" +
        "a=ice-options:trickle\r\n" +
        "a=fingerprint:sha-256 19:79:F5:59:1F:33:34:88:BA:C9:57:7A:DB:0B:46:BE:1E:83:97:17:A8:19:9C:25:DD:48:CA:28:FE:4D:32:12\r\n" +
        "a=setup:actpass\r\n" +
        "a=mid:0\r\n" +
        "a=extmap:13 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time\r\n" +
        "a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid\r\n" +
        "a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id\r\n" +
        "a=extmap:5 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id\r\n" +
        "a=recvonly\r\n" +
        "a=rtcp-mux\r\n" +
        "a=rtcp-rsize\r\n" +
        "a=rtpmap:96 VP8/90000\r\n" +
        "a=rtcp-fb:96 goog-remb\r\n" +
        "a=rtcp-fb:96 transport-cc\r\n" +
        "a=rtcp-fb:96 ccm fir\r\n" +
        "a=rtcp-fb:96 nack\r\n" +
        "a=rtcp-fb:96 nack pli\r\n" +
        "a=rtpmap:97 rtx/90000\r\n" +
        "a=fmtp:97 apt=96\r\n" +
        "a=rid:h recv\r\n" +
        "a=rid:m recv\r\n" +
        "a=rid:l recv\r\n" +
        "a=simulcast:recv h;m;l\r\n" +
        "";

  console.log("offerSdp", offerSdp);
  const offer = { type: 'offer', sdp: offerSdp };

  // client side
  const client = new RTCPeerConnection();
  await client.setRemoteDescription(offer);

  const localStream = await navigator.mediaDevices.getUserMedia({video: true});
  const localVideoTrack = localStream.getVideoTracks()[0];

  const transceiver = client.getTransceivers()[0];
  transceiver.direction = "sendonly";
  transceiver.sender.replaceTrack(localVideoTrack);

  const answer = await client.createAnswer();
  await client.setLocalDescription(answer);

  console.log('-- answer.sdp --', answer.sdp);
  console.log('-- local.sdp --', client.localDescription.sdp);
})()