offerListener.ts

public offerListener() {
        this.socket.offerListen()
            .pipe(takeWhile(() => this.alive))
            .subscribe(async (data) => {
                // receive user id and peer offer
                let id: string = data.id;
                let offer: RTCSessionDescription = data.offer;

                if (!this.peerConnections[id] || this.peerConnections[id] == null) {
                    this.peerConnections[id] = new RTCPeerConnection(this.connectionConfig);
                }

                this.peerConnections[id]
                    .setRemoteDescription(offer)
                    .then(() => {
                        // get current user media stream
                        return this.getUserMediaCommonFunctionCall(id, 2);
                    })
                    .then((stream: MediaStream) => {
                        this.userVideo.srcObject = stream;
                    })
                    .then(() => {
                        // create answer
                        if (!this.peerConnections[id].localDescription || this.peerConnections[id].localDescription == null) {
                            return this.peerConnections[id].createAnswer();
                        }
                        throw "answer created already";
                    })
                    .then((answer: RTCSessionDescriptionInit) => {
                        return this.peerConnections[id].setLocalDescription(answer);
                    })
                    .then(() => {
                        //send back answer to the requester peer
                        this.socket.sendAnswer(id, this.peerConnections[id].localDescription);
                    })
                    .catch((err) => {
                        console.error('err=>', err);
                    });


                this.peerConnections[id].ontrack = (event: RTCTrackEvent) => {
                    if (otherVidElement) {
                        // add stream from requester 
                        if (!otherVidElement.srcObject) {
                            otherVidElement.srcObject = event.streams[0];
                        }

                        // event listener to check if stream has ended
                        event.track.onended = (event: Event) => {
                            console.log('track ended!');
                            otherVidElement.srcObject = otherVidElement.srcObject;
                        }
                    }
                };
            }, (err) => {
                console.log('err=>', err);
            })

    }