RTCRtpStreamStats.trackId - Web APIs
the rtcrtpstreamstats dictionary's trackid property is a string which uniquely identifies the rtcmediastreamtrackstats object which contains the track statistics for the mediastreamtrack for which statistics are provided in this object.
... syntax var trackid = rtcrtpstreamstats.trackid; value a domstring which uniquely identifies the rtcmediastreamtrackstats object that provides statistics for the track for which statistics are being collected by this rtcstatsreport.

Index - Web APIs
3510 rtcrtpstreamstats.trackid api, property, rtcrtpstreamstats, rtp, reference, statistics, stats, webrtc, webrtc api, id, rtc, track, trackid the rtcrtpstreamstats dictionary's trackid property is a string which uniquely identifies the rtcmediastreamtrackstats object which contains the track statistics for the mediastreamtrack for which statistics are provided in this object.

RTCRtpStreamStats - Web APIs
trackid a domstring which uniquely identifies the rtcmediastreamtrackstats object representing the associated mediastreamtrack.