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A PeerJS call can receive without sending, given an empty MediaStream and offerToReceive constraints

added 2026-10-17·view source

peerjswebrtcmedia

Fact

peer.call(id, stream) (PeerJS 1.5) refuses a missing stream, but an empty new MediaStream() is accepted, and the options' constraints are passed to createOffer. With { constraints: { offerToReceiveAudio: true, offerToReceiveVideo: true } } the offer has receive-only audio and video sections, so the answering side's call.answer(itsStream) sends its tracks and the caller's stream event delivers them.

Why it matters

A viewer of a one-way stream (a broadcast, a camera feed) otherwise needs getUserMedia, with a permission prompt and a camera light, or a fake canvas and oscillator stream that wastes upload, just to place the call. Without the constraints the offer has no media sections and the call connects with nothing on it.

How to apply

const call = peer.call(stationId, new MediaStream(), {
  constraints: { offerToReceiveAudio: true, offerToReceiveVideo: true },
});
call.on('stream', (s) => { if (video.srcObject !== s) video.srcObject = s; });

The station answers each call with the same captured stream (video.captureStream(), a camera, a canvas).

Sources