RTCPeerConnection.createAnswer()

The createAnswer() method on the RTCPeerConnection interface creates an SDP answer to an offer received from a remote peer during the offer/answer negotiation of a WebRTC connection. The answer contains information about any media already attached to the session, codecs and options supported by the browser, and any ICE candidates already gathered. The answer is delivered to the returned Promise, and should then be sent to the source of the offer to continue the negotiation process.

options Optional

An optional object providing options requested for the answer. Currently, there are no available options.

In older code and documentation, you may see a callback-based version of this function. This has been deprecated and its use is strongly discouraged. You should update any existing code to use the Promise-based version of createAnswer() instead. The parameters for this form of createAnswer() are described below, to aid in updating existing code.

successCallback Deprecated

A callback function which will be passed a single RTCSessionDescription object describing the newly-created answer.

failureCallback Deprecated

A callback function which will be passed a single DOMException object explaining why the request to create an answer failed.

options Optional

An optional object providing options requested for the answer.

NotReadableError

The identity provider wasn't able to provide an identity assertion.

OperationError

Generation of the SDP failed for some reason; this is a general failure catch-all exception.

A Promise whose fulfillment handler is called with an object conforming to the RTCSessionDescriptionInit dictionary which contains the SDP answer to be delivered to the other peer.

Here is a segment of code taken from the code that goes with the article Signaling and video calling. This code comes from the handler for the message sent to carry an offer to another peer across the signaling channel.

pc.createAnswer().then(function(answer) {
  return pc.setLocalDescription(answer);
})
.then(function() {
  // Send the answer to the remote peer through the signaling server.
})
.catch(handleGetUserMediaError);

This asks RTCPeerConnection to create and return a new answer. In our promise handler, the returned answer is set as the description of the local end of the connection by calling setLocalDescription().

Once that succeeds, the answer is sent to the signaling server using whatever protocol you see fit.

Promise.catch() is used to trap and handle errors.

See Handling the invitation in Signaling and video calling to see the complete code, in context, from which this snippet is derived; that will help you understand the signaling process and how answers work.