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RTCPeerConnection.createAnswer()

This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

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 whreiady been 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.

Syntax

aPromise = RTCPeerConnection.createAnswer([options]);

RTCPeerConnection.createAnswer(successCallback, failureCallback[, options]);  

Parameters

options Optional
An object which contains options which customize the answer; this is based on the RTCAnswerOptions dictionary.

Deprecated parameters

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
An RTCSessionDescriptionCallback which will be passed a single RTCSessionDescription object describing the newly-created answer.
failureCallback
An RTCPeerConnectionErrorCallback which will be passed a single DOMError object explaining why the request to create an answer failed.
options Optional
An optional RTCOfferOptions object providing options requested for the answer.

Exceptions

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.

Return value

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.

Example

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.

Keep in mind that this is part of the signaling process, the transport layer for which is an implementation detail that's entirely up to you. In this case, a WebSocket connection is used to send a JSON message with a type field with the value "video-answer" to the other peer, carrying the answer to the device which sent the offer to connect. The contents of the object being passed to the sendToServer() function, along with everything else in the promise fulfillment handler, depend entirely on your design

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.

Specifications

Specification Status Comment
WebRTC 1.0: Real-time Communication Between Browsers
The definition of 'createAnswer()' in that specification.
Working Draft Initial definition.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Yes) [1][2] (Yes) 22 (22) ? ? ?
Promise-based version 52.0 ? 37 (37) ? ? ?
RTCSessionDescriptionInit returned ? ? 53 (53) ? ? ?
Feature Android Webview Chrome for Android Edge Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) [1][2] (Yes) [1][2] (Yes) 24.0 (24) ? ? ? ?
Promise-based version 52.0 52.0 ? 37.0 (37) ? ? ? ?
RTCSessionDescriptionInit returned ? ? ? 53.0 (53) ? ? ? ?

[1] The callback-based version of this method was deprecated in Chrome 49.

[2] Though this method is not prefixed, the interface it belongs to was until Chrome 56.

Document Tags and Contributors

 Contributors to this page: Sheppy, jpmedley, abbycar
 Last updated by: Sheppy,