The MediaTrackConstraints dictionary's facingMode property is a ConstrainDOMString describing the requested or mandatory constraints placed upon the value of the facingMode constrainable property.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.facingMode as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.
Because RTP doesn't include this information, tracks associated with a WebRTC RTCPeerConnection will never include this property.
An object based on ConstrainDOMString specifying one or more acceptable, ideal, and/or exact (mandatory) facing modes are acceptable for a video track.
An exact value in this case indicates that the specified facing mode is specifically required; for example:
const constraints = {
facingMode: { exact: "user" },
};
This indicates that only a user-facing camera is acceptable; if there is no user-facing camera, or the user declines permission to use that camera, the media request will fail.
The following strings are permitted values for the facing mode. These may represent separate cameras, or they may represent directions in which an adjustable camera can be pointed.
"user"-
The video source is facing toward the user; this includes, for example, the front-facing camera on a smartphone.
"environment"-
The video source is facing away from the user, thereby viewing their environment. This is the back camera on a smartphone.
"left"-
The video source is facing toward the user but to their left, such as a camera aimed toward the user but over their left shoulder.
"right"-
The video source is facing toward the user but to their right, such as a camera aimed toward the user but over their right shoulder.