The getCapabilities() method of the MediaStreamTrack interface returns a MediaTrackCapabilities object which specifies the values or range of values which each constrainable property, based upon the platform and user agent.
Once you know what the browser's capabilities are, your script can use applyConstraints() to ask for the track to be configured to match ideal or acceptable settings. See Capabilities, constraints, and settings for details on how to work with constrainable properties.