The MediaTrackConstraints dictionary's aspectRatio property is a ConstrainDouble describing the requested or mandatory constraints placed upon the value of the aspectRatio constrainable property.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.aspectRatio as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.
A ConstrainDouble describing the acceptable or required value(s) for a video track's aspect ratio. The value is the width divided by the height and is rounded to ten decimal places. For example, the standard high-definition video aspect ratio of 16:9 can be computed as 1920/1080, or 1.7777777778.
If this value is a number, the user agent will attempt to obtain media whose aspect ratio is as close as possible to this number given the capabilities of the hardware and the other constraints specified. Otherwise, the value of this ConstrainDouble will guide the user agent in its efforts to provide an exact match to the required aspect ratio (if exact is specified or both min and max are provided and have the same value) or to a best-possible value.