The MediaTrackConstraints dictionary's frameRate property is a ConstrainDouble describing the requested or mandatory constraints placed upon the value of the frameRate constrainable property.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.frameRate 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 frame rate, in frames per second.
If this value is a number, the user agent will attempt to obtain media whose frame rate 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 frame rate (if exact is specified or both min and max are provided and have the same value) or to a best-possible value.