The MediaTrackSettings dictionary's frameRate property is a double-precision floating-point number indicating the frame rate, in frames per second, of the MediaStreamTrack as currently configured. This lets you determine what value was selected to comply with your specified constraints for this property's value as described in the MediaTrackConstraints.frameRate property you provided when calling either getUserMedia() or MediaStreamTrack.applyConstraints().
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.