The KeyboardEvent.getModifierState() method returns the current state of the specified modifier key: true if the modifier is active (that is the modifier key is pressed or locked), otherwise, false.

API DOM getModifierState KeyboardEvent Method Reference

The MouseEvent.getModifierState() method returns the current state of the specified modifier key: true if the modifier is active (i.e., the modifier key is pressed or locked), otherwise, false.

API DOM DOM Events getModifierState Method MouseEvent Reference