The pointerLockElement property provides the element set as the target for mouse events while the pointer is locked. It is null if lock is pending, pointer is unlocked, or the target is in another document.
The Pointer Lock API (formerly called Mouse Lock API) provides input methods based on the movement of the mouse over time (i.e., deltas), not just the absolute position of the mouse cursor in the viewport. It gives you access to raw mouse movement, locks the target of mouse events to a single element, eliminates limits on how far mouse movement can go in a single direction, and removes the cursor from view. It is ideal for first person 3D games, for example.
The MouseEvent.movementX read-only property provides the shift in the X coordinate of the mouse pointer between that event and the previous mousemove event. In other words, the value of that property is computed that way : currentEvent.movementX = currentEvent.screenX - previousEvent.screenX.
The MouseEvent.movementY read-only property provides the shift in the Y coordinate of the mouse pointer between that event and the previous mousemove event. In other words, the value of that property is computed that way : currentEvent.movementY = currentEvent.screenY - previousEvent.screenY.