XRFrame.getPose()
  The XRFrame method getPose() returns the relative position and
  orientation—the pose—of one XRSpace to that of another space. With this, you can observe the motion of objects relative to each other and to fixed locations throughout the scene.
For example, to get the position of a controller relative to the viewer's head, you would compare the controller's gripSpace to the XRReferenceSpace of type viewer.
Syntax
getPose(space, baseSpace)
Parameters
Return value
  An XRPose object specifying the position and orientation, relative to
  the XRSpace indicated by baseSpace.
Specifications
| Specification | 
|---|
| WebXR Device API # dom-xrframe-getpose | 
Browser compatibility
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