The Path2D.addPath() method of the Canvas 2D API adds to the path the path given by the argument.
The Path2D interface of the Canvas 2D API is used to declare paths that are then later used on CanvasRenderingContext2D objects. The path methods of the CanvasRenderingContext2D interface are present on this interface as well and are allowing you to create paths that you can retain and replay as required on a canvas.
The Path2D() constructor returns a newly instantiated Path2D object, optionally with another path as an argument (creates a copy), or optionally with a string consisting of SVG path data.