Property getBBox does not exist on type SVGElement
Not all SVG elements have bounding boxes, <defs> for instance doesn't, neither does <title>, so yes SVGElement is the wrong type to use. You want SVGGraphicsElement
Instead of SVGElement, SVGSVGElement is the correct type to use for accessing <svg> elements, where getBBox() method is available as well because of the inheritance from SVGGraphicsElement.
(d3Selection.node() as SVGSVGElement).getBBox()Or if d3Selection is defined as
let d3Selection: D3.Selection<SVGSVGElement, {}, HTMLElement, any>d3Selection.node().getBBox()could be directly into usage.