The SVGAnimatedRect interface represents an SVGRect attribute that can be animated.
SVGAnimatedRect
MDN Reference
The animVal read-only property of the SVGAnimatedRect interface represents the current animated value of the viewBox attribute of an SVG element as a read-only DOMRectReadOnly object.
animVal
viewBox
The baseVal read-only property of the SVGAnimatedRect interface represents the current non-animated value of the viewBox attribute of an SVG element.
baseVal