https://developer.mozilla.org/en-US/docs/Web/API/SVGLengthList/numberOfItems
SVGLengthList: numberOfItems property - Web APIs | MDN
The numberOfItems property of the SVGLengthList interface returns the number of items in the list. length is an alias of it.
web apispropertymdn
https://developer.mozilla.org/en-US/docs/Web/API/SVGLengthList
SVGLengthList - Web APIs | MDN
The SVGLengthList interface defines a list of SVGLength objects. It is used for the baseVal and animVal properties of SVGAnimatedLengthList.
web apismdn