The DOMRectList interface represents a collection of DOMRect objects, typically used to hold the rectangles associated with a particular element, like bounding boxes returned by methods such as Element.getClientRects.
DOMRectList
MDN Reference
The read-only length property of the DOMRectList interface returns the number of DOMRect objects in the list.
length
The DOMRectList method item() returns the DOMRect at the specified index within the list, or null if the index is out of range.
item()
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