Calculate the position of an element relative to another element or event. It has also functionality to handle collision within the viewport of a given container. It does not manipulate CSS or DOM, it only calculates values. It is a basic technology to support implementation of tooltips, popups, popovers, scrollspies, sticky navi and many more.

.closestDescendant( selector [, findAll ] )

Get the first element(s) that matches the selector by traversing down through descendants in the DOM tree level by level. It use a breadth first search (BFS), that mean it will stop going deeper in a subtree if the first matching descendant was found in the current subtree.