It's based on an optimized R-tree data structure with bulk insertion support.
Spatial index is a special data structure for points and rectangles that allows you to perform queries like
"all items within this bounding box" very efficiently (e.g. hundreds of times faster than looping over all items).
This version of RBush uses a fixed min/max accessor structure of
[ '.left', '.top', '.right', '.bottom' ].
This is to avoid the eval like function creation that the original library used, which caused CSP policy violations.