d3.nest() Creates a new nest operator. The set of keys is initially empty.

map.keys() Returns an array of string keys for every entry in this map. The order of the returned keys is arbitrary.

nest.rollup(function) Specifies a rollup function to be applied on each group of leaf elements. The return

nest.map(array) Applies the nest operator to the specified array, returning a nested

set.add(value) Adds the specified value string to this set. Returns the set, allowing chaining. For example:

set.values() Returns an array of the string values in this set. The order of the returned values is arbitrary. Can be used

nest.sortValues(comparator) Sorts leaf elements using the specified comparator function, such as

nest.entries(array) Applies the nest operator to the specified array, returning an array of key-values entries

set.has(value) Returns true if and only if this set has an entry for the specified value string.

map.size() Returns the number of entries in this map.