nest.key()

nest.key(key)

Registers a new key function. The key function will be invoked for each element in the input array and must return a string identifier to assign the element to its group. Most often, the function is a simple accessor, such as the year and variety accessors above. (Keys functions are not passed the input array index.) Each time a key is registered, it is pushed onto the end of the internal array of keys, and the nest operator applies an additional level of nesting.