stratify.id()

stratify.id([id])

If id is specified, sets the id accessor to the given function and returns this stratify operator. Otherwise, returns the current id accessor, which defaults to:

function id(d) {
  return d.id;
}

The id accessor is invoked for each element in the input data passed to the stratify operator, being passed the current datum (d) and the current index (i). The returned string is then used to identify the node’s relationships in conjunction with the parent id. For leaf nodes, the id may be undefined; otherwise, the id must be unique. (Null and the empty string are equivalent to undefined.)