y.y()

y.y([y])

If y is specified, sets the y-coordinate accessor to the specified number or function, re-evaluates the y-accessor for each node, and returns this force. If y is not specified, returns the current y-accessor, which defaults to:

function y() {
  return 0;
}

The y-accessor is invoked for each node in the simulation, being passed the node and its zero-based index. The resulting number is then stored internally, such that the target y-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new y, and not on every application of the force.