x.x()

x.x([x])

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

function x() {
  return 0;
}

The x-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 x-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new x, and not on every application of the force.