y.strength()

y.strength([strength])

If strength is specified, sets the strength accessor to the specified number or function, re-evaluates the strength accessor for each node, and returns this force. The strength determines how much to increment the node’s y-velocity: (y - node.y) × strength. For example, a value of 0.1 indicates that the node should move a tenth of the way from its current y-position to the target y-position with each application. Higher values moves nodes more quickly to the target position, often at the expense of other forces or constraints. A value outside the range [0,1] is not recommended.

If strength is not specified, returns the current strength accessor, which defaults to:

function strength() {
  return 0.1;
}

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