simulation.force()

simulation.force(name[, force])

If force is specified, assigns the force for the specified name and returns this simulation. If force is not specified, returns the force with the specified name, or undefined if there is no such force. (By default, new simulations have no forces.) For example, to create a new simulation to layout a graph, you might say:

var simulation = d3.forceSimulation(nodes)
    .force("charge", d3.forceManyBody())
    .force("link", d3.forceLink(links))
    .force("center", d3.forceCenter());