score:1

Accepted answer

you must not add a new link force but modify the existing link force

function reset() {
  simulation.force("link").strength(1);
  simulation
    // .force("link", d3.forceLink()
    //   //.id(function(d) {return d.id;})
    //   .strength(1))
    .force("charge", d3.forceManyBody())
    .force('x', d3.forceX(width / 2))
    .force('y', d3.forceY(height / 2))
    .force('r', null)

  simulation.alpha(1).restart()
}

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