let nodes = [1, 2, 3, 4, 5]
let edges = ['1-3', '1-4', '2-1', '2-3', '2-4', '3-2', '3-5', '4-2', '4-5']

let nodeWeights = {1: 20, 2: 30, 3: 25, 4: 50, 5: 45}
let edgeWeights = {
  '1-3': 3,
  '1-4': 5,
  '2-1': 2,
  '2-3': 9,
  '2-4': 4,
  '3-2': 1,
  '3-5': 2,
  '4-2': 8,
  '4-5': 6
}

let clusters = {
  1: [1],
  2: [2],
  5: [5],
  6: [3, 4]
}
let mergeHash = { 3: 6, 4: 6 }

let newNodes = []
let newNodeWeights = {}
for (let key in clusters) {
  newNodes.push(key)
  newNodeWeights[key] = 0
  for (let nodeID of clusters[key]) {
    newNodeWeights[key] += nodeWeights[nodeID]
  }
}

let newEdges = []
let newEdgeWeights = {}
for (let edgeKey of edges) {
  let [i, j] = edgeKey.split('-').map(x => parseInt(x))
  if (mergeHash[i]) {
    i = mergeHash[i]
  }
  if (mergeHash[j]) {
    j = mergeHash[j]
  }
  let newKey = i + '-' + j
  if (!newEdges.includes(newKey)) { newEdges.push(newKey) }
  if (newEdgeWeights[newKey]) {
    newEdgeWeights[newKey] += edgeWeights[edgeKey]
  } else {
    newEdgeWeights[newKey] = edgeWeights[edgeKey]
  }
}

{
  "1-6": 8,
  "2-1": 2,
  "2-6": 13,
  "6-2": 9,
  "6-5": 8
}