Using flatMap to traverse graphs.

flatMapGraph.js

const Graph = {
  A: ['B', 'C'],
  B: ['D'],
  C: ['A', 'B'],
  D: ['E']
}

const nextSteps = node => Graph[node]

// Forgive me, padre
Array.prototype.flatMap = function (f) {
  return this.map(f).reduce(
    (xs, ys) => xs.concat(ys)
  )
}

nextSteps('A')        // [B, C]
  .flatMap(nextSteps) // [D, A, B]
  .flatMap(nextSteps) // [E, B, C, D]