roadmap.js

function buildGraph(edges) {
  let graph = Object.create(null);
  function addEdge(from, to) {
    if (graph[from] == null) {
      graph[from] = [to];
    } else {
      graph[from].push(to);
    }
    console.log(from, to);
    console.log(graph[from]); //vs console.log(graph);  why does printing the entire graph here print the completed graph on the second call, even though it shouldn't be fully constructed until the last call?
  }

  for (let [from, to] of edges.map(r => r.split("-"))) {
    addEdge(from, to);
    addEdge(to, from);
  }
  return graph;
}

const roads = [
  "Alice's House-Bob's House",   "Alice's House-Cabin",
  "Alice's House-Post Office",   "Bob's House-Town Hall",
  "Daria's House-Ernie's House", "Daria's House-Town Hall",
  "Ernie's House-Grete's House", "Grete's House-Farm",
  "Grete's House-Shop",          "Marketplace-Farm",
  "Marketplace-Post Office",     "Marketplace-Shop",
  "Marketplace-Town Hall",       "Shop-Town Hall"
];
const roadGraph = buildGraph(roads);