gistfile1.js

const roads = [
  'a-b', // ['a', 'b']
  'a-c', // ['a', 'c']
  'a-d', // ['a', 'd']
  'b-a', // ['b', 'a']
  'b-d',
  'b-e',
  'c-a',
  'c-d',
  'd-a',
  'd-e',
  'e-b',
  'e-d'
]
const example = {
  a: ['b', 'c', 'd'],
  b: ['a', 'd', 'e'],
  c: ['a', 'd'],
  d: ['a', 'e'],
  e: ['b', 'd']
}

function createGraph (roads) {
  let graph = {}
  for (let x of roads) {
    let array = x.split('-')
    
    // v1
    // let start = array[0]
    // let end = array[1]
    
    // v2
    // let [start, end] = array
    if (start in graph) {
      graph[start].push(end)
    } else {
      graph[start] = [end]
    }
  }
  return graph
}
const graph = createGraph(roads)
console.log('graph', graph)