Drill 17: Real project — Trees, graphs and traversal

function breadthFirst(graph, start) {
  const visited = new Set([start])
  const queue = [start]
  const order = []

  while (queue.length) {
    const node = queue.shift()
    order.push(node)
    for (const neighbour of graph[node] ?? []) {
      if (!visited.has(neighbour)) {
        visited.add(neighbour)
        queue.push(neighbour)
      }
    }
  }
  return order
}