Fix the traversal without a visited set — find the bug

const graph = new Map([["a", ["b", "c"]], ["b", ["a"]], ["c", []]]);
const order = [];
const queue = ["a"];
let head = 0;
while (head < queue.length && order.length < 4) {
  const node = queue[head++];
  order.push(node);
  for (const next of graph.get(node) || []) queue.push(next);
}
document.querySelector('#out').textContent = "visited " + order.length;