▶1function mergeKStreams(streams, key) {
2  // Only the k heads live in the heap, never the whole feed.
3  const heads = new Heap((a, b) => a.key - b.key);
4  for (let i = 0; i < streams.length; i++) {
5    const first = streams[i][0];
6    if (first !== undefined) {
7      heads.push({ key: key(first), stream: i, pos: 0 });
8    }
9  }
10 
11  const merged = [];
12  for (;;) {
13    const head = heads.pop();          // smallest of the k heads
14    if (head === undefined) break;     // every stream exhausted
15 
16    merged.push(streams[head.stream][head.pos]);
17 
18    const next = streams[head.stream][head.pos + 1];
19    if (next !== undefined) {
20      heads.push({ key: key(next), stream: head.stream, pos: head.pos + 1 });
21    }
22  }
23  return merged;
24}