A small heap implementation

heap.js

function Heap(data = [], compare = (a, b) => a < b) {
  const heap = [null];
  const size = () => heap.length - 1;

  const leftIdx = (i) => i * 2
  const rightIdx = (i) => i * 2 + 1
  const parentIdx = (i) => Math.floor(i / 2);

  const left = (i) => heap[leftIdx(i)];
  const right = (i) => heap[rightIdx(i)];
  const parent = (i) => heap[parentIdx(i)];

  const childIdx = (i) => {
    if (!right(i)) return leftIdx(i);
    return compare(left(i), right(i)) ? leftIdx(i) : rightIdx(i)
  }
  const child = (i) => heap[childIdx(i)]

  const swapWithParent = (i) => {
    const idx = parentIdx(i);
    [heap[i], heap[idx]] = [heap[idx], heap[i]]
    return idx;
  }

  const swapWithChild = (i) => {
    const idx = childIdx(i);
    swapWithParent(idx);
    return idx;
  }

  const push = (val) => {
    heap.push(val);

    let idx = size();
    while (idx >= 2 && compare(heap[idx], parent(idx)))
      idx = swapWithParent(idx);
  }

  const pop = () => {
    const val = heap[1];
    heap[1] = heap[size()]
    heap.length--

    let idx = 1;
    while (left(idx) && compare(child(idx), heap[idx]))
      idx = swapWithChild(idx);

    return val
  }

  const peek = () => heap[1]

  for (let i = 0; i < data.length; i++)
    push(data[i])

  return { push, pop, peek, size }
}