data-structures-and-algorithms/heapq

import {siftdown} from './core/index.js';

export default function heappushpop(heap, item) {
    const x = heap.data;
    const n = x.length;

    if (n === 0) return item;

    const compare = heap.compare;

    if (compare(item, x[0]) <= 0) return item;

    const smallest = x[0];

    x[0] = item;

    // Sift down the new root

    siftdown(compare, x, 0, n, 0);

    return smallest;
}