upsert-array-items.js

// Replace items in `as` with a matching `id` with the 
// respective item in `bs`, otherwise append new items.
const abs = Object.fromEntries([
  ...as.map(a => [a.id, a]),
  ...bs.map(b => [b.id, b])
]);

// The 'trick' is that if you have two entries with the same id
// (in this case, implying that the value was updated within `bs`)
// then with this method the value of the right-most entry will be used.

If I get time I will implement an immutable version of Counter.

function total(map) {
  return map.values().reduce((acc, count) => acc + count, 0);
}

function mostCommon(map, n) {
  // ...
  // Not trivial to do well, so I'll come back to this... :)
}