proposal-upsert: `emplace` cannot be correctly and incrementally polyfilled

The problem is that since emplace is defined not to call get and set, it can only be correctly polyfilled when completely reimplementing Map from scratch, as core-js does. In other words, it is impossible to build the functionality correctly by extending the Map prototype because you would not have the requisite privileged access to the underlying data structure.

My impression is that that would make this one of those features that we can all use in ~5 years when everyone is finally using engines that natively support it.

About this issue

Original URL
State: closed
Created 2 years ago
Reactions: 1
Comments: 24 (12 by maintainers)

Most upvoted comments

const has = Function.call.bind(Map.prototype.has);
const get = Function.call.bind(Map.prototype.get);
const set = Function.call.bind(Map.prototype.set);

function emplace(key, handler) {
  const M = this;
  if (has(M, key)) { // incl. RequireInternalSlot(M, [[MapData]])
    let value = get(M, key);
    if ('update' in handler) {
      value = handler.update(value, key, M);
      set(map, key, value);
    } return value;
  }
  const inserted = handler.insert(key, M);
  set(M, key, inserted);
  return inserted;
}

Object.defineProperty(Map.prototype, 'emplace', { writable: true, configurable: true, value: emplace });

zloirock on Nov 26, 2022

If you want to make a rule it seems reasonable to start a thread in a place where the people who will be affected (and who can effect rule changes) would see it.

This is a thread based on a technical misunderstanding, and I am closing it.

conartist6 on Nov 26, 2022

@ljharb Thanks for the succinct clarification. I’ve never needed to use that pattern so I didn’t recognize it.

conartist6 on Nov 26, 2022

I’m going to take 24 hours to cool off from this. Thanks.

conartist6 on Nov 26, 2022