d3-array's group and groups functions

group.js

class InternMap extends Map {
  constructor(entries, key = keyof) {
    super();
    Object.defineProperties(this, {_intern: {value: new Map()}, _key: {value: key}});
    if (entries != null) for (const [key, value] of entries) this.set(key, value);
  }
  get(key) {
    return super.get(intern_get(this, key));
  }
  has(key) {
    return super.has(intern_get(this, key));
  }
  set(key, value) {
    return super.set(intern_set(this, key), value);
  }
  delete(key) {
    return super.delete(intern_delete(this, key));
  }
}

function intern_get({_intern, _key}, value) {
  const key = _key(value);
  return _intern.has(key) ? _intern.get(key) : value;
}

function intern_set({_intern, _key}, value) {
  const key = _key(value);
  if (_intern.has(key)) return _intern.get(key);
  _intern.set(key, value);
  return value;
}

function intern_delete({_intern, _key}, value) {
  const key = _key(value);
  if (_intern.has(key)) {
    value = _intern.get(key);
    _intern.delete(key);
  }
  return value;
}

function keyof(value) {
  return value !== null && typeof value === "object" ? value.valueOf() : value;
}

function nest(values, map, reduce, keys) {
  return (function regroup(values, i) {
    if (i >= keys.length) return reduce(values);
    const groups = new InternMap();
    const keyof = keys[i++];
    let index = -1;
    for (const value of values) {
      const key = keyof(value, ++index, values);
      const group = groups.get(key);
      if (group) group.push(value);
      else groups.set(key, [value]);
    }
    for (const [key, values] of groups) {
      groups.set(key, regroup(values, i));
    }
    return map(groups);
  })(values, 0);
}

function identity(d) {
	return d;
}

function group(values, ...keys) {
  return nest(values, identity, identity, keys);
}

function groups(values, ...keys) {
  return nest(values, Array.from, identity, keys);
}