Multiple group by

multiGroupBy.mjs

function groupBy(iterable, prop) {
  const dict = { __proto__: null };
  for (const item of iterable) {
    const group = item[prop];
    (dict[group] || (dict[group] = [])).push(item);
  }
  return dict;
}

export function multiGroupBy(iterable, props) {
  const prop = props.shift();
  const dict = groupBy(iterable, prop);
  if (props.length !== 0) {
    for (const key of Object.keys(dict)) {
      dict[key] = multiGroupBy(dict[key], props);
    }
  }
  props.unshift(prop);
  return dict;
}