groupby.js

export const groupByUsing = (iterable, key, callback) => {
  const mapper = typeof key !== "function" ? x => x[key] : key;
  const map = new Map();

  let i = -1;
  for (const value of iterable) {
    const mappedKey = mapper(value, ++i, key);
    const mappedValue = callback(value, i, key);

    const group = map.get(mappedKey);
    if (group == null) {
      map.set(mappedKey, [mappedValue]);
    } else {
      group.push(mappedValue);
    }
  }

  return map;
};

export const groupBy = (iterable, key) => groupByUsing(iterable, key, x => x);