groupby-polyfill/lib/polyfill.js

small zero dependent polyfill for `Object.groupBy` and `Map.groupBy`

/*! groupby-polyfill. MIT License. Jimmy Wärting <https://jimmy.warting.se/opensource> */

/**
 * Groups elements from an iterable into an object based on a callback function.
 *
 * @template T, K
 * @param {Iterable<T>} iterable - The iterable to group.
 * @param {function(T, number): K} callbackfn - The callback function to
 * determine the grouping key.
 * @returns {Object.<string, T[]>} An object where keys are the grouping keys
 * and values are arrays of grouped elements.
 */
Object.groupBy ??= function groupBy (iterable, callbackfn) {
  const obj = Object.create(null)
  let i = 0
  for (const value of iterable) {
    const key = callbackfn(value, i++)
    key in obj ? obj[key].push(value) : (obj[key] = [value])
  }
  return obj
}

/**
 * Groups elements from an iterable into a Map based on a callback function.
 *
 * @template K, T
 * @param {Iterable<T>} iterable - The iterable to group.
 * @param {function(T, number): K} callbackfn - The callback function to
 * determine the grouping key.
 * @returns {Map<K, T[]>} A Map where keys are the grouping keys and values are
 * arrays of grouped elements.
 */
Map.groupBy ??= function groupBy (iterable, callbackfn) {
  const map = new Map()
  let i = 0
  for (const value of iterable) {
    const key = callbackfn(value, i++), list = map.get(key)
    list ? list.push(value) : map.set(key, [value])
  }
  return map
}