Function reduce

src/CollectionHelperMethods.ts:4–27

(
  collection: C,
  reducer: (reduction: V | R, value: V, key: K, iter: C) => R,
  reduction: V | R | undefined,
  context: unknown,
  useFirst: boolean,
  reverse: boolean
)

import assertNotInfinite from './utils/assertNotInfinite';

export function reduce<K, V, R, C extends Collection<K, V>>(
  collection: C,
  reducer: (reduction: V | R, value: V, key: K, iter: C) => R,
  reduction: V | R | undefined,
  context: unknown,
  useFirst: boolean,
  reverse: boolean
): V | R | undefined {
  // @ts-expect-error Migrate to CollectionImpl in v6
  assertNotInfinite(collection.size);
  // @ts-expect-error Migrate to CollectionImpl in v6
  collection.__iterate((v: V, k: K, c: C) => {
    if (useFirst) {
      useFirst = false;
      reduction = v;
    } else {
      // `reduction` has already been seeded here (either with the provided
      // initial value or with the first iterated value), so it is never the
      // `undefined` placeholder — only a `V` or a `R`.
      reduction = reducer.call(context, reduction!, v, k, c);
    }
  }, reverse);
  return reduction;
}

export function keyMapper<K, V>(v: V, k: K): K {
  return k;