countBy

Counts array values by a derived property key.

Code

countBy.ts

export type CountByKeySelector<Value, Key extends PropertyKey> = (
  this: void,
  value: Value,
  index: number
) => Key;

/**
 * Counts are optional because a selector's return type describes every key it
 * may produce, not a guarantee that every possible key occurs in the input.
 */
export type CountByResult<Key extends PropertyKey> = Partial<Record<Key, number>>;

/** Counts values by a derived property key. */
export const countBy = <Value, Key extends PropertyKey>(
  array: readonly Value[],
  getKey: CountByKeySelector<Value, Key>
): CountByResult<Key> => {
  const result = Object.create(null) as CountByResult<Key>;
  for (let index = 0; index < array.length; index += 1) {
    if (!Object.hasOwn(array, index)) continue;
    const value = array[index] as Value;
    const key = getKey(value, index);
    const count = Reflect.get(result, key) as number | undefined;
    Reflect.set(result, key, (count ?? 0) + 1);
  }
  return result;
};

Download

wget

Terminal

wget -O src/lib/countBy.ts https://raw.githubusercontent.com/jrTilak/lazykit/HEAD/registry/functions/countBy.ts

degit

Terminal

temp_dir="$(mktemp -d)" && bunx degit jrTilak/lazykit/registry/functions "$temp_dir" && mv "$temp_dir/countBy.ts" src/lib/countBy.ts && rm -r "$temp_dir"

Examples

countBy.example.ts

import { countBy } from "./countBy";

const totals = countBy(
  ["api", "web", "api"] as const,
  (value) => value
);
// { api: 2, web: 1 }

const apiCount = totals.api ?? 0;
// 2

Props

• array (readonly T[]) — Values to count. Empty slots are skipped.
• getKey (CountByKeySelector<T, Key>) — Selects a bucket and is invoked without a receiver.
• Returns Partial<Record<Key, number>> with a null prototype. A count is undefined when that key did not occur.