keyValMap.js

// @flow strict

import type { ObjMap } from './ObjMap';

/**
 * Creates a keyed JS object from an array, given a function to produce the keys
 * and a function to produce the values from each item in the array.
 *
 *     const phoneBook = [
 *       { name: 'Jon', num: '555-1234' },
 *       { name: 'Jenny', num: '867-5309' }
 *     ]
 *
 *     // { Jon: '555-1234', Jenny: '867-5309' }
 *     const phonesByName = keyValMap(
 *       phoneBook,
 *       entry => entry.name,
 *       entry => entry.num
 *     )
 *
 */
export default function keyValMap<T, V>(
  list: $ReadOnlyArray<T>,
  keyFn: (item: T) => string,
  valFn: (item: T) => V,
): ObjMap<V> {
  return list.reduce((map, item) => {
    map[keyFn(item)] = valFn(item);
    return map;
  }, Object.create(null));
}