オブジェクトの和

sumObjects.ts

/**
 * sumObjects([
 *   {a: 1, b: 2},
 *   {b: 3, c: 4},
 * ])
 * => {a: 1, b: 5, c: 4}
 */
const sumObjects = (
  objects: Record<string, number>[]
): Record<string, number> =>
  objects.reduce((acc, cur) => {
    Object.entries(cur).forEach(([k, v]) => {
      acc[k] = (acc[k] ?? 0) + v
    })
    return acc
  }, {})