Group array of objects by property (Typed)

group-by.ts

/**
 * Groups array by given key
 * @param items
 * @param fn
 * @returns
 */
function groupBy<T, K extends string | number>(
  items: T[],
  fn: (item: T) => K,
): { [key: string]: T[] } {
  return items.reduce(
    (result, item) => ({
      ...result,
      [fn(item)]: [...(result[fn(item).toString()] || []), item],
    }),
    {},
  );
}

// Example usage
// ==========================
const plants = [
  {
    name: 'Velour',
    type: 'Cactus'
  },
  {
    name: 'Sunburst',
    type: 'Cactus'
  },
  {
    name: 'Chamomile',
    type: 'Herb'
  }
];

const grouped = groupBy(plants, (plant) => plant.type);
console.log(grouped);
/*
{
  Cactus: [
    {
      name: 'Velour',
      type: 'Cactus'
    },
    {
      name: 'Sunburst',
      type: 'Cactus'
    },
  ],
  Herb: [
    {
      name: 'Chamomile',
      type: 'Herb'
    },
  ]
}
*/