A groupBy function in TypeScript.

groupBy.es5.js

function groupBy(fn) {
  return function(items) {
    return items.reduce(function(groups, item) {
      var key = fn(item);
      if (!(key in groups))
        // eslint-disable-next-line no-param-reassign
        groups[key] = [];
      groups[key].push(item);
      return groups;
    }, Object.create(null));
  };
}

export default groupBy;

groupBy.ts

const groupBy = <T, K extends string>(fn: (item: T) => K) => (items: T[]) =>
  items.reduce((groups, item) => {
    const key = fn(item);
    return {
      ...groups,
      [key]: [...(groups?.[key] ?? []), item]
    };
  }, {} as Record<K, T[]>);

export default groupBy;