TypeScript groupBy function

group_by_function.ts

/**
 * Groups an array of objects by object property
 */
const groupBy = <T>(array: T[], key: keyof T): { [key: string]: T[] } => {
  return array.reduce((accumulator: { [key: string]: T[] }, currentElement: T) => {
    const keyValue = currentElement[key] as unknown as string;

    if (keyValue in accumulator) {
      accumulator[keyValue].push(currentElement);
    } else {
      accumulator[keyValue] = [currentElement];
    }

    return accumulator;
  }, {} as { [key: string]: T[] });
};