TypeScript groupBy

utils.ts

/**
 * StackOverflow Question = https://stackoverflow.com/questions/14446511
 * Based on StackOverflow Anser = https://stackoverflow.com/a/66987261/1832572
 */

/**
 * Groups items on given criteria. The criteria can be a property or a function
 * which is applied on each item.
 * 
 * @param list The elements to be grouped ona criteria
 * @param groupOn The criteria to group the elements together. Can bee a property
 * or a function applied on each items.
 * @return a map of items group on the given criteria.
 */
export function groupBy<T, K extends keyof T>(list: T[], groupOn: K | ((i: T) => any)): Map<any, T[]> {
    const groupFn = typeof groupOn === "function" ? groupOn : (o: T) => o[groupOn];
    return list.reduce((map, elm) => {
        const key = groupFn(elm);
        let group = map.get(key);
        if (!group) {
            group = [];
            map.set(key, group);            
        }
        group.push(elm);
        return map;
    }, new Map<any, T[]>());
}