groupBy in Typescript (for use in Angular)

groupBy.ts

declare global {
    interface Array<T> {
        groupBy(prop: T): Array<T>;
    }
}

if (!Array.prototype.groupBy) {
    Array.prototype.groupBy = (prop: string) => {
        return this.reduce(function(groups, item) {
            const val = item[prop];
            groups[val] = groups[val] || [];
            groups[val].push(item);
            return groups;
        }, {});
    }
}