Group Array By Id in TypeScript

Array.extension.ts

/* tslint:disable */
export {};

type TGroup<T> = {Id: any; Values: T[]};
declare global {
    interface Array<T> {
        groupBy(aKey: string): TGroup<T>[];
    }
}

Array.prototype.groupBy = function<T>(aKey: string): TGroup<T>[]
{
    const lReducer: any = (lPrevious: TGroup<T>[], lCurrentValue: T) =>
    {
        const lExisting: TGroup<T> | undefined
            = lPrevious.find((aValue: TGroup<T>) => aValue.Id === lCurrentValue[aKey]);
        if (lExisting)
        {
            lExisting.Values.push(lCurrentValue);
        }
        else
        {
            lPrevious.push(
            {
                Id: lCurrentValue[aKey],
                Values: [lCurrentValue],
            });
        }
        return lPrevious;
    };

    return this.reduce(lReducer, []);
};

require("./Array.extension.ts") --> Use correct path to load on your start up file

type T = {
    id: number;
    qty: number;
};
 const arr: T[] = [];
arr.push({id: 1, qty: 10});
arr.push({id: 1, qty: 100});
arr.push({id: 2, qty: 200});
arr.push({id: 2, qty: 2});
arr.groupBy("id"));

Result:

[
    {
        Id: 1,
        Values: [
            {
                id: 1,
                qty: 10
            },
            {
                id: 1,
                qty: 100
            }
        ]
    },
    {
        Id: 2,
        Values: [
            {
                id: 2,
                qty: 200
            },
            {
                id: 2,
                qty: 2
            }
        ]
    }
]