Group-By typescript

groupBy is a light weight npm typescript package for grouping items in javascript arrays the function expects a javascript array, field name to be grouped by, a preaction (function of your choice that will return a new K Object) and a postAction optional (function of your choice that will iterate the new K[] list and do additional actions and return the new processed data as same type K[])

example

import {test,expect} from "vitest";
import { groupBy } from "./index";

type Pupil = {
    name: string;
    grade: number;
    major: string;
}


type GroupedPupil = {
    name: string;
    grade: number;
}


const arr: Pupil[] = [{ name: "Alice", grade: 70, major: "physics" }, { name: "Alice", grade: 80, major: "chemistry" }, { name: "Alice", grade: 60, major: "Mathematics" }
    , { name: "Bob", grade: 85, major: "physics" }, { name: "Bob", grade: 80, major: "chemistry" }
    , { name: "Charlie", grade: 100, major: "Literature" }, { name: "Charlie", grade: 90, major: "History" },
{ name: "David", grade: 95, major: "Computer Science" }, { name: "David", grade: 85, major: "Mathematics" }];


function sum(newValue: Pupil, previousValue: GroupedPupil): GroupedPupil {
    const previousGrade = previousValue.grade ? previousValue.grade : 0;
    return {
        name: newValue.name,
        grade: previousGrade + Number(newValue.grade)
    }
}

function avarage(newItems: GroupedPupil[]): GroupedPupil[] {

    return newItems.map(item => {
        const count = arr.filter(pupil => pupil.name === item.name).length;
        return {
            name: item.name,
            grade: item.grade / count // Example average calculation (replace with actual logic)
        };
    });
}

test("groupBy function test", () => {
    expect(groupBy(arr, "name", sum, avarage)).toEqual([
        { name: "Alice", grade: 70 },
        { name: "Bob", grade: 82.5 },
        { name: "Charlie", grade: 95 },
        { name: "David", grade: 90 }
    ]);

});