JSFiddle - React, Tailwind, and code Playground

JavaScript

var table = [
    {group:"a", stuff:"new"},
    {group:"a", stuff:"old"},
    {group:"a", stuff:"new"},
    {group:"b", stuff:"newOld"},
    {group:"b", stuff:"old"},
    {group:"b", stuff:"old"},
    {group:"c", stuff:"new"},
    {group:"c", stuff:"old"},
    {group:"c", stuff:"newOld"},
];

    var groups = {all:{}};

table.forEach(function(a){
    if (!groups[a.group]){ groups[a.group] = {}; }
    var stuff = groups["all"][a.stuff];
    groups["all"][a.stuff] = !stuff ? 1 : ++stuff;
    stuff = groups[a.group][a.stuff];
    groups[a.group][a.stuff] = !stuff ? 1 : ++stuff;
});

console.log("Direct Call: "+Object.keys(groups.a));

var selected = "a";
console.log("Variable Pass "+Object.keys(groups.selected));