JSFiddle - React, Tailwind, and code Playground

JavaScript

function groupBy(list, keyGetter) {
    const map = new Map();
    list.forEach((item) => {
        const key = keyGetter(item);
        if (!map.has(key)) {
            map.set(key, [item]);
        } else {
            map.get(key).push(item);
        }
    });
    return map;
}

const pets = [
  	{type:"Dog", name:"Spot"},
    {type:"Cat", name:"Tiger"},
    {type:"Dog", name:"Rover"}, 
    {type:"Cat", name:"Leo"}
];

const grouped = groupBy(pets, pet => pet.type);

console.log(grouped.get("Dog"));
console.log(grouped.get("Cat"));