let arr = [{
	"birthdate": "1993",
	"name": "Ben"
},
{
	"birthdate": "1994",
	"name": "John"
},
{
	"birthdate": "1995",
	"name": "Larry"
},
{
	"birthdate": "1995",
	"name": "Nicole"
},
{
	"birthdate": "1996",
	"name": "Jane"
},
{
	"birthdate": "1996",
	"name": "Janet"
},
{
	"birthdate": "1996",
	"name": "Dora"
},
];

const res = arr.reduce((ac, a) => {
let temp = ac.find(x => x.birthdate === a.birthdate);
if (!temp) ac.push({ ...a,
	name: [a.name]
})
else temp.name.push(a.name)
return ac;
}, [])
console.log(res);

export const groupArrayBy = (arr, groupBy) => {

    let newArr = []
    arr.map((item) => {
        if(item[groupBy]) {
            let finded = newArr.filter((newItem) => newItem[groupBy] === item[groupBy])
            if(finded.length > 0) {
                finded[0].products.push(item)
            } else {
                newArr.push({category: item[groupBy], products: [item]})
            }
        }
    })

    return newArr
}