Answers for "object.group by"

javascript group by property array of objects

function groupArrayOfObjects(list, key) {
  return list.reduce(function(rv, x) {
    (rv[x[key]] = rv[x[key]] || []).push(x);
    return rv;
  }, {});
};

var people = [
    {sex:"Male", name:"Jeff"},
    {sex:"Female", name:"Megan"},
    {sex:"Male", name:"Taylor"},
    {sex:"Female", name:"Madison"}
];
var groupedPeople=groupArrayOfObjects(people,"sex");
console.log(groupedPeople.Male);//will be the Males 
console.log(groupedPeople.Female);//will be the Females

groupby javascript by instances

let people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    let key = obj[property]
    if (!acc[key]) {
      acc[key] = []
    }
    acc[key].push(obj)
    return acc
  }, {})
}

let groupedPeople = groupBy(people, 'age')
// groupedPeople is:
// {
//   20: [
//     { name: 'Max', age: 20 },
//     { name: 'Jane', age: 20 }
//   ],
//   21: [{ name: 'Alice', age: 21 }]
// }