groupBy

groupBy.ts

let seniors = [
    {
      "id": 1,
      "name": "Jack",
      "active": true,
      "level": "senior"
    },
    {
      "id": 2,
      "name": "Suzan",
      "active": true,
      "level": "senior"
    },
    {
      "id": 3,
      "name": "Joana",
      "level": "senior",
      "active": true
    },
    {
      "id": 4,
      "name": "Mark",
      "level": "senior",
      "active": false
    }
  ];

interface User { 
    id: number
    name: string
    active: boolean
    level: Level
}

type Grouped<T> = {
    [groupname: string]: Array<T>
}

const groupBy_ = <T extends object>(list: Array<T>, key: keyof T) => list.reduce( (acc:Grouped<T>, item:T):Grouped<T> => (
{
  ...acc, 
[ item[key] ]: [
    ...( acc[ item[key] ] || [ ] ), item
    ]
}

), {} /*inicial state*/ );

const seniors_: Array<User> = seniors;

const ouput: Grouped<User> = groupBy_(seniors_, "active");

// expected output -> result === output

const result = {
  "true": [
    {
      "id": 1,
      "name": "Jack",
      "active": true,
      "level": "senior"
    },
    {
      "id": 2,
      "name": "Suzan",
      "active": true,
      "level": "senior"
    },
    {
      "id": 3,
      "name": "Joana",
      "level": "senior",
      "active": true
    }
  ],
  "false": [
    {
      "id": 4,
      "name": "Mark",
      "level": "senior",
      "active": false
    }
  ]
}