javascript - group result using date from object keys and only get count the results

Question

Hello I'm having a hard time getting this complex return using MongoDB nor Javascript. Hope can anyone teach me how to get this return.

Admin.

group result by id

user flatten result

Here's the data example.

let user = [
    {
  _id: 123,
  name: 'John',
  createdAt: "2015-08-12T00:00:00Z"
  },
  {
  _id: 124,
  name: 'Jane',
  createdAt: "2015-09-12T00:00:00Z"
  },
  {
  _id: 125,
  name: 'Robert',
  createdAt: "2015-09-12T00:00:00Z"
  },
  {
  _id: 126,
  name: 'Samson',
  createdAt: "2016-11-12T00:00:00Z"
  }
]

Expected Result

user

[
{
 "15-8": 1 //yyyy-mm: number of data for the month of august
},
{
 "15-9": 2
},
{
 "16-11": 1
}
]

admin

[
    {
     "15-8": 
   {
    _id: 123,
    count: 1
   }
    },
    {
     "15-9": {
     _id: 124,
     count: 1,
   },{
     _id: 125,
     count: 1
    },
    {
     "16-11": {
     _id: 126,
     count: 1
     }
    }
    ]

question from:https://stackoverflow.com/questions/65947873/group-result-using-date-from-object-keys-and-only-get-count-the-results

Answer 1

1. You should have the function to get key from date string named getKeyFromDate with format result YY-MM

2. Loop user data to aggregate your data by using reduce, for example.

let user = [
    {
      _id: 123,
      name: 'John',
      createdAt: "2015-08-12T00:00:00Z"
    },
    {
      _id: 124,
      name: 'Jane',
      createdAt: "2015-09-12T00:00:00Z"
    },
    {
      _id: 125,
      name: 'Robert',
      createdAt: "2015-09-12T00:00:00Z"
    },
    {
      _id: 126,
      name: 'Samson',
      createdAt: "2016-11-12T00:00:00Z"
    }
];

const getKeyFromDate = (dateString) => {
  var date = new Date(dateString);
  var year = date.getFullYear().toString().substr(2, 3);
  var month = date.getMonth() + 1;
  return `${year}-${month}`; // YY-MM
};

var result = user.reduce((acc, {createdAt}) => {
  var key = getKeyFromDate(createdAt);
  acc[key] = acc[key] || {[key]: 0}; //Also use shortcut like: acc[key] ??= {[key]: 0};
  acc[key][key] += 1;
  return acc; 
  
}, {});
console.log(Object.values(result));