Combining and grouping arrays using JavaScript

Question

I have two sets of data that need to be combined and grouped a certain way.

The first set of data:

let overallCert =
[
  {
      "id": "1",
      "entity_id": "3",
      "status": "Certified",
  },
  {
      "id": "379",
      "entity_id": "417",
      "status": "Certified",
  }
];

The second set of data:

let userCerts =
[
  {
      "id": "640",
      "entity_id": "417",
      "method": "Field Study",
      "strand": "",
      "date_completed": "2016-07-15T08:00:00.000Z"
  },
  {
      "id": "814",
      "entity_id": "417",
      "method": "Field Study",
      "date_completed": "2019-07-15T08:00:00.000Z"
  },
  {
      "id": "844",
      "entity_id": "3",
      "method": "Online",
      "date_completed": "2022-03-28T08:00:00.000Z"
  },
  {
      "id": "845",
      "entity_id": "3",
      "method": "Field Study",
      "date_completed": "2022-03-28T08:00:00.000Z"
  }
];

I want to merge and group these arrays of objects by entity_id to create this output below...

Desired output:

let desiredOutput = 
[
  [
    [
      {
        "id": "640",
        "entity_id": "417",
        "method": "Field Study",
        "date_completed": "2016-07-15T08:00:00.000Z"
      },
      {
        "id": "814",
        "entity_id": "417",
        "method": "Field Study",
        "date_completed": "2019-07-15T08:00:00.000Z"
      },
    ],
    [
      {
        "id": "379",
        "entity_id": "417",
        "status": "Certified",
      }
    ]
  ],
  [
    [
      {
        "id": "844",
        "entity_id": "3",
        "method": "Online",
        "date_completed": "2022-03-28T08:00:00.000Z"
      },
      {
        "id": "845",
        "entity_id": "3",
        "method": "Field Study",
        "date_completed": "2022-03-28T08:00:00.000Z"
      }
    ],
    [
      {
        "id": "379",
        "entity_id": "417",
        "status": "Certified",
      }
    ]
  ]
];

So far, I have managed this:

let certsDataCombined = overallCert.map(item => ({ ...item,
  0 : userCerts.filter(c => c.entity_id == item.entity_id)
}));

let userCertsGroupBy = groupBy(certsDataCombined, "entity_id");

function groupBy(arr, prop) {
  const map = new Map(Array.from(arr, obj => [obj[prop], []]));
  arr.forEach(obj => map.get(obj[prop]).push(obj));
  return Array.from(map.values());
}

This code almost does the trick, but I need to encapsulate the overallCert data in its own array, plus the nesting is a little off. is the current output:

Can you tell me why the object with entity_id 417 { "id": "379", "entity_id": "417", "status": "Certified" } is there in second array in desired output? Also why the element with "id": "1" is not there in the desired output? — Nitheesh
– Nitheesh, Commented Mar 31, 2022 at 4:29
What should the overall ordering of the result be? Does it matter? — Phil
– Phil, Commented Mar 31, 2022 at 4:29
@Nitheesh That was a copy and paste mistake when I was creating the array in VS Code. I'd update the question, but the edit feature is not working for me right now — Kurt Leadley
– Kurt Leadley, Commented Mar 31, 2022 at 4:43

Phil · Accepted Answer · 2022-03-31 04:55:58Z

Seems you could build the output by mapping each overallCert to a new array and then pulling the matching userCerts using a filter

const overallCert = [{"id":"1","entity_id":"3","status":"Certified"},{"id":"379","entity_id":"417","status":"Certified"}];
const userCerts = [{"id":"640","entity_id":"417","method":"Field Study","strand":"","date_completed":"2016-07-15T08:00:00.000Z"},{"id":"814","entity_id":"417","method":"Field Study","date_completed":"2019-07-15T08:00:00.000Z"},{"id":"844","entity_id":"3","method":"Online","date_completed":"2022-03-28T08:00:00.000Z"},{"id":"845","entity_id":"3","method":"Field Study","date_completed":"2022-03-28T08:00:00.000Z"}];

const result = overallCert.map(cert => [
  userCerts.filter(({ entity_id }) => cert.entity_id === entity_id),
  [ cert ]
]);

console.log(result);

.as-console-wrapper { max-height: 100% !important; }

Note the order is based on overallCert and not userCerts like in your question.

The above is O(n^2) time complexity which isn't fantastic. You could improve this by first grouping both arrays by entity_id for O(n). This would also let you order by userCerts like in your examples.

const overallCert = [{"id":"1","entity_id":"3","status":"Certified"},{"id":"379","entity_id":"417","status":"Certified"}];
const userCerts = [{"id":"640","entity_id":"417","method":"Field Study","strand":"","date_completed":"2016-07-15T08:00:00.000Z"},{"id":"814","entity_id":"417","method":"Field Study","date_completed":"2019-07-15T08:00:00.000Z"},{"id":"844","entity_id":"3","method":"Online","date_completed":"2022-03-28T08:00:00.000Z"},{"id":"845","entity_id":"3","method":"Field Study","date_completed":"2022-03-28T08:00:00.000Z"}];

// Helper function since Map is missing this
Map.prototype.getOrDefault = function(key, defaultValue) {
  return (this.has(key) ? this : this.set(key, defaultValue)).get(key);
};

// Groups certs by entity_id
const reducer = (map, cert) => (
  map.getOrDefault(cert.entity_id, []).push(cert),
  map
);

const overallCertsByEntityId = overallCert.reduce(reducer, new Map());
const userCertByEntityId = userCerts.reduce(reducer, new Map());

const result = Array.from(userCertByEntityId, ([key, certs]) => [
  certs,
  overallCertsByEntityId.get(key)
]);

console.log(result);

.as-console-wrapper { max-height: 100% !important; }

This is exactly what I was looking for (order didn't matter, it's going into a db via an api). Thanks!

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