combine data on a common field from multiple arrays in node

Question

I have the below user data.The data was nested on 2 levels and flattening it doesn't work

[
  [
    { class_id: "Grade 1", roll_number: "1", name: "alex" },
    { class_id: "Grade 1", roll_number: "2", name: "bob" },
  ],
  [
    { class_id: "Grade 2", roll_number: "7", name: "peter" },
    { class_id: "Grade 3", roll_number: "6", name: "lia" },
    { class_id: "Grade 2", roll_number: "5", name: "mary" },
    { class_id: "Grade 3", roll_number: "1", name: "violet" },
  ],
]

Requirement is that I have to generate data taking into account of the class_id and generate information as below

[
  {
    class_id: "Grade",
    students: [
      { roll_number: "1", name: "alex" },
      { roll_number: "2", name: "boc" },
    ],
  },
  {
    class_id: "Grade 2",
    students: [
      { roll_number: "7", name: "peter" },
      { roll_number: "5", name: "mary" },
    ],
  },
  {
    class_id: "Grade 3",
    students: [
      { roll_number: "6", name: "lia" },
      { roll_number: "1", name: "violet" },
    ],
  },
]

I have tried using the below piece of code

var response = [];
data.map(item => {
  var students = [];
  for (var i = 0; i < item.length; i++) {
    const stu_obj = {
      name: item[i].name,
      roll_number: item[i].roll_number,
    };
    students.push(stu_obj);
    if (i == 0) {
      const class_obj = {
        class_id: item[0].class_id,
        studentsList: students,
      };
      response.push(class_obj);
      continue;
    }
  }
});

But it doesn't work for cases where the item in array has multiple class_ids. What should I do?

Does this answer your question? [Group array items using object](https://stackoverflow.com/questions/31688459/group-array-items-using-object) — pilchard, Jul 20 '23 at 18:52
combined with [JavaScript flattening an array of arrays of objects](https://stackoverflow.com/questions/29158723/javascript-flattening-an-array-of-arrays-of-objects) — pilchard, Jul 20 '23 at 18:53

Andy · Accepted Answer · 2023-07-21T08:59:51.173

If you use the flat method on the data array you can iterate over the objects in the flattened array to process them.

If you're trying to assign things to an object with a specific id one of the simpler methods is to create an object with that id as its key name, and then assign the object you want to contain the relevant data as its value. Then, at the end of the iteration, you can grab the Object.values as an array of objects matching your criteria.

Here's an example using reduce.

const data = [[{"class_id":"Grade 1","roll_number":"1","name":"alex"},{"class_id":"Grade 1","roll_number":"2","name":"bob"}],[{"class_id":"Grade 2","roll_number":"7","name":"peter"},{"class_id":"Grade 3","roll_number":"6","name":"lia"},{"class_id":"Grade 2","roll_number":"5","name":"mary"},{"class_id":"Grade 3","roll_number":"1","name":"violet"}]];

// Flatten the array and then `reduce` over the
// array. For iterated object...
const out = data.flat().reduce((acc, c) => {
  
  // ...destructure its relevant properties
  const { class_id, roll_number, name } = c;

  // If a property on the accumulator doesn't exist
  // create it and assign it an object with the class_id
  // and and empty students array
  acc[class_id] ??= { class_id, students: [] };

  // Push an object containing the roll_number and name
  // to the students array of the corresponding object
  // identified by the class_id
  acc[class_id].students.push({ roll_number, name });

  // Return the accumulator for the next
  // round of iteration
  return acc;
}, {});

// Since the result of `reduce` is an object
// use `Object.values` to get an array containing its
// object values
console.log(Object.values(out));

Additional documentation

This is just a standard group-by with more dupes than one could count — pilchard, Jul 20 '23 at 18:51

score 0 · Answer 2 · answered Jul 20 '23 at 18:54

So you want to group your students by class_id and then organize that data as an array.

I'm not sure what the original structure is, can it only be nested 1 level deep? If so, then code like this should work:

function groupStudentsByClass(data) {
    // Flatten the array (assumes data is only nested a single level)
    let flatData = data.flat();

    // Group by class_id
    const objByClassId = flatData.reduce((acc, curr) => {
        if (!acc[curr.class_id]) {
            acc[curr.class_id] = [];
        }

        acc[curr.class_id].push({ "roll_number": curr.roll_number, "name": curr.name });

        return acc;
    }, {});

    // Convert the object to the desired array format
    return Object.keys(objByClassId).map(class_id => {
        return { "class_id": class_id, "students": objByClassId[class_id] };
    });
}

console.log(groupStudentsByClass([
    [
        { "class_id": "Grade 1", "roll_number": "1", "name": "alex" },
        { "class_id": "Grade 1", "roll_number": "2", "name": "bob" }
    ],
    [
        { "class_id": "Grade 2", "roll_number": "7", "name": "peter" },
        { "class_id": "Grade 3", "roll_number": "6", "name": "lia" },
        { "class_id": "Grade 2", "roll_number": "5", "name": "mary" },
        { "class_id": "Grade 3", "roll_number": "1", "name": "violet" }
    ]
]));

Thanks...but originally it is nested on 2 levels. I have provided a more refined form of the data and flatten is not an option — sumitra_gangula, Jul 20 '23 at 19:02
@sumitra_gangula In what way does flattening not work? It works fine here: https://jsfiddle.net/5tkqjbro/ — Unmitigated, Jul 20 '23 at 19:17

score -1 · Answer 3 · answered Jul 20 '23 at 18:51

You can flatten the array, then iterate over it while using an object to store an array for each class_id.

const arr = [[{"class_id":"Grade 1","roll_number":"1","name":"alex"},{"class_id":"Grade 1","roll_number":"2","name":"bob"}],[{"class_id":"Grade 2","roll_number":"7","name":"peter"},{"class_id":"Grade 3","roll_number":"6","name":"lia"},{"class_id":"Grade 2","roll_number":"5","name":"mary"},{"class_id":"Grade 3","roll_number":"1","name":"violet"}]];
const res = Object.entries(arr.flat().reduce((acc, {class_id, ...rest}) => {
  (acc[class_id] ??= []).push(rest);
  return acc;
}, {})).map(([class_id, students]) => ({class_id, students}));
console.log(res);

combine data on a common field from multiple arrays in node

3 Answers3