Select nested array of objects inside an array of objects

Question

My data structure:

const obj = {
    "data": [
        {
            "id": 1,
            "name": "Genre 1",
            "books": [
                { "id": 1, "name": "book 1" },
                { "id": 2, "name": "book 2" },
                { "id": 3, "name": "book 3" }
            ]
        },
        {
            "id": 2,
            "name": "Genre 2",
            "books": [
                { "id": 1, "name": "book 1" },
                { "id": 2, "name": "book 2" },
                { "id": 3, "name": "book 3" }
            ]
        },
        {
            "id": 3,
            "name": "Genre 3",
            "books": [
                { "id": 1, "name": "book 1" },
                { "id": 2, "name": "book 2" },
                { "id": 3, "name": "book 3" }
            ]
        }
    ]
};

If given an id, how can I assign the appropriate array of books to a 1 const?

You should edit your post to have valid JSON, b/c right now people are spending half of their effort on the fact that this is not valid JSON, so it would be nice to know if you go with var <whatever> = [....] or var <whatever> = { "data": [...] }. If you updated this, you could get better responses for how to find an object within an array that is the value to the key called data (the way I think you intended this to be, OR the object that has the correct id in a variable called data with an array as it's value (which some might argue would be simpler). Either way, it would help :) — Joshua Michael Calafell
– Joshua Michael Calafell, Commented Apr 9, 2018 at 13:06

Brandon Taylor · Accepted Answer · 2018-04-09 01:10:15Z

You can use .find() to get to the correct object, then access the books array.

const books = data.find(d => d.id === id).books;

This assumes data is an array of objects. If you literally have an object with a key of "data", it would be closer to this:

const obj = {
  "data": [
    {
      "id": 1,
      "name": "Genre 1",
      "books": [
          { "id": 1, "name": "book 1" },
          { "id": 2, "name": "book 2" },
          { "id": 3, "name": "book 3" }
       ]
    },
    {
      "id": 2,
      "name": "Genre 2",
      "books": [
          { "id": 1, "name": "book 1" },
          { "id": 2, "name": "book 2" },
          { "id": 3, "name": "book 3" }
      ]
    },
    {
      "id": 3,
      "name": "Genre 3",
      "books": [
        { "id": 1, "name": "book 1" },
        { "id": 2, "name": "book 2" },
        { "id": 3, "name": "book 3" }
      ]
  }
]};

const books = obj.data.find(d => d.id === id).books;

Reference for .find()

Joshua Michael Calafell · Accepted Answer · 2018-04-09 19:32:05Z

0

Basic example of how to find the books with a particular ID

const id_you_are_looking_for = 1;

const answer = obj.data.find(x => x.id === id_you_are_looking_for).books;

You could make a function out of it...

function returnBooksById(obj, id) {
    return obj.data.find(x => x.id === id).books;
}

Or using arrow functions...

const returnBooksById = (obj, id) => obj.data.find(x => x.id === id).books;

Or, if you're using a library like Lodash with ES6, it's as easy as this...

import { find } from 'lodash';
const id = 1
const books = find(obj, { id }).books;

edited Apr 9, 2018 at 19:32

answered Apr 9, 2018 at 1:23

Joshua Michael Calafell

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2 Comments

kctang Over a year ago

Avoid filter() if you are trying to find a specific entry. Filter will iterate through the entire array even after finding what you are seeking. Filter should be used when you are trying to filter one or more matching elements. Use the find() solution above by @Brandon.

Joshua Michael Calafell Over a year ago

You're right, for some reason I had not thought about the performance of .filter() and do agree with you now. Thanks!

Naveed Akram · Accepted Answer · 2020-12-29 08:54:42Z

0

Basic example of how to find the books with a particular ID

const result = data.books.filter(x => x.id == 1);
console.log(result);

answered Dec 29, 2020 at 8:54

Naveed Akram

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1 Comment

Tomer Shetah Over a year ago

Hello and welcome to SO! While this code may answer the question, providing additional context regarding how and/or why it solves the problem would improve the answer's long-term value. Please read the tour, and How do I write a good answer?

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