Extract objects from nested array of objects

Question

I have the following data that is an array of nested objects:

"categories": [
  {
   "id": 1,
   "name": "Category 1",
   "years": [
    { "id": 1, "name": "1" },
    { "id": 2, "name": "2" }
   ]
  },
  {
   "id": 2,
   "name": "Category 2",
   "years": [
    { "id": 2, "name": "2" },
    { "id": 3, "name": "3" }
   ]
  }
 ]

I want to extract unique years in a separate array (desired output):

[
  { "id": 1, "name": "1" },
  { "id": 2, "name": "2" },
  { "id": 3, "name": "3" },
]

When I map out the years, I'm getting an array of arrays, how should I extract the unique objects for years?

let years = categories.map( (c) => { return c.years })

Use categories.reduce to build up the collection of unique years — jalynn2
– jalynn2, Commented Aug 22, 2019 at 17:49

Patrick Roberts · Accepted Answer · 2019-08-22 18:02:11Z

2

You can use a Map to filter duplicate years from the array of values using id as the key, and reduce() on both categories and years using the map as the accumulator:

const categories = [{
    "id": 1,
    "name": "Category 1",
    "years": [
      { "id": 1, "name": "1" },
      { "id": 2, "name": "2" }
    ]
  },
  {
    "id": 2,
    "name": "Category 2",
    "years": [
      { "id": 2, "name": "2" },
      { "id": 3, "name": "3" }
    ]
  }
];

const years = categories.reduce(
  (map, category) => category.years.reduce(
    (map, year) => map.set(year.id, year),
    map
  ),
  new Map()
);

console.log([...years.values()]);

edited Aug 22, 2019 at 18:02

answered Aug 22, 2019 at 17:53

Patrick Roberts

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Code Maniac · Accepted Answer · 2019-08-22 17:51:35Z

2

You can use reduce and Map

let data = [{"id": 1,"name": "Category 1","years": [{ "id": 1, "name": "1" },{ "id": 2, "name": "2" }]},{"id": 2,"name": "Category 2","years": [{ "id": 2, "name": "2" },{ "id": 3, "name": "3" }]}]
  
let final = data.reduce((op,{years}) => {
  years.forEach(({id, name}) => {
    let key = id + '-' + name
    op.set(key, op.get(key) || {id, name})
  })
  return op
},new Map())

console.log([...final.values()])

answered Aug 22, 2019 at 17:51

Code Maniac

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