Combine objects of an array's subcategories based on its category

Question

Q: Here is an array of multiple objects (category and sub categories).

let cats = [
  { cat: 'letter', subCat: 'A'}, 
  { cat: 'letter', subCat: 'B'}, 
  { cat: 'number', subCat: '2'}, 
  { cat: 'number', subCat: '3'}, 
  { cat: 'number', subCat: '4'}, 
  { cat: 'other', subCat: '+'}, 
]

need to convert this cats object like this -

cats = [
    { cat: 'letter', subCat: ['A', 'B'] },
    { cat: 'number', subCat: ['2', '3', '4'] },
    { cat: 'other', subCat: ['+'] }
  ]

Answer 1

You can achieve this using a simple reduce

let cats=[{cat:'letter',subCat:'A'},{cat:'letter',subCat:'B'},{cat:'number',subCat:'2'},{cat:'number',subCat:'3'},{cat:'number',subCat:'4'},{cat:'other',subCat:'+'},]

const merged = cats.reduce((a, {cat, subCat})=>{
  a[cat] = a[cat] || {cat, subCat:[]}
  a[cat]["subCat"].push(subCat)
  return a;
},{})

console.log(Object.values(merged))

Create an accumulator with each unique cat as key and the object you need in the final array as value like this:

{
  "letter": {
    "cat": "letter",
    "subCat": [
      "A",
      "B"
    ]
  }
}

Then use Object.values to get only the values in an array.

You could do the whole thing in one go:

let cats = [{cat:'letter',subCat:'A'},{cat:'letter',subCat:'B'},{cat:'number',subCat:'2'},{cat:'number',subCat:'3'},{cat:'number',subCat:'4'},{cat:'other',subCat:'+'},]

const merged = Object.values(cats.reduce((a,{cat, subCat}) => (
  (a[cat] = a[cat] || {cat, subCat:[]})["subCat"].push(subCat), a
),{}))

console.log(merged)