Sorting array of objects into an array of paired objects with Javascript

Question

I have an array of objects and I want to be able to sort them by their "site" value into pairs. There can't be more that 2 objects in each child array so if there is 3 matches I get 1 child array with 2 objects and 1 child array with 1 object.

I have:

[{site:'A'}, {site:'A'}, {site:'B'}, {site:'B'}, {site:'B'}];

I want:

[[{site:'A'}, {site:'A'}],[{site:'B'}, {site:'B'}], [{site:'B'}]]

Whats the best way to do this? any help is appreciated.

[Split array into chunks](https://stackoverflow.com/q/8495687)? — VLAZ, Jul 15 '21 at 20:35
Can a chunk have both one A and one B? What would happen then? — trincot, Jul 15 '21 at 20:55

trizin · Answer 1 · 2021-07-15T21:17:58.993

3

This should work for you

function sortArray(arr){
    arr.sort((a,b)=>a.site > b.site ? 1 : -1) // Sorting the array to have consecutive values
    let chunks = [];
    for(let i = 0;i<arr.length;i+=2){
        if(arr[i]?.site == arr[i+1]?.site) chunks.push(arr.slice(i,i+2));
         else {
               chunks.push([arr[i]]);
               i--;
         }
    }
    return chunks;
}

let arr = [{site:'A'}, {site:'A'}, {site:'B'}, {site:'B'}, {site:'B'}];
console.log(sortArray(arr))

edited Jul 15 '21 at 21:17

answered Jul 15 '21 at 20:44

trizin

347
2
13

@MikeM Thanks for asking that. I've updated my answer, it should work now. – trizin Jul 15 '21 at 20:54
Yes, seems to work, but you're not likely to get many votes as it doesn't use `reduce`. – MikeM Jul 15 '21 at 20:55

score 2 · Accepted Answer · answered Jul 15 '21 at 21:14

Using reduce ;) :

const a = [{
  site: 'A'
}, {
  site: 'A'
}, {
  site: 'B'
}, {
  site: 'B'
}, {
  site: 'B'
}];

var r = a.reduce((ac, x) => ({
  ...ac,
  [x.site]: [...(ac[x.site] || []), x]
}), {})

var r2 = Object.values(r).flatMap(x =>
  x.reduce((ac, z, i) => {
    if (i % 2) {
      ac[i - 1].push(z)
      return ac
    }
    return [...ac, [z]]
  }, []))


console.log(r2)

PS: Since this is hard to read I'd suggest to use lodash (specifically groupBy and chunk methods)

score 0 · Answer 3 · answered Jul 15 '21 at 21:09

It's kind of a 'groupBy' operation (as seen in underscore or lodash). Those produce an object keyed by the values being grouped. Consider writing it that way for general use. To get the shape the OP is looking for, strip out the values of that result...

function groupBy(array, key) {
  return array.reduce((acc, el) => {
    let value = el[key];
    if (!acc[value]) acc[value] = [];
    acc[value].push(el);
    return acc;
  }, {});
}

let array = [{site:'A'}, {site:'A'}, {site:'B'}, {site:'B'}, {site:'B'}];
let grouped = groupBy(array, 'site');  // produces { A: [{site:'A'} ...], B: ... }
let groups = Object.values(grouped)
console.log(groups)

Not quite the right shape: the second array element contains three objects. — MikeM, Jul 15 '21 at 21:13

Sorting array of objects into an array of paired objects with Javascript

3 Answers3