How to merge and return new array from object in es6

Question

Suppose there are two objects.

const a = [
  { id: '1-1-1', name: 'a111' },
  { id: '1-1-2', name: 'a112' },
  { id: '1-2-1', name: 'a121' },
  { id: '1-2-2', name: 'a122' },
  { id: '2-1-1', name: 'a211' },
  { id: '2-1-2', name: 'a212' }
]
const b = ['1-1', '1-2', '2-1']

and the result

  {

      '1-1':[
        { id: '1-1-1', name: 'a111' },
        { id: '1-1-2', name: 'a112' },
      ],
      '1-2':[
          { id: '1-2-1', name: 'a121' },
          { id: '1-2-2', name: 'a122' },
      ],
      '2-1':[
        { id: '2-1-1', name: 'a211' },
        { id: '2-1-2', name: 'a212' },
      ]
    }

Basically, I want to group the data.

I use includes to check if the item from b to match the id from a. Then construct the new array.

This is my attempt(fiddle):

return b.map(item => a.map(jtem => {
  if(jtem.id.includes(item)){
    return {
      [item]: jtem
    }
  }
}))

For somehow, it doesn't work.

and, is there a clever way to avoid the nested for loop or map function?

Shouldn't the result be an object? – Jack Bashford Mar 25 '19 at 07:02 — Jack Bashford, Mar 25 '19 at 07:02
@JackBashford Hey man, sry, u r right, I just updated it. – SPG Mar 25 '19 at 07:08 — SPG, Mar 25 '19 at 07:08
Do you really want `includes`? I'd recommend `startsWith` – Bergi Mar 25 '19 at 08:26 — Bergi, Mar 25 '19 at 08:26
@Bergi thx, I think `startWith` is better – SPG Mar 26 '19 at 00:30 — SPG, Mar 26 '19 at 00:30

Maheer Ali · Accepted Answer · 2019-03-25T15:19:11.787

You can do that in following steps:

Apply reduce() on the array b
During each iteration use filter() on the the array a
Get all the items from a which starts with item of b using String.prototype.startsWith()
At last set it as property of the ac and return ac

const a = [
  { id: '1-1-1', name: 'a111' },
  { id: '1-1-2', name: 'a112' },
  { id: '1-2-1', name: 'a121' },
  { id: '1-2-2', name: 'a122' },
  { id: '2-1-1', name: 'a211' },
  { id: '2-1-2', name: 'a212' }
]
const b = ['1-1', '1-2', '2-1']

let res = b.reduce((ac,b) => {
  
  ac[b] = a.filter(x => x.id.startsWith(b));
  return ac;

},{})
console.log(res)

As suggested by @Falco is the comments that It would be better to scan over the a once as its large. So here is that version.Actually its better regarding performance

const a = [
  { id: '1-1-1', name: 'a111' },
  { id: '1-1-2', name: 'a112' },
  { id: '1-2-1', name: 'a121' },
  { id: '1-2-2', name: 'a122' },
  { id: '2-1-1', name: 'a211' },
  { id: '2-1-2', name: 'a212' }
]
const b = ['1-1', '1-2', '2-1']


let res = a.reduce((ac,x) => {
  let temp = b.find(y => x.id.startsWith(y))
  if(!ac[temp]) ac[temp] = [];
  ac[temp].push(x);
  return ac;
},{})

console.log(res)

Note: startsWith is not supported by I.E. So you can create polyfill using indexOf

if(!String.prototype.startWith){
  String.prototype.startsWith = function(str){
    return this.indexOf(str) === 0
  }
}

While it is specifically said in the question that the op wants to use es6, and that IE don't support es6 features, I just want to mention that startsWith() don't work in IE (while reduce, filter, and setting a property of an object is totally fine if IE > 9) and if someone wants to do the same thing that startsWith do, they can implment their own with some substring :) — Neyt, Mar 25 '19 at 10:07
For big `a` and small `b` I would probably go with `a.reduce(...)` because of locality and only scan over the big array once. — Falco, Mar 25 '19 at 11:13
@MaheerAli Thank you - here is a benchmark comparing the two :-) https://jsbench.me/dfjtoadysr/1 — Falco, Mar 25 '19 at 11:47

How to merge and return new array from object in es6

1 Answers1