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?

Answer 1

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
  }
}