Returning a nested object from a map

Question

Current approach:

function stuff() {
   const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday']
   const names ['tom', 'dave', 'kate', 'jane', 'james']

   const result = {}

   days.map(day => {
      const innerResult = {}

      names.map(name => {
         // some logic
         const res = {
            result: 'some result'
         }

         innerResult[name] = res
      })

      result[day] = innerResult
   })

   return result
}

which would look something like:

{
   "monday": {
      "tom": {
         result: 'some result'
      },
      "dave": {
         result: 'some result'
      },
      "kate": {
         result: 'some result'
      },
      "jane": {
         result: 'some result'
      },
      "james": {
         result: 'some result'
      },
   },
   "tuesday": {
      // ...etc
   }
}

is this possible to achieve without the use of mutable variables such as result and innerResult, can I return this structure straight from the map or something along those lines...

For instance, something like:

function stuff() {
   const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday']
   const names ['tom', 'dave', 'kate', 'jane', 'james']

   return days.map(day => {
      return { [day]: names.map(name => {
         return {
            [name]: { result: 'some result' }
         }
      })}
   })
}

obviously the structure on those won't match but something along those lines, any help would be appreciated.

NEVER use map if you do not need an array as output. Use forEach to just loop. In this case you could use reduce — mplungjan, Sep 01 '22 at 07:21
Does this answer your question? "[Create an object from an array of keys and an array of values](/q/39127989/90527)", "[How create an object with keys which come from an array of keys without loops?](/q/67807806/90527)" — outis, Sep 01 '22 at 07:55

adiga · Accepted Answer · 2022-09-01T07:11:28.290

You could use Object.fromEntries() to create an object from an array of entries.

const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday']
const names = ['tom', 'dave', 'kate', 'jane', 'james']

const output = Object.fromEntries(
    days.map(day => 
       [ 
        day, 
        Object.fromEntries( names.map(n => [n, { result: 'value' }]) ) 
       ]
     )
  )

console.log(output)

You could create an array of individual objects as in the questions. And then use Object.assign() to merge them into a single object

const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday'],
      names = ['tom', 'dave', 'kate', 'jane', 'james'],
     
      // merges an array of objects into one
      merge = arr => Object.assign(...arr), 
      getNested = names => names.map(n => ({ [n]: {result: 'value' } }) ),
      output2 = merge( days.map(day => ({ [day]: merge(getNested(names)) })) )

console.log(output2)

score 2 · Answer 2 · answered Sep 01 '22 at 07:21

Instead of map, you can use reduce

function stuff() {
   const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday'];
   const names = ['tom', 'dave', 'kate', 'jane', 'james'];

    return days.reduce((previousValue, currentValue) => { 
        previousValue[currentValue] = names.reduce((previous, current) => {
            previous[current] = { result: 'some result' };
            return previous;
        }, {});

        return previousValue;
    }, {});    
}

console.log(stuff())

I would not use the word `previousValue` for the accumulator because it is not a previous value — mplungjan, Sep 01 '22 at 07:22

Returning a nested object from a map

2 Answers2