Map function to build an array from filtered object

Question

I'm trying to get my head around map functions currently without much success.

I currently have a list of objects and am trying to remap to a new object using the map function to return a map indexed by the created property with a list of wooids.

Having written my map function, I only seem to be returning the last time.

const variants = [
{
   created: '2022-03-06',
    hashname: 'c78ba80402290724609a5e98c369c90984494152',
    hashobject: '80864e6329d5e305a512ace872ad7d56a3f41095',
    hashparent: '19c5d50ddddeb7c9a92469df78c47d9d611f1599',
    action: 'added',
    wooid: 7288
  },
{
   created: '2022-03-06',
    hashname: 'c78ba80402290724609a5e98c369c90984494152',
    hashobject: '80864e6329d5e305a512ace872ad7d56a3f41095',
    hashparent: '19c5d50ddddeb7c9a92469df78c47d9d611f1599',
    action: 'added',
    wooid: 7289
  }   
]

const res = Object.fromEntries(variants.map(k => [k.created, [k.wooid]]))
console.log(res)

Current output

{ '2022-03-06': [ 7289 ] }

Desired output:

{ '2022-03-06': [ 7289, 7288 ] }

Any help would be greatly appreciated as I'm unsure how to dynamically populate the array to include all other entires (6 in total).

Sure the code works - however not as I'm anticipating. I'm only getting back the last result - ideally I'm looking to return an object like so: { '2022-03-06': [ 7289, 7288 ] } — user1387363
– user1387363, Commented Mar 7, 2022 at 1:06
What determines the ordering in the resulting array? Why is 7289 first and not 7288 which appears first in the original array? — Phil
– Phil, Commented Mar 7, 2022 at 1:14
Nothing determines the ordering - I've just added to illustrate it capturing multiple values. — user1387363
– user1387363, Commented Mar 7, 2022 at 1:15
Ordering in arrays is significant so which order should the values be in? — Phil
– Phil, Commented Mar 7, 2022 at 1:15

Phil · Accepted Answer · 2022-03-07 01:21:23Z

1

Use Array.prototype.reduce() to build up your object. Check if you already have a key for each created value and if not, create an empty array. Then append the wooid value

const variants = [{"created":"2022-03-06","hashname":"c78ba80402290724609a5e98c369c90984494152","hashobject":"80864e6329d5e305a512ace872ad7d56a3f41095","hashparent":"19c5d50ddddeb7c9a92469df78c47d9d611f1599","action":"added","wooid":7288},{"created":"2022-03-06","hashname":"c78ba80402290724609a5e98c369c90984494152","hashobject":"80864e6329d5e305a512ace872ad7d56a3f41095","hashparent":"19c5d50ddddeb7c9a92469df78c47d9d611f1599","action":"added","wooid":7289}]

const res = variants.reduce((acc, { created, wooid }) => ({
  ...acc,
  [ created ]: [
    ...acc[created] ?? [], // init to an empty array if unset
    wooid
  ]
}), {})

console.log(res)

This will collect wooid values by created in the order they appear in the original data.

answered Mar 7, 2022 at 1:21

Phil

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2 Comments

user1387363 Over a year ago

Ah ok - so reduce is the better option in this instance - thanks for your help!

Phil Over a year ago

@user1387363 Object.fromEntries() is just a simplified reduce anyway. You need something like the above in order to track existing created keys

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