Help me create a better 2d array, please

README.md

My request to the API returns an array of objects. I need to group each item in this array based on the value of one of its keys. Thus, I need to end up with a 2d array of arrays.

what_I_have_now.js

// API returns an array
const array = [
  { foreign_key: 1 },
  { foreign_key: 2 },
  { foreign_key: 1 },
  { foreign_key: 1 },
  { foreign_key: 3 },
  { foreign_key: 2 },
  { foreign_key: 3 },
  { foreign_key: 1 },
  { foreign_key: 2 },
  { foreign_key: 3 },
  { foreign_key: 3 },
  { foreign_key: 1 }
]

// Reduce to an object with keys of the foreign_key
const groupByForeignKeys = items.reduce((acc, cur) => {
  if (!acc[cur.foreign_key]) {
    acc[cur.foreign_key] = []
  }

  acc[cur.foreign_key].push(cur)

  return acc
}, {})

// Create our empty 2d array
let twoDArray = []

// For each key, push the value of arrays to our now 2d array
for (let key in groupByForeignKeys) {
  twoDArray.push(groupByForeignKeys[key])
}

// This is what I'm getting and expecting
// twoDArray === [
//  [ { foreign_key: 1 }, { foreign_key: 1 }, { foreign_key: 1 }, { foreign_key: 1 } ],
//  [ { foreign_key: 2 }, { foreign_key: 2 }, { foreign_key: 2 } ],
//  [ { foreign_key: 3 }, { foreign_key: 3 }, { foreign_key: 3 }, { foreign_key: 3 } ]
// ]

What should the data look like when its done?

The data should end up looking like this:

const 2dArray = [
  [ { foreign_key: 1 }, { foreign_key: 1 }, { foreign_key: 1 }, { foreign_key: 1 } ],
  [ { foreign_key: 2 }, { foreign_key: 2 }, { foreign_key: 2 } ],
  [ { foreign_key: 3 }, { foreign_key: 3 }, { foreign_key: 3 }, { foreign_key: 3 } ]
]

If all the keys are the same, does in need to be an array of objects, could it not be an array of strings/numbers?

Well, these objects have other keys as well. I didn't want to write out full objects. The array I'm getting is a bunch of records from an API, so there are other keys like id, name, primary_image, etc.

Looks like instead of for...in I can use Object.values(obj) to achieve my 2dArray in this case.

So it would be:

const twoDArray = Object.values(groupByForeignKeys)

I'd use lodash.

Something like this...

var result = _.groupBy(data, "foreign_key");

That will give you an object where the keys are the unique "foreign_key" values. Each one of those keys will be an array of objects that contain that key.

That would be a good solution, too. Frankly, more legible than Object.values(), too. Thanks for your help, Paul.

So I think the overall solutions would look like this...

const _ = require('lodash');

const array = [
  { foreign_key: 1 },
  { foreign_key: 2 },
  { foreign_key: 1 },
  { foreign_key: 1 },
  { foreign_key: 3 },
  { foreign_key: 2 },
  { foreign_key: 3 },
  { foreign_key: 1 },
  { foreign_key: 2 },
  { foreign_key: 3 },
  { foreign_key: 3 },
  { foreign_key: 1 }
];

const groupByForeignKeys = _.groupBy(array, "foreign_key");

const twoDArray = Object.values(groupByForeignKeys);