Create an object based on the keys in array of objects

Question

I am having an array of objects and I want to create another object based on their keys. For example I am having an array as

var arr = [{1: 36011, 2: 18320, 3: 36011, 4: 10570},
            {1: 19754, 2: 6722, 3: 19754, 4: 6699},
            {1: 15711, 2: 10039, 3: 15711, 4: 4172}]

and I want my result array as

var result = {1:[36011,19754,15711], 2:[18320,6722,10039],..}

I was suggested to use lodash, I am new to this so I've tried using reduce

var i = 1, new_arr = {};
_.reduce(arr, function(key, val){
     new_arr[i++] = temp1.key 
return new_arr;
},{})

I am getting the values as undefined. What is the mistake, Can anyone help me with this?

There is no need for lodash, reduce is widely supported. This is almost 2020! The i in your code is global, so it will count all the values. The reduce in your code cannot have key/val pair since you are applying it to an array. — kornieff
– kornieff, Commented Dec 1, 2019 at 4:58
Neither of the questions indicated in "close as duplicate" had answers to this question. Both of those were about grouping on a single, specific property, whereas this is about cross-merging multiple objects on arbitrary properties. — JLRishe
– JLRishe, Commented Dec 1, 2019 at 7:08

Ori Drori · Accepted Answer · 2019-12-01 07:18:15Z

2

with lodash you can use _.mergeWith() and concat the values in the customiser function:

const arr = [{"1":36011,"2":18320,"3":36011,"4":10570},{"1":19754,"2":6722,"3":19754,"4":6699},{"1":15711,"2":10039,"3":15711,"4":4172}]
            
const result = _.mergeWith({}, ...arr, (ov = [], sv) => ov.concat(sv))

console.log(result)

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.js"></script>

answered Dec 1, 2019 at 7:18

Ori Drori

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Comments

kornieff · Accepted Answer · 2019-12-02 08:51:32Z

2

This might look intimidating at first. Start reducing the dataset from an empty object. Then reduce each row starting with result of outer reduce and build on the result so far.

const addToBucket = (bucket = {}, k, v, prev = bucket[k] || []) => ({
  ...bucket,
  [k]: [...prev, v],
})

const toBucket = list =>
  list.reduce(
    (bucket, row) =>
      Object.entries(row).reduce((b, [k, v]) => addToBucket(b, k, v), bucket),
    {},
  )

const data = [
  { 1: 36011, 2: 18320, 3: 36011, 4: 10570 },
  { 1: 19754, 2: 6722, 3: 19754, 4: 6699 },
  { 1: 15711, 2: 10039, 3: 15711, 4: 4172 },
]

console.log(toBucket(data))

// Nested list, as requested
console.log(Object.entries(toBucket(data)).map(([_, v]) => v))

edited Dec 2, 2019 at 8:51

answered Dec 1, 2019 at 4:30

kornieff

2,57721 silver badges29 bronze badges

5 Comments

Lahari Over a year ago

Thanks! This was really helpful and I learned a new piece of code today! @kornieff

kornieff Over a year ago

Glad I could help. If my code is still scary, just read up on ES6 deconstruct and spread features. Welcome to 2020 🥳!

Lahari Over a year ago

If I want to create the whole in an array, not an object, How should I change it? @kornieff

kornieff Over a year ago

Can you give me an example of what you want it to look like?

Lahari Over a year ago

I've added an image to my question. check it @kornieff

hygull · Accepted Answer · 2019-12-01 04:51:10Z

0

You can try like this in simplest way.

This is also available online at https://rextester.com/IPI35964 to try and see output.

var arr = [{1: 36011, 2: 18320, 3: 36011, 4: 10570},
        {1: 19754, 2: 6722, 3: 19754, 4: 6699},
        {1: 15711, 2: 10039, 3: 15711, 4: 4172}]

var output = {}
for(var obj of arr){
    for(var key in obj){
        if(key in output){
            output[key].push(obj[key])
        } else {
            output [key] = [obj[key]]
        }
    }
}
console.log(output)

Output

{ '1': [ 36011, 19754, 15711 ],
  '2': [ 18320, 6722, 10039 ],
  '3': [ 36011, 19754, 15711 ],
  '4': [ 10570, 6699, 4172 ] }

edited Dec 1, 2019 at 4:51

answered Dec 1, 2019 at 4:31

hygull

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Comments

JLRishe · Accepted Answer · 2019-12-01 07:20:22Z

One fairly clean way of doing this:

Create one big array of key-value pairs
Group those key-value pairs by their keys
Extract the values from the grouped key-value pairs

var arr = [
  { 1: 36011, 2: 18320, 3: 36011, 4: 10570 },
  { 1: 19754, 2: 6722, 3: 19754, 4: 6699 },
  { 1: 15711, 2: 10039, 3: 15711, 4: 4172 }
];

// Get one big array of all key-value pairs
var pairs = _.flatMap(arr, _.entries);

// Group by the keys (index 0 of the pairs)
var groups = _.groupBy(pairs, 0);

// Extract the values
var final = _.mapValues(groups, _.partial(_.map, _, 1));

//console.log(pairs);
console.log(final);


// Using chaining:
var final2 = _.chain(arr)
  .flatMap(_.entries)
  .groupBy(0)
  .mapValues(_.partial(_.map, _, 1));

console.log(final2);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js"></script>

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