Getting unique objects from array of objects

Question

I have an array of objects that looks like this:

[
  {
    "places": "NEW YORK",
    "locations": "USA"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  }
]

I want to filter and get only the unique entries. I tried the following function, but it doesn´t work:

function onlyUnique(value, index, self) { 
  return self.indexOf(value) === index;
}

Using a Set didn't seem to work either. Any ideas?

You need to deep compare. Unique instances of Objects (even if they have the exact same property values) won't be considered equal. — zero298, Apr 04 '19 at 19:31
Why is the property `country` missing in the first object? Is it a mistake? — Damien, Apr 04 '19 at 19:45
It´s not a mistake, some objects got two keys, others got three. — , Apr 04 '19 at 19:46
Ideally, when mapping in the first case, I would like to return only the first two objects — , Apr 04 '19 at 19:48

score 3 · Answer 1 · answered Apr 04 '19 at 19:51

A simple solution using new Set().

const data = [
            {
                "places": "NEW YORK",
                "locations": "USA"
            },
            {
                "places": "MONTREAL",
                "locations": "QUEBEC",
                "country": "Canada"
            },
  {
                "places": "MONTREAL",
                "locations": "QUEBEC",
                "country": "Canada"
            },

        ];
        
const uniqueArray = a => [...new Set(a.map(o => JSON.stringify(o)))].map(s => JSON.parse(s));

console.log(uniqueArray(data));

I was thinking on something similar using `JSON.stringify()`, but what would happen if you have two objects that have same pairs of `(key, value)` but they are presented in different order when you `stringify` they? — Shidersz, Apr 04 '19 at 20:03

score 0 · Answer 2 · answered Apr 04 '19 at 19:50

0

You can do it like this

let data = data.filter(function (a) {
    let key = a.places + '|' + a.country;
    if (!this[key]) {
        this[key] = true;
        return true;
    }
}, Object.create(null));

console.log(data);

answered Apr 04 '19 at 19:50

Sergo Kupreishvili

444
9
21

Can you please explain how this works? (Also I think you're going to get an "identifier already declared error for `let data = data.filter(...`) – Cat Apr 04 '19 at 20:00

score 0 · Answer 3 · answered Apr 04 '19 at 19:56

if you are using underscore, you can some thing like this :

var list = [
  {
    "places": "NEW YORK",
    "locations": "USA"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },

];

var uniqList = _.uniq(list, function (item) {
 return item.places+'_'+item.locations+'_'+item.country;
});

console.log(uniqList);

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore.js"></script>

zfrisch · Answer 4 · 2019-04-05T23:23:35.773

Solution:

You can use filter with a local Set object. By comparing the stringified objects and the size property before and after trying to add to the Set, you can return a unique array by asking if the two numbers are not equal.

Code:

place_array.filter(v => b.size != b.add(JSON.stringify(v)).size, b = new Set());

Why use this code?

Unlike many of the other answers it relies on a singular pass through the array using the filter iterator. This means that it's more performant than multiple loops through an array. It also uses only localized variables, meaning that their is no global clutter and, since the filter method is functional, absolutely no side effects.

Note: There is always a hit to performance when using a Set Object in any situation. This comes down to Browser internals, however, when finding unique Objects/Arrays the stringify into a Set method is likely the best solution. Though with incredibly small data-sets you may find it practical to use an Array or build your own task-specific Object.

Working Example:

let place_array = [{
    "places": "NEW YORK",
    "locations": "USA"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },

];

let result = place_array.filter(v => b.size != b.add(JSON.stringify(v)).size, b = new Set());

console.log(result);

Alternative Solution:

You can use filter with a local Array object. By determining whether or not the stringified objects are in the Array already and pushing if they're not, you can return a unique array by asking if the stringified object is not included in the array.

Code:

place_array.filter(v => !b.includes(s(v)) && !!b.push(s(v)), s = JSON.stringify, b = []);

Why use this code?

Effective though the Set object may be, it is ultimately slower due to backend processes than utilizing an Array object. You can prove this quite easily by running a test like a jsperf: https://jsperf.com/set-vs-ray-includes-battle/1

The Results of a Test:

Working Example:

let place_array = [{
    "places": "NEW YORK",
    "locations": "USA"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },
  {
    "places": "MONTREAL",
    "locations": "QUEBEC",
    "country": "Canada"
  },

];

let result = place_array.filter(v => !b.includes(s(v)) && !!b.push(s(v)), s = JSON.stringify, b = []);

console.log(result);

@David I updated my answer with an alternative solution that utilizes `Array.prototype.includes` and is much faster, while still maintaining all the benefits of the `Set` method. — zfrisch, Apr 05 '19 at 23:25

score 0 · Answer 5 · answered Apr 04 '19 at 20:07

Something like this:

 var distObj = {};
 for(let i = 0; i < obj.places.lenght; i++){
    let tmp = obj.places;
    distObj[i].places = obj[i].places.filter(item => item !== tmp);
 }

If you combine this with for( k in Obj) and callback you can iterate over every deep and attr in Object

benvc · Answer 6 · 2021-09-29T21:51:25.833

If your key values are simple as in your example, then you could do something like the following to handle instances where objects in your array have the same keys and values but not in the same order (since object property order is not guaranteed across browsers).

Convert objects to arrays using Object.entries, sort, and convert to strings with JSON.stringify
Remove duplicates with Set and convert strings back to arrays using JSON.parse and then to objects using reduce to convert array pairs to object key value pairs

const arr = [{ "places": "NEW YORK", "locations": "USA" }, { "places": "MONTREAL", "locations": "QUEBEC", "country": "Canada" }, { "country": "Canada", "places": "MONTREAL", "locations": "QUEBEC" }];
const strings = arr.map((obj) => JSON.stringify(Object.entries(obj).sort()));
const result = [...new Set(strings)].map((s) => {
  return JSON.parse(s).reduce((acc, [k, v]) => {
    acc[k] = v;
    return acc;
  }, {});
});

console.log(result);
// [{"locations":"USA","places":"NEW YORK"},{"country":"Canada","locations":"QUEBEC","places":"MONTREAL"}]

Shakil Alam · Answer 7 · 2021-09-28T14:33:02.773

0

You can try this.

let uniqueObjects= [];
objects.map(item => {
    let exists = uniqueProperties.some((object)=>{
        // Do check here
        return object.type == item.type;
    })
    if (!exists) {
        uniqueObjects.push(item);
    }
})

edited Sep 28 '21 at 14:33

answered Sep 28 '21 at 14:20

Shakil Alam

308
4
9

Getting unique objects from array of objects

7 Answers7

Solution:

Code:

Why use this code?

Working Example:

Alternative Solution:

Code:

Why use this code?

The Results of a Test:

Working Example: