es6 unique array of objects with set

Question

I came across this example for creating unique arrays with es6

[ ...new Set(array) ]

Which seems to work fine until I tried it with an array of objects and it didn't return unique array.

i.e.

let item = [ ...new Set([{id:123,value:'test'},{id:123,value:'test'}]) ];

Why is that ?

What do you mean by "doesnt work"? `[ ... new Set([{ val: 1 }, { val: 2 }, { val: 3 }]) ]` gives me an array of objects... — James Monger, Oct 12 '16 at 11:09
by Using Set I was hoping the get a filtered Unique array back — StevieB, Oct 12 '16 at 11:10
This has nothing to do with `Set`, two objects are **never** the same, even if they contain the same values — adeneo, Oct 12 '16 at 11:15

Vic · Answer 1 · 2018-03-01T13:31:01.010

25

you can try to do

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

I know its ugly as hell but in most cases works apart from where you have new Date() in your object param then that on stringify be converted to ISO string.

so then do

let arr = [{id:1},{id:1},{id:2}];
uniqueArray(arr) //[{id:1},{id:2}]

edited Mar 01 '18 at 13:31

answered Mar 01 '18 at 13:24

Vic

404
4
7

I came across this question and I realised I've written exaclty the same with exactly the same syntax in my console :-) – ekqnp Dec 04 '18 at 13:33
However, this solution won't work with regexp: `a = [{b: /foo/},{b: /bar/}]` will return `[{b:{}}]` – ekqnp Dec 04 '18 at 15:33
Only this one is woking with Mongoose document. – Homan Huang Sep 01 '19 at 11:21

score 19 · Accepted Answer · answered Oct 12 '16 at 11:13

Why is that ?

As per documentation

The Set object lets you store unique values of any type, whether primitive values or object references.

Now reference for each of those arrays inside that Set constructor will be different so they are not considered to be a unique value by the constructor.

score 5 · Answer 3 · answered Apr 27 '17 at 20:13

This will work:

let objectReference = {id:123,value:'test'}
let uniqueArray = [...new Set([objectReference, objectReference])]

>> [{id:123,value:'test'}]

What you're doing:

let objRef1 = {id:123,value:'test'} // creates a reference to a location in memory
let objRef2 = {id:123,value:'test'} // creates a new reference to a different place in memory

let uniqueArray = [...new Set([objRef1, objRef2])]

>> [{id:123,value:'test'},{id:123,value:'test'}]

score 0 · Answer 4 · answered Jul 26 '23 at 15:05

If you don't use a library like lodash or radash for example. You can use Set as answered by @Vic. Just a bug detected is about objects without same keys order. For example {a: '1', b: '2'} and {b: '2', a: '1'} are not string equals. Here is a working solution with covered cases

Implementation

uniq(source) {
  if (!Array.isArray(source)) {
     return [];
  }
  return [...new Set(source.map(o => {
    const sortedObjectKeys = Object.keys(o).sort();
    const obj = Object.assign({}, ...sortedObjectKeys.map(k => ({[k]: o[k]})) as any);
    return JSON.stringify(obj);
  }))]
  .map(s => JSON.parse(s));
}

Test cases

describe(`uniq`, () => {
  it('should return unique collection values', () => {
    expect(uniq([{v: 1}, {v: 2}, {v: 1}])).toEqual([{v: 1}, {v: 2}]);
  });

 it('should return unique collection values for unsorted properties', () => {
   expect(uniq([{a: 'test', v: 1}, {v: 2}, {v: 1, a: 'test'}])).toEqual([{a: 'test', v: 1}, {v: 2}]);
 });

 it('should return empty array for non array source', () => {
   expect(uniq({v: 1})).toEqual([]);
   expect(uniq('aString')).toEqual([]);
   expect(uniq(125)).toEqual([]);
   expect(uniq(true)).toEqual([]);
   expect(uniq([])).toEqual([]);
   expect(uniq(undefined)).toEqual([]);
   expect(uniq(null)).toEqual([]);
});

});

es6 unique array of objects with set

4 Answers4

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