How to return the duplicate objects of two arrays?

Question

Let's say we have:

var array1 = [{ id: 1 }, { id: 4}, { id: 3 }] 
var array2 = [{ id: 1 }, { id: 2}]

I know you can concat the two arrays like this (without having duplicates):

Array.from(new Set(array1.concat(array2)))

Now, how to create a new array with only the objects that share the same values?

var array2 = [{ id: 1 }]

Not exactly, I think, as here there isn't proof that the objects are really the same, just equivalent. — Oscar Paz, Mar 21 '18 at 10:25
Anyways, use lodash for this sort of stuff `_.intersectionBy(array1, array2, _.eq)` — georg, Mar 21 '18 at 10:37

Mohammad Usman · Answer 1 · 2018-03-21T10:35:13.027

2

You can use .filter() and .some() to extract matching elements:

let array1 = [{ id: 1 }, { id: 4}, { id: 3 }] 
let array2 = [{ id: 1 }, { id: 2}] 

let result = array1.filter(({id}) => array2.some(o => o.id === id));

console.log(result);

Useful Resources:

edited Mar 21 '18 at 10:35

answered Mar 21 '18 at 10:26

Mohammad Usman

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I just want them to be equivalent, without relying in any value. – alex Mar 21 '18 at 10:28
@alex I didn't get your point. Can you be more specific what you wants? – Mohammad Usman Mar 21 '18 at 10:29
@alex Do you wants to match all properties (not just one) of an object in `array1` with any object in `array2`? – Mohammad Usman Mar 21 '18 at 10:38

Nina Scholz · Accepted Answer · 2018-03-21T10:29:49.083

1

You could take a set with the id of the objects and filter array2

var array1 = [{ id: 1 }, { id: 4}, { id: 3 }] ,
    array2 = [{ id: 1 }, { id: 2}],
    s = new Set(array1.map(({ id }) => id)),
    common = array2.filter(({ id }) => s.has(id));
    
console.log(common);

The requested sameness with identical objects.

var array1 = [{ id: 1 }, { id: 4}, { id: 3 }] ,
    array2 = [array1[0], { id: 2}],
    s = new Set(array1),
    common = array2.filter(o => s.has(o));
    
console.log(common);

edited Mar 21 '18 at 10:29

answered Mar 21 '18 at 10:27

Nina Scholz

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I just want them to be equivalent, without relying in any value. – alex Mar 21 '18 at 10:28

score 1 · Answer 3 · answered Mar 21 '18 at 10:30

Assuming, by your definition, that the objects, even if they have the same structure, are not really the same object, I define an 'equality function', and then, with filter and some:

var array1 = [{ id: 1 }, { id: 4}, { id: 3 }] 
var array2 = [{ id: 1 }, { id: 2}];

var equal = function(o1, o2) { return o1.id === o2.id };

var result = array2.filter(function(item1) {
  return array1.some(function(item2) { return equal(item1, item2) });
});

console.log(result);

How to return the duplicate objects of two arrays?

3 Answers3