Javascript: Remove elements with a duplicate key-value pair from an array of unique objects except first such one (NOT DUPLICATE)

Question

Note: This is not a duplicate question, as the elements are all unique. If the elements were the same I wouldn't have asked. I am talking about a single key values...

I'm trying to find an efficient way to remove elements with a duplicate key-value pair from an array of unique objects except first such one. i.e.

Let's say I have an array of object like below:

    const array = [
        { checkID: 'aaa', animal: 'cat'},
        { checkID: 'bbb', animal: 'dog'},
        { checkID: 'ccc', animal: 'monkey'},
        { checkID: 'ddd', animal: 'horse'},
        { checkID: 'eee', animal: 'cow'},
        { checkID: 'fff', animal: 'cat'}
    ]

Here is the logic:

The target key is 'animal'
If there is no other element with the same value for the field, keep it.
If there are duplicate elements with the same value for the field but it is the first one, keep it.
If there are duplicate elements with the same value for the field and it is not the first one, delete it.

As a result, the array should be:

    const result = [
        { checkID: 'aaa', animal: 'cat'},
        { checkID: 'bbb', animal: 'dog'},
        { checkID: 'ccc', animal: 'monkey'},
        { checkID: 'ddd', animal: 'horse'},
        { checkID: 'eee', animal: 'cow'}
    ]

The elements are unique and it makes the job complicated. Tried creating a separate array with the 'animal' values and then using the filter method but the code got messy and thought there might be a better way.

Any suggestions? Thanks

Please add your attempt, also why weren't the dog elements filtered out? what's the filter logic exactly? — Majed Badawi, Sep 11 '22 at 11:19
you can take a look at this link https://stackoverflow.com/questions/2218999/how-to-remove-all-duplicates-from-an-array-of-objects — Rashad Musmar, Sep 11 '22 at 11:24
I specifically mentioned that it is an array of unique objects, but someone closed it thinking it is a duplicate of a question about removing duplicate objects...sigh — J.Ko, Sep 11 '22 at 11:37

score 1 · Answer 1 · answered Sep 11 '22 at 11:23

You can use a map to filter out the duplicate as follows:

const array = [ { checkID: 'aaa', animal: 'cat'}, { checkID: 'bbb', animal: 'dog'}, { checkID: 'ccc', animal: 'monkey'}, { checkID: 'ddd', animal: 'horse'}, { checkID: 'eee', animal: 'cow'}, { checkID: 'fff', animal: 'cat'} ];

const unique = [...
  array.reduce((map, obj) => {
    if(!map.has(obj.animal)) map.set(obj.animal, obj);
    return map;
  }, new Map)
  .values()
];

console.log(unique);

score 1 · Accepted Answer · answered Sep 11 '22 at 11:30

You could take a Set in a closure with the wanted key for filtering.

const
    uniqueBy = (k, s = new Set) => o => !s.has(o[k]) && s.add(o[k]),
    array = [{ checkID: 'aaa', animal: 'cat' }, { checkID: 'bbb', animal: 'dog' }, { checkID: 'ccc', animal: 'monkey' }, { checkID: 'ddd', animal: 'horse' }, { checkID: 'eee', animal: 'cow' }, { checkID: 'fff', animal: 'cat' }],
    result = array.filter(uniqueBy('animal'));

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Doing it in four lines of code...fabulous. – J.Ko Sep 11 '22 at 11:40 — J.Ko, Sep 11 '22 at 11:40

Javascript: Remove elements with a duplicate key-value pair from an array of unique objects except first such one (NOT DUPLICATE)

2 Answers2