Listing duplicate entries in an array of objects in JavaScript/Node.js

Question

I have an array offers with objects that look like so:

{ 
    sellItem: {
        id: _, 
        quantity: _
    }, 
    buyItem: {
        id: _, 
        quantity: _
    }, 
    volume: _
}

I would like to find duplicates - meaning offers with the same 'sell id' and 'buy id'. I would also like to log the index of these items in the original "offers" array.

I tried doing this for 2 days but did not manage to get anywhere because I found it too difficult to write what I'd like to do in a manageable amount of lines. The other questions on StackOverflow were concerned with only one object rather than with nested objects.

Example of what my offers array looks like:

{sellItem: {id: Pizza, quantity: 2}, buyItem: {id: Dollar, quantity: 1}, volume: 1}
{sellItem: {id: Pizza, quantity: 3}, buyItem: {id: Dollar, quantity: 2}, volume: 1}
{sellItem: {id: Banana, quantity: 2}, buyItem: {id: Pound, quantity: 1}, volume: 1}
{sellItem: {id: Apple, quantity: 2}, buyItem: {id: Euro, quantity: 1}, volume: 1}
{sellItem: {id: Pizza, quantity: 5}, buyItem: {id: Dollar, quantity: 3}, volume: 1}

And the expected result here:

0: Selling 2x Pizza for 1x Dollar
1: Selling 3x Pizza for 2x Dollar
4: Selling 5x Pizza for 3x Dollar

All other entries from the "offers" array should be ignored because they are not duplicate offers.

Unmitigated · Accepted Answer · 2020-08-05 18:26:17Z

1

You can use filter along with find to search for another element with the same id.

const arr = [{sellItem: {id: 'Pizza', quantity: 2}, buyItem: {id: 'Dollar', quantity: 1}, volume: 1},
{sellItem: {id: 'Pizza', quantity: 3}, buyItem: {id: 'Dollar', quantity: 2}, volume: 1},
{sellItem: {id: 'Banana', quantity: 2}, buyItem: {id: 'Pound', quantity: 1}, volume: 1},
{sellItem: {id: 'Apple', quantity: 2}, buyItem: {id: 'Euro', quantity: 1}, volume: 1},
{sellItem: {id: 'Pizza', quantity: 5}, buyItem: {id: 'Dollar', quantity: 3}, volume: 1}];
const res = arr.filter(({sellItem: {id}},idx)=>
   arr.find(({sellItem:{id:id2}},idx2) => idx !== idx2 && id === id2));
console.log(res);

answered Aug 5, 2020 at 18:26

Unmitigated

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2 Comments

vxern Over a year ago

Wow! It took you only a few dozen seconds to come up with something that took me 2 days of ripping my hair out. Thanks so much! I will study this code to find out exactly how it works.

Unmitigated Over a year ago

@vxern Happy to help.

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