How comparing object item elements in an array with reduce?

Question

I have array

 const example = [{ day: 5, month: 1, key: 1 },{ day: 5, month: 1, key: 2 },{ day: 3, month: 3, key: 3 },{ day: 5, month: 1, key: 4 },{ day: 3, month: 3, key: 5 },{ day: 11, month: 4, key: 6 }];

how to check the elements of the day and month when the same ? if the year and month are repeated I would like the result return

{err:'the same',index:[0,1,2,3]}

My code not working.

const duplicates = Object.entries(array).reduce((acc, item, index) => {
  if (acc[1].year === item[1].year && acc[1].month === item[1].month) {
    return [
      {
        index:[acc[0],acc[1]],
        err: "the same"
      }
    ];
  }
return acc;});

Majed Badawi · Accepted Answer · 2021-04-26 10:55:29Z

1

Iterate over the array using Array#reduce to get a Map of the day-month as the key and its occurrence indices as the value.
Using Array#filter, get the entries that have more than one index.
After that, return the list using Array#map.

const array = [ { day: 5, month: 1, key: 1 }, { day: 5, month: 1, key: 2 }, { day: 3, month: 3, key: 3 }, { day: 5, month: 1, key: 4 }, { day: 3, month: 3, key: 5 }, { day: 11, month: 4, key: 6 } ];

const duplicates = [...array
.reduce((map, { day, month }, index) => {
  const key = `${day}-${month}`;
  map.set(key, [...(map.get(key) || []), index]);
  return map;
}, new Map)
.values()]
.filter(indices => indices.length > 1)
.map(indices => ({ err: "the same", index: indices }));

console.log(duplicates);

edited Apr 26, 2021 at 10:55

answered Apr 25, 2021 at 21:27

Majed Badawi

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Comments

trincot · Accepted Answer · 2021-04-26 09:02:47Z

I would use the Map constructor to build a map keyed by (unique) year-month values, and where the associated values are arrays of error objects, initialised with an empty index array. Then populate those index arrays, and get those that have more than one entry. This involves no reduce:

const example = [{ day: 5, month: 1, key: 1 },{ day: 5, month: 1, key: 2 },{ day: 3, month: 3, key: 3 },{ day: 5, month: 1, key: 4 },{ day: 3, month: 3, key: 5 },{ day: 11, month: 4, key: 6 }];

const map = new Map(example.map(o => 
    [o.month * 100 + o.day, {err: "the same", index: []}]
));
example.forEach((o, i) => map.get(o.month * 100 + o.day).index.push(i));
const dupes = Array.from(map.values()).filter(o => o.index.length > 1);
console.log(dupes);

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