javascript flatten array and remove duplicates with reduce

Question

I am attempting to flatten an array and remove duplicates in one step using reduce and a Set but the array is flattened but the duplicates remain.

The related answer speaks to removing duplicates from an array but I would like to flatten and remove duplicates in one step, so I would suggest this might not be a duplicate question.

const wells = [
  [{
      id: 1,
      name: 'well one'
    },
    {
      id: 2,
      name: 'well two'
    },
    {
      id: 3,
      name: 'well three'
    },
  ],
  [{
      id: 2,
      name: 'well two'
    },
    {
      id: 3,
      name: 'well three'
    },
    {
      id: 4,
      name: 'well four'
    },
  ],
];


const uniqueflattenedWells = wells.reduce(
  (a, b) => Array.from(new Set(a.concat(b))), [],
);

console.log(uniqueflattenedWells)

Objects aren't `===` to each other, so a Set won't de-duplicate — CertainPerformance, Dec 09 '20 at 17:53
I edited the question to have it re opened as the suggested answer talks of removing duplicates but I would like to flatten and remove duplicates in one step. so hopefully it will be reopened — Bryan Dellinger, Dec 09 '20 at 17:59

blex · Accepted Answer · 2020-12-09T18:07:03.070

You can use filter and every if IDs are ok to determine whether items are duplicated:

const wells = [[{"id":1,"name":"well one"},{"id":2,"name":"well two"},{"id":3,"name":"well three"}],[{"id":2,"name":"well two"},{"id":3,"name":"well three"},{"id":4,"name":"well four"}]];

const uniqueflattenedWells = wells.reduce(
  (a, b) => a.concat(b.filter(o => a.every(x => x.id !== o.id))),
  [],
);

console.log(uniqueflattenedWells)

Note: this method won't dedupe items coming from the same inner Array

score 0 · Answer 2 · answered Dec 09 '20 at 17:57

Objects are only equal when they are the exact same reference. One way to solve this problem is by using JSON.stringify to compare the objects, but note that this is not the fastest method.

const wells = [
  [{
      id: 1,
      name: 'well one'
    },
    {
      id: 2,
      name: 'well two'
    },
    {
      id: 3,
      name: 'well three'
    },
  ],
  [{
      id: 2,
      name: 'well two'
    },
    {
      id: 3,
      name: 'well three'
    },
    {
      id: 4,
      name: 'well four'
    },
  ],
];


const uniqueflattenedWells = ((set) => wells.reduce((a,b)=>a.concat(b), []).filter(obj => {
    const str = JSON.stringify(obj);
    return !set.has(str) && set.add(str);
  })
)(new Set);

console.log(uniqueflattenedWells)

javascript flatten array and remove duplicates with reduce

2 Answers2