Get unique values from array of objects using reduce function

Question

I have an array of objects

const data = [{
    productId: 7000254,
    quantity: 1
}, {
    productId: 7000255,
    quantity: 1
}, {
    productId: 7000256,
    quantity: 1
}, {
    productId: 7000257,
    quantity: 1
}, {
    productId: 7000254,
    quantity: 1
}];

I need to get unique values from it using the reduce function.

I made it using below code

data.map((rp) => {
      if (products.map(({ productId }) => productId).indexOf(rp.productId) === -1) {
        products.push({ productId: parseInt(rp.productId), quantity: 1 })
      }
    })

but as you can see it's a lengthy process because I have to iterate over the array multiple times. So is there any way using reduce function?

var unique = data.reduce((a, b ,c,d) => {
  if (a.map(({productId}) => productId).indexOf(b.productId) === -1) {
    return [a,b]
  }
})
console.log(unique)

Expected output

0: {productId: 7000254, quantity: 1}
1: {productId: 7000255, quantity: 1}
2: {productId: 7000256, quantity: 1}
3: {productId: 7000257, quantity: 1}

DecPK · Accepted Answer · 2021-09-01 06:44:03Z

2

You can efficiently achieve this result using filter and Set.

const data = [{
    productId: 7000254,
    quantity: 1,
  },
  {
    productId: 7000255,
    quantity: 1,
  },
  {
    productId: 7000256,
    quantity: 1,
  },
  {
    productId: 7000257,
    quantity: 1,
  },
  {
    productId: 7000254,
    quantity: 1,
  },
];

const set = new Set();
const result = data.filter((o) => {
  if (set.has(o.productId)) return false;
  set.add(o.productId);
  return true;
});

console.log(result);

/* This is not a part of answer. It is just to give the output fill height. So IGNORE IT */

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

answered Sep 1, 2021 at 6:44

DecPK

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7 Comments

Totati Over a year ago

Jeah, I would choose this over reduce. It's easier to read.

DecPK Over a year ago

@Totati Time complexity is also O(n) as compared to reduce if used with find.

Totati Over a year ago

You coudl use Set with reduce too, but I'd still choose filter :D You coudl use Map too, but that would mean 2 iterations, as at the end you need to map the values back to an array.

DecPK Over a year ago

@malarres set.has Will gives the result in constant time O(1).

malarres Over a year ago

nice to know @HR01M8055, I got from the spec that it required to be O(n) "as much" but I didn't know that it was implemented as O(1) source:

Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection.

262.ecma-international.org/6.0/#sec-set-objects

|

Nitheesh · Accepted Answer · 2021-09-01 06:44:59Z

0

Array.reduce implementation.

Logic

Loop through the array.
Before pushing that to accumulator, verify there is aleady a node present in the accumulator.
If a node exist, dont push, or else push it to accumulator.

const data = [{
  productId: 7000254,
  quantity: 1
}, {
  productId: 7000255,
  quantity: 1
}, {
  productId: 7000256,
  quantity: 1
}, {
  productId: 7000257,
  quantity: 1
}, {
  productId: 7000254,
  quantity: 1
}];
const output = data.reduce((acc, curr) => {
  const matchingNode = acc.find(node => node.productId === curr.productId);
  if(!matchingNode) {
    acc.push(curr);
  }
  return acc;
}, []);
console.log(output)

answered Sep 1, 2021 at 6:44

Nitheesh

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4 Comments

DecPK Over a year ago

find inside reduce makes its worst time complexity to n * 2. You could have used Set here to make it O(n)

Nitheesh Over a year ago

@HR01M8055 Question is regarding functionality, not regarding performace.

DecPK Over a year ago

I totally agree with you. But If we get functionality with performance then that will be even better. What is the use of the functionality if it is not efficient. Don’t you think

Nitheesh Over a year ago

@HR01M8055 Agree. We have a lot of performace optimizations. One is provided by you. Do I ned to find all he performace improvement as fix them?

Greg K · Accepted Answer · 2023-05-12 12:15:31Z

0

Best way to do this without iterating multiple times is to use reduce something like this

const output = data.reduce((pv, cv) => (
  pv.array.indexOf(cv.productId) === -1) // new value
    ? {
        array: [...pv.array, cv.productId],
        output: [...pv.output, cv]
      }
    : pv
), { array: [], output: [] })

console.log({ output })

edited May 12, 2023 at 12:15

Greg K

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answered Sep 2, 2021 at 7:49

Profer

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