Sum of same objects with Array.reduce in Javascript

December 3, 2021

I have an array like this:

const inputArray = [
  {
    userId: 1,
    sum: 30,
  },
  {
    userId: 1,
    sum: 20,
  },
  {
    userId: 2,
    sum: 50,
  },
  {
    userId: 2,
    sum: 80,
  },
];

Then I wrote a function that sums the values by key and got the following result:

const output = [
  {
    userId: 1,
    sum: 50,
  },
  50,
  {
    userId: 2,
    sum: 130,
  },
  130,
];

How can this error be corrected? Function code below:

const output = inputArray.reduce((accumulator, currentItem) => {
  const index = accumulator.findIndex((item) => item.userId === currentItem.userId);

  if (index < 0) {
    return [...accumulator, currentItem];
  } else {
    return [...accumulator, (accumulator[index].sum += currentItem.sum)];
  }
}, []);

>Solution :

Try

const inputArray = [
  {
    userId: 1,
    sum: 30,
  },
  {
    userId: 1,
    sum: 20,
  },
  {
    userId: 2,
    sum: 50,
  },
  {
    userId: 2,
    sum: 80,
  },
];

const output = inputArray.reduce((accumulator, currentItem) => {
  const matchedItem = accumulator.find((item) => item.userId === currentItem.userId);

  if (!matchedItem) {
    return [...accumulator, currentItem];
  } else {
    matchedItem.sum += currentItem.sum
    return accumulator
  }
}, []);

console.log(output)

you were appending a new item to accumulator even when the item already existed, you just needed to mutate it. Also, since you dealing with an array of objects, you can use find as it will return a reference to the matched object itself, letting you update the object directly, rather than having to use findIndex then use the index to find it in the array again