Array Groupby And Sum Values

I have an array like this: let purchases = [{ year: 2019, sum: 3000 }, { year: 2019, sum: 5500 }, { year: 2020, sum: 2300 } ] I nee

Solution 1:

This can be done using Array.prototype.reduce.

let purchases = [{
    year: 2019,
    sum: 3000
  },
  {
    year: 2019,
    sum: 5500
  },
  {
    year: 2020,
    sum: 2300
  }
];

const output = purchases.reduce((acc, cur) => {
  acc[cur.year] ? acc[cur.year] += cur.sum : acc[cur.year] = cur.sum;
  return acc;
}, {});
console.log(output);